Transcriber’s Notes
Text printed in italics has been transcribed _between underscores_,
bold face text has been transcribed =between equal signs=. Small
capitals have been replaced with all capitals.
Single letters in square brackets, such as [T] and [V], represent
shapes rather than letters.
More Transcriber’s Notes may be found at the end of this text.
Harper’s Practical Books for Boys
A SERIES OF NEW HANDY-BOOKS FOR AMERICAN BOYS
_Each Crown 8vo, with many Illustrations._
I
HARPER’S OUTDOOR BOOK FOR BOYS
By Joseph H. Adams. With Additional Contributions by Kirk Munroe,
Tappan Adney, Capt. Howard Patterson, L. M. Yale, and others. Cloth,
$1.75.
II
HARPER’S ELECTRICITY BOOK FOR BOYS
Written and Illustrated by Joseph H. Adams. With a Dictionary of
Electrical Terms. Cloth, $1.75.
_IN PRESS_
III
HARPER’S HOW TO UNDERSTAND ELECTRICAL WORK
A Simple Explanation of Electric Light, Heat, Power, and Traction in
Daily Life. By Joseph B. Baker, Technical Editor, U. S. Geological
Survey, formerly of the General Electric Company.
IV
HARPER’S INDOOR BOOK FOR BOYS
By Joseph H. Adams and others. Cloth, $1.75.
V
HARPER’S MACHINERY BOOK FOR BOYS
The Boy’s Own Book of Engines and Machinery. Cloth, $1.75.
HARPER & BROTHERS, PUBLISHERS, NEW YORK
[Illustration: Copyright, 1907, by Joseph H. Adams, N. Y.
THOMAS A. EDISON DICTATING TO HIS GRAPHOPHONE]
HARPER’S
ELECTRICITY BOOK
FOR BOYS
WRITTEN AND ILLUSTRATED BY
=JOSEPH H. ADAMS=
AUTHOR OF
“HARPER’S OUTDOOR BOOK FOR BOYS”
WITH AN EXPLANATION OF ELECTRIC LIGHT, HEAT
POWER, AND TRACTION BY JOSEPH B. BAKER
TECHNICAL EDITOR, U. S. GEOLOGICAL SURVEY
AND
=A DICTIONARY OF ELECTRICAL TERMS=
[Illustration]
HARPER & BROTHERS PUBLISHERS
NEW YORK AND LONDON
MCMVII
Copyright, 1907, by HARPER & BROTHERS.
_All rights reserved._
Published November, 1907.
CONTENTS
PAGE
INTRODUCTION xi
Part I
CHAPTER I.--SOME GENERAL EXPLANATIONS 3
AN INVISIBLE WORLD-POWER -- GENERATING ELECTRICITY -- WHAT A BOY
CAN DO -- INEXPENSIVE TOOLS -- SOME PRACTICAL ADVICE
CHAPTER II.--CELLS AND BATTERIES 12
SIMPLE AND INEXPENSIVE CELLS -- HOW TO MAKE CELLS AND BATTERIES
-- A PLUNGE-BATTERY -- A STORAGE-BATTERY -- DRY-CELLS AND
BATTERIES
CHAPTER III.--PUSH-BUTTONS AND SWITCHES 33
HOW TO MAKE PUSH-BUTTONS -- SWITCHES AND CUT-OUTS -- TABLE-JACK
SWITCHES -- BINDING-POSTS AND CONNECTORS -- LIGHTNING-ARRESTERS
AND FUSE-BLOCKS -- SOME PRACTICAL PRECAUTIONS
CHAPTER IV.--MAGNETS AND INDUCTION-COILS 54
SIMPLE AND HORSESHOE MAGNETS -- INDUCTION-COILS -- AN ELECTRIC
BUZZER -- ELECTRIC BELLS -- A LARGE INDUCTION-COIL --
CIRCUIT-INTERRUPTERS
CHAPTER V.--ANNUNCIATORS AND BELLS 78
A DRUM-SOUNDER -- A SIMPLE ANNUNCIATOR -- A DOUBLE ELECTRIC BELL
-- AN ELECTRIC HORN -- HOW TO MAKE A BURGLAR-ALARM -- ELECTRIC
CALL-SIGNALS -- CLOCK-ALARMS -- A DINING-TABLE CALL
CHAPTER VI.--CURRENT-DETECTORS AND GALVANOMETERS 102
HOW TO MAKE DETECTORS -- AN ASTATIC CURRENT-DETECTOR -- AN
ASTATIC GALVANOMETER -- A TANGENT GALVANOMETER
Part II
CHAPTER VII.--ELECTRICAL RESISTANCE 125
GOVERNING THE ELECTRIC CURRENT -- OHM’S LAW -- RESISTANCE-COILS
AND RHEOSTATS -- HOW TO MAKE SIMPLE APPARATUS -- LIQUID
RESISTANCE -- IMPORTANCE OF SWITCHES -- USES OF A HOUSE-CURRENT
-- RUNNING A SEWING-MACHINE, FAN, OR TOYS -- AN EASY METHOD FOR A
BOY’S USE
CHAPTER VIII.--THE TELEPHONE 156
VIBRATORY WAVES -- A BLADDER TELEPHONE -- A SINGLE (RECEIVER)
LINE -- PLAN OF INSTALLATION -- A DOUBLE-POLE RECEIVER -- THE
TRANSMITTER -- ANOTHER FORM OF TRANSMITTER -- THE WIRING SYSTEM
-- A TELEPHONE INDUCTION-COIL -- AN INSTALLATION PLAN -- A
PORTABLE APPARATUS
CHAPTER IX.--LINE AND WIRELESS TELEGRAPHS 190
A GROUND TELEGRAPH -- HOW TO TALK FROM HOUSE TO HOUSE -- THE
MORSE TELEGRAPH CODE -- A STORY OF EDISON -- HOW DETECTIVES USED
THE CODE -- WIRELESS TELEGRAPHY -- ITS TRUE CHARACTER -- HOW A
BOY CAN MAKE A PRACTICAL APPARATUS -- RECEIVING AND SENDING POLES
-- INDUCTION-COILS, BATTERIES, COHERERS AND DE-COHERERS, ETC. --
WORKING PLANS IN DETAIL -- AËROGRAMS ACROSS THE ATLANTIC AND,
PERHAPS, AROUND THE WORLD
CHAPTER X.--DYNAMOS AND MOTORS 229
DEPENDENCE OF MODERN ELECTRICITY UPON THE DYNAMO -- A FIELD OF
FORCE CUTTING ANOTHER FIELD OF FORCE -- VARIETIES OF DYNAMOS --
SIMPLER FORMS OF GENERATORS AND MOTORS -- HOW TO MAKE A
UNI-DIRECTION CURRENT MACHINE -- PERMANENT MAGNET, ARMATURE,
SHAFTS, WHEELS, ETC. -- A SMALL DYNAMO -- MACHINES TO LIGHT
LAMPS, RUN MOTORS, ETC. -- A SPLIT-RING DYNAMO -- A SMALL MOTOR
-- THE FLAT-BED MOTOR -- MOTORS OF OTHER TYPES
CHAPTER XI.--GALVANISM AND ELECTRO-PLATING 266
A FASCINATING USE OF ELECTRICITY -- A SIMPLE ELECTRO-PLATING
OUTFIT -- THE SULPHATE OF COPPER BATH -- HOW TO MAKE THE TANK AND
OTHER APPARATUS -- A VARIETY OF BEAUTIFUL AND USEFUL RESULTS --
EXPLANATIONS OF VARIOUS BATTERIES -- THE CLEANSING PROCESS -- THE
PLATING-BATH -- SILVER-PLATING -- GOLD-PLATING -- NICKEL-PLATING
-- FINISHING -- ELECTROTYPING -- PRACTICAL DETAILS OF INTERESTING
WORK
CHAPTER XII.--MISCELLANEOUS APPARATUS 294
MAKING A ROTARY GLASS-CUTTER -- TO SMOOTH GLASS EDGES -- CUTTING
HOLES IN GLASS -- ANTI-HUM DEVICE FOR METALLIC LINES -- A
REEL-CAR FOR WIRE -- INSULATORS -- JOINTS AND SPLICES --
“GROUNDS” -- THE EDISON ROACH-KILLER -- AN ELECTRIC MOUSE-KILLER
CHAPTER XIII.--FRICTIONAL ELECTRICITY 312
ITS NATURE -- LIMITED USES -- SIMPLICITY OF APPARATUS -- A
“WIMSHURST INFLUENCE MACHINE” -- MATERIALS REQUIRED -- GLASS,
TIN-FOIL, SPINDLES, UPRIGHTS, WHEELS, ETC. -- A LARGE LEYDEN-JAR
-- APPARATUS FOR INTERESTING EXPERIMENTS -- NECESSITY OF CAUTION
CHAPTER XIV.--FORMULÆ 327
ACID-PROOF CEMENTS -- HARD CEMENT -- SOFT CEMENT -- VERY HARD
CEMENT -- CLARK’S COMPOUND -- BATTERY FLUID -- GLASS RUBBING --
ACETIC GLUE -- INSULATORS -- NON-CONDUCTORS -- INSULATING VARNISH
-- BATTERY WAX
CHAPTER XV.--ELECTRIC LIGHT, HEAT AND POWER 334
(By Joseph B. Baker)
THE WORK OF THE DYNAMO -- THE ELECTRIC LIGHT -- USES OF THE
ARC-LIGHT -- INCANDESCENT AND OTHER LAMPS -- ELECTRIC HEAT --
ELECTRIC FURNACES -- WELDING METALS -- ELECTRIC CAR-HEATERS --
HOUSEHOLD USES -- ELECTRIC POWER -- POWER FROM WATER-WHEELS --
TRANSFORMERS -- ROTARY CONVERTERS -- OIL-SWITCHES -- ELECTRIC
TRACTION -- THE TROLLEY-CAR -- THE CONTINUOUS-CURRENT MOTOR --
THE CONTROLLER -- ELECTRIC LOCOMOTIVES -- OTHER FORMS OF ELECTRIC
TRACTION
A DICTIONARY OF ELECTRICAL TERMS 359
INTRODUCTION
If a handy-book of electricity like this had fallen into the hands of
Thomas A. Edison when he was a newsboy on the Grand Trunk Railway, or
when he was a telegraph operator, he would have devoured it with the
utmost eagerness. To be sure, at that time, in the early sixties, all
that we knew of electricity and its applications could have been told in
a very brief compass. It was an almost unknown field, and the crude form
of the telegraph then in use represented its most important application.
There were no electric lights; there was no telephone or phonograph;
there were no electric motors. Telegraphing, itself, was a slow and
difficult process. All the conditions were as far removed as possible
from the broad field of applied electricity indicated in this book.
But this does not mean that we have now accomplished all that there is
to be done. On the contrary, the next half-century will be full of
wonderful advances. This makes it more than ever essential that we
should become acquainted with the principles and present conditions of a
science which is being applied more and more closely to the work of
every-day life. It is necessary to know this from the inside, not simply
from general descriptions. Theory is all very well, but there is nothing
like mastering principles, and then applying them and working out
results for one’s self. Any active and intelligent boy with an
inquiring mind will find a new world opened to him in the satisfaction
of making electrical devices for himself according to the suggestions
given in this book. This will show him the reasons for things in
concrete form, will familiarize him with principles, and will develop
his mechanical ingenuity. He may be laying the foundation for inventions
of his own or for professional success in some of the many fields which
electricity now offers. Work of this kind brings out what is in one, and
there is no satisfaction greater than that of winning success by one’s
own efforts.
The boy who makes a push-button for his own home, or builds his own
telephone line or wireless telegraph plant, or by his own ingenuity
makes electricity run his mother’s sewing-machine and do other home
work, has learned applications of theory which he will never forget. The
new world which he will enter is a modern fairyland of science, for in
the use of electricity he has added to himself the control of a powerful
genie, a willing and most useful servant, who will do his errands or
provide new playthings, who will give him manual training and a vast
increase in general knowledge. The contents of this book, ranging from
the preparation of simple cells to the making of dynamos and motors, and
the delightful possibilities of electro-plating, shows the richness of
the field which is made accessible by Mr. Adams’ practical explanations,
his carefully tested working plans, and his numerous and admirable
drawings--all of which have been made for this book.
It is in keeping with the practical character of the _Electricity Book_
that pains are taken throughout to show the simplest and most
inexpensive way of choosing materials and securing results. The actual
working out of these directions can be done at very small expense.
Furthermore, there need be no concern whatever as to possible danger if
the book is read with reasonable intelligence. Mr. Adams has taken pains
to place danger-signals wherever special precautions are advisable, and,
as a father of boys who are constantly working with electricity in his
laboratory, he may be relied upon as a safe and sure counsellor and
guide.
While this book shows boys what they can do themselves, its scope has
been enlarged by Mr. Baker’s chapter explaining briefly the working of
electricity all about us, in light and heat, in the trolley-car, and
other daily applications. In addition, Mr. Adams has prepared a
Dictionary of Electrical Terms, and these brief definitions will be
found peculiarly helpful in the first reading of the book. It is
believed that there is no book in this particular field comparable to
Harper’s _Electricity Book_ in its comprehensiveness, practical
character, and the number and usefulness of its illustrations. It
follows the successful _Out-door Book for Boys_ in Harper’s series of
_Practical Books for Boys_, and it will be followed by _How to
Understand Electrical Work_, a book, not of instructions in making
electrical apparatus, but of explanations of the commercial uses of
electricity all about us.
Part I
ELECTRICITY BOOK FOR BOYS
Chapter I
SOME GENERAL EXPLANATIONS
We are living in the age of electricity, just as our fathers lived in
the age of steam. Electricity is the world-power, the most powerful and
terrible of nature’s hidden forces. Yet, when man has learned how to
harness its fiery energies, electricity becomes the most docile and
useful of his servants. Unquestionably, electricity is to-day the most
fascinating and the most profitable field for the investigator and the
inventor. The best brains of the country are at work upon its problems.
New discoveries are constantly being recorded, and no labor is thought
too great if it but add its mite to the sum total of our knowledge. And
yet, ridiculous as the statement may seem, we do not know what
electricity is. We only know certain of its manifestations--what it can
do. All we can say is that it does our bidding; it propels our trains,
lights our houses and streets, warms us, cooks for us, and performs a
thousand and one other tasks at the turn of a button or at the thrust of
a switch. But _what_ it is, we do not know. Electricity has no weight,
no bulk, no color. No one has seen it; it cannot be classified, nor
analyzed, nor resolved into its ultimate elements by any known process
of science. We must content ourselves with describing it as one
manifestation of the energy which fills the universe and appears in a
variety of forms--such as heat, light, magnetism, chemical affinity, and
mechanical motion. In all probability it is one of those phenomena of
nature that are destined to remain forever secret. Thus it stands in
line with gravitation, magnetism, the active principle of radium, and
the perpetual motion of the solar system.
Electricity was known to the early Greeks; indeed, it derives its name
from the Greek word for amber (electron). For many centuries amber was
credited with certain special or magical powers. When it was rubbed with
a flannel cloth, “the hidden spirit” came out and laid hold of small
detached objects, such as bits of paper, thread, chips, or pith-balls.
No one could explain this phenomenon. It was looked upon with
superstitious awe and the amber itself was regarded as possessing the
special attributes of divinity. But as time went on, it was discovered
that in various other substances this mysterious attractive power could
be excited, at will, through the agency of friction. Rubbing a piece of
glass rod with silk or leather generated an “electricity” identical with
that of the amber; or the same result could be obtained by exciting hard
rubber with catskin. The conclusion followed that electricity was not a
property of the special materials employed to generate it, but that it
came from without, from that great reservoir of energy, the atmosphere.
Then came Franklin with his experiment of the kite, and the invention
of the Leyden-jar and the chemical production of the electric fluid by
means of batteries. It was shown that the properties of the new and
strange force were the same, whether it was produced by the static
(frictional) process or by the galvanic (chemical) method. Electrical
science as a science, had begun.
And yet, for many years, electricity was hardly more than a scientific
toy. It was not supposed to possess any practical usefulness. The
entertaining experiments with the static machine and the Leyden-jar
(chapter xiii.) were confined to the laboratory and the lecture hall.
Electricity was an amusing display of unknown energy, but no one ever
dreamed that it could ever be made to serve the practical ends of life.
It was not until about 1850 that electrical science became anything more
than a name. The galvanic and voltaic batteries (chapter ii.) opened the
way for “current” electricity, which flowed continuously, instead of
jumping and disappearing like the spark from a Leyden-jar. When the
continuous current became an established fact, the telegraph and
telephone headed the line of a long series of developments. Finally, the
generation of electricity in greater volume, and cheaply, made possible
the application of its power for heating, light, traction, and the other
forms of activity in which it now does so large a share of the world’s
work.
How electricity works is a question often asked, but not easily
answered. There are certain so-called laws, but we shall best arrive at
a conclusion by simply stating a few of the facts that have been
established through the observation and investigation of scientists and
electrical engineers.[1]
[1] Explanations of any technical names or phrases used in the text
will be found in the simple dictionary of electrical terms which
appears as an appendix.
For example, electricity is always alert, ready to move, and continually
on the lookout for a chance to obtain its freedom. It will never go the
longest way round if there is a short cut; and it will heat, light, or
fuse anything in its path that is too weak to carry or resist it. For
this reason, it must be generated in small volume--that is, just
sufficient to do the work required of it. If produced in larger volume,
it must be held in check by resistance, and only so much allowed to
escape as may be needed for the specified work.
Again, when electricity is generated this must be done in one of two
ways--by friction or chemically. But in both processes there must be air
surrounding the generators, and the fluid must be of a nature through
which oxygen and hydrogen can circulate freely. Water fluids are
suitable for this purpose, but oils cannot be used, as they contain
hydro-carbon in large quantities and are non-conductors.
Batteries are chemical generators, dynamos are magneto-electric, and
static machines are frictional. Now the theory is that electricity is
drawn from the ether and, in its normal state, is quiet. If it be
disturbed and collected by mechanical or chemical means, it is always on
the alert to escape and again take its place in the atmosphere. As its
volume is increased, so its energy to get away is multiplied, and this
energy may be transformed, at will, into power, heat, or light. To
express the idea in the simplest language, it wants to go home, and in
its effort to do so it expresses itself in the form of stored-up power,
precisely like water behind a dam. It is for man’s cunning brain to
devise all sorts of tasks that this power must perform before it can
gain its release. It can’t go home until its work is done.
Nearly every boy has experimented, at one time or another, with
electricity and electrical apparatus, and whether it was with some of
the simple frictional or galvanic toys, or with the more complicated
induction-coils and motors, he has undoubtedly found it a most
interesting amusement and an ever new and widening field for study. Then
again, many boys would like to know something about simple electrical
apparatus and how to make and use it. But his school-books relating to
the general subject of electricity are hardly definite enough to serve
as a practical manual. And yet there are many things in the way of
electrical machinery and equipment that a boy can easily construct and
use. In this book it is my purpose to show him just what can be done
with the aid of the tools that are usually in his possession. While some
things may have to be purchased from an electrical supply-house or other
sources, there is still much material to be found about the house that
may be put to good use by the amateur electrician.
It is not possible or desirable to describe every variety of electrical
equipment. We must confine ourselves to apparatus which can be readily
understood and operated. The “practical” idea is the one to be borne in
mind. This book shows a boy how to use his brains and the simple tools
and material that may be at his command. Care and thought in the
construction of the apparatus are the important qualifications for
success. The instructions are given in the clearest possible language;
the diagrams and drawings are intelligible to any one who will take the
trouble to study them. If your finished apparatus does not work
properly, read the description again and see if you have not made some
error. A misplaced or broken wire, a wrong connection, or a short
circuit will mean all the difference between success and failure.
Save in one short chapter, static or frictional electricity (see
Appendix) is not considered; for outside of laboratory experimenting and
electro-medical apparatus, frictional electricity is but a
toy--interesting and useful when generated in small volume, but very
dangerous and difficult of control when in great volume. For example,
the bolt of lightning is but the many times multiplied spark stored in
the Leyden-jar by the static machine. For all practical purposes,
galvanic electricity, in its various phases of direct and alternating
current, meets the requirements of man. With the improved apparatus and
the rapid advancement along the line of invention, electricity is as
easily controlled to-day as steam--in fact, its economical use is even
more fully under control and its adaptability more practical.
In the following pages there are probably illustrations and descriptions
of many things that will seem strange to the boy who has not heard of
them; but if a book were written each year on the subject of
electricity, every new one would include principles and facts not known
before. The field of electrical research is so broad and so many are
working in it that new discoveries are being made continually.
To those familiar with the application of electricity, it is clearly
evident that, as yet, we are only beginning to deal with this unknown
force. For generations to come, developments will take place and
invention follow invention until electricity assumes its rightful place
as the motive force of the world. To the boy interested in this subject
a wide field is open, and the youth of to-day, who are taking up this
study, are destined to become the successful electrical engineers and
inventors of the future. There is no better education for any boy, in
the application and principles of electricity, than to begin at the very
bottom of the ladder and climb up, constructing and studying as he
progresses. When he attempts to design more technical and difficult
apparatus the lessons learned in a practical way will be of inestimable
value, greater by far than any theoretical principles deduced from
books; he knows his subject from the ground up; he understands his
machine because he has constructed it with his own hands.
As I have said already, the necessary tools are few in number and not
expensive. They may include a hammer, a plane, awls, pliers,
wire-cutters, and tin-shears. The raw material is also cheap--lead, tin,
wire, wood, and simple chemicals. The laboratory may be a corner in the
attic, or even in a boy’s bedroom, so far as the finer work is
concerned, while the hammering and sawing may be done in the cellar. The
other best plan, of course, is to get the use of a spare room which may
be fitted with shelves, drawers, and appliances for serious work. To
enthusiastic beginners, as well as to those who have had some experience
in electricity, a needed warning may be given in three words: “Take no
chances.” Electricity, the subtle, stealthy, and ever-alert force, will
often deal a blow when least expected. For that reason, a boy should
never meddle with a high-tension current or with the mains from dynamos.
The current in the house, used for lighting, cooking, or heating
purposes, is always an attractive point for the young electrician, but
the wires should never be touched in any way. Too many accidents have
happened, and the conductors, lamp-sockets, and plugs should be
carefully avoided.
The boy should keep strictly to his batteries, or small dynamos run by
water-power from a faucet; in no case should the wire from power-houses
be tampered with. One little knows what a current it may be carrying and
what a death-dealing force it possesses. Always bear in mind that a
naked wire falling from a trolley equipment carries enough force to kill
anything it strikes.
Special attention is called to the dictionary of electrical terms given
in the Appendix. The young student should never pass over a word or a
term that he does not thoroughly understand. Always look it up at once
and _every time_ it occurs, until you are sure that its meaning is fixed
in your mind. This is an education in itself, at least so far as the
theoretical knowledge of our subject is concerned.
As a final word, I should like every boy interested in electricity to
hear what Thomas A. Edison once said to me when I was a boy working in
his laboratories. I often recall it when things do not go just right at
first.
I asked the great inventor one day if invention was not made up largely
of inspiration. He looked at me quizzically for a moment, and then
replied: “My boy, I have little use for a man who works on inspiration.
Invention is two parts inspiration and ninety-eight per cent.
perspiration.”
You will never get what you are after unless you work hard for it. You
must stick to it until you produce results. If the history of the
world’s most valuable inventions could be fully known, the fact would be
clearly established that the vital spark of inspiration is but the
starting-point. Then follow the days, weeks, and sometimes years of
industrious toil, failures, and disappointments, until finally the
desired end is attained. One must work for success; there is no other
means of winning it.
As the table of contents shows, Part I. of this book explains principles
and the simpler forms of electrical appliances. From this we advance to
Part II., which deals with more complex forms of electrical work, most
of which, however, are within the reach of intelligent boys who have
followed the chapters carefully from the first. In a final chapter we
have simple explanations of the great commercial uses of electricity,
which we see all about us, although very few of us have a clear idea as
to their operation.
Chapter II
CELLS AND BATTERIES
Simple Cells
In order to generate electricity it is necessary to employ cells,
batteries, or dynamos. Since the construction and operation of a dynamo
is somewhat intricate, it will be better to start with the simpler
methods of electric generation, and so work up to the more complicated
forms. For small apparatus, such as electric bells and light magnets and
motors, the zinc-carbon-sal-ammoniac cell will answer very well; but for
larger machinery, where more current is required, the bluestone and the
bi-chromate batteries will be found necessary.
[Illustration: FIG. 1
FIG. 2
FIG. 3
FIG. 5
SIMPLE BATTERY ELEMENTS]
A simple and inexpensive cell may be made from electric-light carbons,
with the copper coating removed, and pencils of zinc, such as are used
for electric-bell batteries and which can be purchased for five cents
each. Copper wire is to be bound around the top of each pencil of carbon
and zinc, and firmly fastened with the pliers, so that it will not pull
off or become detached. It will be well to cut a groove with a file
around the top of both the carbon and zinc, into which the wire will
fit. The elements should then be clamped between two pieces of wood
and held with screws, as shown in Fig. 1. A more efficient carbon pole
is made by strapping six or more short carbon pencils around one long
one, as shown in Fig. 3. The short pieces of electric-light carbons are
bound to the longest carbon with heavy elastic bands, or cotton string
dipped in paraffine or wax, to make the cotton impervious to water and
the sal-ammoniac solution.
Another arrangement of elements is shown in Fig. 2, where a zinc rod is
suspended between two carbons, the carbons being connected by a wire
that must not touch the zinc.
A fruit-jar, or a wide-necked pickle-bottle, may be employed for a cell,
but before the solution is poured in, the upper edge of the glass should
be coated with paraffine. This should be melted and applied with a
brush, or the edge of the glass dipped in the paraffine.
The solution is made by dissolving four ounces of sal-ammoniac in a pint
of water, and the jar should be filled three-fourths full. In this
solution the carbons and zinc may be suspended, as shown in the
illustration (Fig. 4) of the sal-ammoniac cell. The wood clamps keep the
carbon and zinc together, and the extending ends rest on the top of the
jar and hold the poles in suspension. Plates of zinc and carbon may be
clamped on either side of a square stick and suspended in the
sal-ammoniac solution, as shown in Fig. 5, taking care, however, that
the screws used for clamping do not touch each other.
If one cell is not sufficiently powerful, several of them may be made
and coupled up in series--that is, by carrying the wire from the zinc
of one to the carbon of the next cell, and so on to the end, taking care
that the wire from the carbon in the first cell and that from the zinc
of the last cell will be the ones in hand, as shown in Fig. 6. This
constitutes a battery. Be sure and keep the ends of the wire apart, to
prevent galvanic action and to save the power of the batteries.
This battery is an excellent one for bells and small experimental work,
and when inactive the zincs are not eaten away (as they would be if
suspended in a bi-chromate solution), for corrosion takes place only as
the electricity is required, or when the circuit is closed. A series of
batteries of this description will last about twelve months, if used for
a bell, and at the end of that time will only require a new zinc and
fresh solution.
The cell in which the plates shown in Fig. 5 are used may contain a
bi-chromate solution; and for experimental work, where electricity is
required for a short time only, this will produce a stronger current.
But remember that the solution eats the zinc rapidly, and the plates
must be removed as soon as you have finished using them.
The bi-chromate solution is made by slowly pouring four ounces of
commercial sulphuric acid into a quart of cold water. This should be
done in an earthen jar, since the heat generated by adding acid to water
is enough to crack a glass bottle. Never pour the water into the acid.
When the solution is about cold, add four ounces of bi-chromate of
potash, and shake or mix it occasionally until dissolved; then place it
in a bottle and label it:
BI-CHROMATE BATTERY FLUID
POISON
Before the zincs are immersed in the bi-chromate solution they should be
well amalgamated to prevent the acid from eating them too rapidly.
The amalgamating is done by immersing the zincs in a diluted solution of
sulphuric acid for a few seconds, and then rubbing mercury (quicksilver)
on the surfaces. The mercury will adhere to the chemically cleaned
surfaces of any metal except iron and steel, and so prevent the
corroding action of the acid. Do not get on too much mercury, but only
enough to give the zinc a thin coat, so that it will present a silvery
or shiny surface.
A two-fluid cell is made with an outer glass or porcelain jar and an
inner porous cup through which the current can pass when the cup is wet.
Fig. 7.
[Illustration: FIG. 4]
[Illustration: FIG. 7]
[Illustration: FIG. 8]
[Illustration: FIG. 6]
A porous cup is an unglazed earthen receptacle, similar to a flower-pot,
through which moisture will pass slowly. The porous cup contains an
amalgamated plate of zinc immersed in a solution of diluted sulphuric
acid--one ounce to one pint of water. The outer cell contains a
saturated solution of sulphate of copper in which a cylindrical piece of
thin sheet-copper is held by a thin copper strap, bent over the edge of
the outer cell. A few lumps or crystals of the copper sulphate, or
bluestone, should be dropped to the bottom of the jar to keep the copper
solution saturated at all times. When not in use, the zinc should be
removed from the inner cell and washed off; and if the battery is not to
be employed for several days, it would be well to pour the solutions
back into bottles and wash the several parts of the battery, so that it
may be fresh and strong when next required. When in action, the
solutions in both cups should be at the same level, and be careful never
to allow the solutions to get mixed or the copper solution to touch the
zinc. Coat the top of the porous cell with paraffine to prevent
crystallization, and also to keep it clean. Take great care, in handling
the acid solutions, to wear old clothes, and do not let the liquids
spatter, for they are strong enough to eat holes in almost anything, and
even to char wood. The two-fluid cells are much stronger than the
one-solution cells, and connected up in series they will develop
considerable power.
For telegraph-sounders, large electric bells, and as accumulators for
charging storage-batteries, the gravity-cell will give the most
satisfactory results. The one shown in Fig. 8 consists of a deep glass
jar, three strips of thin copper riveted together, and a zinc crow-foot
that is caught on the upper edge of the glass jar. These parts will have
to be purchased at a supply-house, together with a pound or two of
sulphate of copper (bluestone).
To set up the cell, place the copper at the bottom and drop in enough of
the crystals to generously cover the bottom, but do not try to imbed the
metallic copper in the crystals; then fill the jar half full of clear
water. In another jar dissolve two ounces of sulphate of zinc in enough
water to complete the filling of the jar to within two inches of the
top; then hang the zinc crow-foot on the edge of the jar so that it is
immersed in the liquid and is suspended about three inches above the top
of the copper strip. The wire that leads up from the copper should be
insulated with a water-proof coating and well covered with paraffine. A
number of these cells may be connected in series to increase the power
of the current, and for a working-battery this will show a high
efficiency. Note that at first the solutions will mingle. To separate
them, join the two wires and start the action; then, in a few hours, a
dividing line will be seen between the white, or clear, and the blue
solutions, and the action of the cell will be stronger. After
long-continued use it may be necessary to draw off some of the clear
zinc sulphate, or top solution, and replace it with pure water. The
action of the acids reduces the metallic zinc to zinc sulphate and
deposits metallic copper on the thin copper strips, and in this process
an electrical current is generated.
A Plunge-battery
When two or more cells (in which sulphuric acid, bi-chromate of potash,
or other strong electropoions are employed) are coupled in series, it
would be well to arrange the copper and zinc, or the zinc and carbon,
poles on a board, so that all of them may be lowered together into the
solutions contained in the several jars. A simple arrangement of this
kind is shown in Fig. 9, where a rack is built for the jars and at the
top of the end boards a projecting piece of wood, supported by a
bracket, is made fast. A narrow piece of board nearly the length of the
jar-rack is fitted with the battery-poles, as shown at Fig. 9 A. The
carbon and zinc, or copper and zinc, poles are attached to small blocks
of wood (as described for Fig. 5), and this block in turn is fastened to
the under side of the board with brass screws. The poles of the cells
are to be connected (as explained in Fig. 6), and when the battery is in
use the poles are immersed in the solution contained in the jars. When
the battery is at rest the narrow board should be lifted up and placed
on the projecting arms of the rack, so that the liquid on the poles may
drain into the jars directly underneath. One or more of these
battery-racks may be constructed, but they cannot be made to hold
conveniently more than four or six cells each; if more cells are
required, those contained in each rack must be coupled up in series.
[Illustration: FIG. 9]
[Illustration: FIG. 10]
A simpler plunge-battery is shown in Fig. 10. A cell-rack is made of
wood and given two or three coats of shellac. The narrow board (to the
under side of which the battery-poles are attached, as explained in Fig.
9) is hung on chains or flexible wires, which in turn are made fast to
an iron shaft running the entire length of the cell-rack. This shaft is
of half-inch round iron, and is held in place, at one end, by a pin and
washer; while at the other the end is filed with a square shoulder, and
a handle and crank is fitted to it, so that the shaft may be turned. A
small hole, made at the side of the crank when it is hanging down, will
receive a hard-wood peg, or a steel nail, and this will prevent the
crank from slipping when the board holding the poles is raised. If a
gear-wheel and tongue can be had to fit on the shaft, it will then be
possible to check the shaft securely at any part of a turn of the crank.
The battery-poles are to be connected in series along the top of the
portable board, as explained for Fig. 6. When two or more of these
plunge-batteries are used at one time, the wire from the carbon of one
is to be connected with the zinc pole of the next, and so on. The wire
from the zinc of the first battery, and the wire from the carbon of the
last battery, will be the ones available for use.
A Storage-battery
When more current is desired than the simple batteries will give, a
storage-battery should be employed as an accumulator. This result can be
secured by coupling primary cells in series, so that they will be
constantly generating and feeding the battery. Storage-batteries are too
heavy to be shifted about, like single cells or small plunge-batteries;
they should be placed in a cellar, where the charging or primary cells
can be located close by, and, unless positively necessary, the battery
of cells and the accumulator should not be moved.
With sufficiently large insulated wires (Nos. 12, 14, or 16 copper), the
current may be carried to any part of the house for use in various
ways--such as running a light motor or a fan, lighting a lamp-circuit,
or fusing metals and chemicals for experimental purposes. While the
battery to be described is not a light one in weight, nor as economical
as the improved new Edison storage-battery, it is a good and constant
one, and, if not overcharged or abused, will last for several years.
The component parts of a storage-battery are lead in metallic and
chemical form, the electrolyte, or fluid, in which the plates are
immersed, and the water-tight and chemical-proof cell or container. From
a plumber, a supply-house, or a lead-works, obtain a quantity of
three-eighth by one-quarter-inch strip-lead of the kind called chemical,
or desilverized; also a larger quantity of lead-tape, one-sixty-fourth
of an inch thick and three-eighths of an inch wide. This last is also
known as torpedo-lead, and is kept by electrical supply-houses.
If the three-eighths by quarter-inch strip-lead cannot be had, then
purchase eight or ten pounds of heavy sheet-lead, and, with a
tin-shears, divide it into strips three-eighths of an inch wide and
twenty-nine inches long, taking care to cut it of uniform width and with
true edges. From hard-wood three-eighths or half an inch thick, cut a
block six by seven inches and make four countersunk holes in it, so that
it may be screwed fast to a table or bench, as shown in Fig. 11 A.
Around this the lead strips should be shaped and beaten at the corners
to make the angles sharp.
From the three-eighths by quarter-inch, or sheet-lead strips, make seven
frames as shown in Fig. 12. This is done by binding a strip of the lead
around the block, as shown at Fig. 11 B. Where the ends come together
insert a short piece of lead, three-eighths or half-inch, as shown at
Fig. 12 A, and solder it fast. A soldering-iron may be heated with a
Bunsen-burner gas-flame or in a charcoal fire. However, if gas is
available, it would be better to use the blue flame from a Bunsen
burner and direct the hot blast directly on the work with a blow-pipe,
and so fuse the lead points together. After a little practice with the
blow-pipe it will be used for many pieces of work in preference to the
soldering-iron. If the sheet-lead is used for the frames in place of the
three-eighths by quarter-inch strips, two or three strips will have to
be taken, so as to build up the band of the frame to about a quarter of
an inch in thickness. When soldered together, or fused at the edges,
these built-up frames will be as rigid as the solid metal.
[Illustration: FIG. 11]
[Illustration: FIG. 12]
[Illustration: FIG. 13]
Now cut a number of strips of the thin lead-tape six inches and a half
long, and others that will necessarily be somewhat longer, for each
frame is to be filled with straight and crimped pieces, as shown in Fig.
13. If there is a fluting-iron in the house, the crimping may be done in
the brass gears at one end of the machine. Or two wheels may be cut from
hard-wood with a fret-saw, and made fast to a block with screws, as
shown in Fig. 14. A handle, attached to one wheel, will make it possible
to turn the gears; and they should be placed just far enough apart to
allow the tape to pass through without tearing or squeezing. Put a
washer between the wheel and the block to prevent friction.
When a frame is in the position shown in Fig. 13, and lying on a piece
of slate or flat stone, you will first put in a crimped piece of tape,
as shown at Fig. 13 A, and under this arrange a straight piece (Fig. 13
B); then, with the blow-pipe and flame, fuse fast to the frame and catch
the flutes of the crimped piece to the straight one every inch or two.
Add alternate crimped and straight strips until the frame is filled and
presents the appearance of Fig. 13. When the seven frames are ready, lay
three of them aside for the positives and four for the negatives. Note
that the positives are red and the negatives a dark yellow when they are
filled with the active material.
There are several methods of depositing the active material in the mesh
or net-work of the plates, but some of them are too technical, others
too complicated, and still others require charging machinery. The
following plan will be the simplest and easiest for the amateur:
At a paint-store, or from a wholesale druggist, obtain several pounds of
oxide of lead (red-lead) and a similar quantity of litharge
(yellow-lead). In an earthen vessel, or large jar, make a solution
composed of water, twenty ounces, and commercial sulphuric acid, two
ounces. This is the mixture commonly known as “one to ten.” Place some
red-lead (dry) in an old saucepan or soup-plate, and add a little of the
acid solution: then, with an old table-knife or small trowel, mix the
lead into a stiff paste, like soft putty. Do not get it too thin or it
will run; nor too thick, as then it will not properly adhere to the
lead-mesh of the frames. With the frame lying on its side, plaster in
the red composition between the flutes and fill up the frame solid with
it. Treat all three of the positive frames in the same manner, taking
care that the exposed surfaces of the composition-filling is smooth and
flush with the edges of the lead frame and mesh. Do not disturb these
plates for a while, but let them remain in position, so as to set and
partially dry. Add acid solution to the yellow-lead in a similar manner,
and fill the four negative plates. When partially dry, the plates will
be ready to combine in a pile.
[Illustration: FIG. 14]
[Illustration: FIG. 15]
[Illustration: FIG. 16]
[Illustration: FIG. 17]
At a supply-house obtain some sheets of cellulous fibre,
three-sixteenths of an inch thick, or some asbestos cloth. If neither
can be had, then soak some pieces of ordinary brown card-board in a
solution of silicate of soda and let them dry. Lay a negative (yellow)
plate on the table with the lug at the left (Fig. 13 C). On this place a
square of the fibre, asbestos, or card-board; and on top of it lay a
positive (red) plate with the lug at the right side. Continue in this
manner until the seven plates are stacked, the four negative lugs being
at the left and the three positives at the right. Tie the plates
securely together with cotton string bound about them in both
directions; then stand the pile up so that the lugs are at the top, as
shown at Fig. 15, with every alternate lug in an opposite direction.
Obtain two lead bars three-eighths of an inch square, or cut strips from
the sheet-lead and solder them together, turning the ends as shown at
Fig. 13 D. Drop one of these bars into the lugs of the positive plates,
as shown in Fig. 15 H, and solder it fast at the three unions. Repeat
this with the other bar in the lugs of the negative plates, and the pile
will then be ready for immersion in the electrolyte. To both ends of
each plate-bar solder binding-posts, so that the conductor-wires can be
attached at one end and the feed-wires at the other. If a hard rubber or
glass cell can be had for the battery so much the better; if not, a
stout box may be made from pine, white-wood, or cypress, and thoroughly
coated with asphaltum varnish or asphaltick. At an electrical
supply-house you can purchase some “P and B” compound, which is acid and
water proof. This is excellent for the inside coating as well as for the
outside of the box.
The box should be made of wood not less than three-quarters of an inch
thick, and the sides, ends, and bottom should be in one piece, free from
knots, sappy places, or cracks. Brass screws should be used to hold the
boards together, and before the joints are made the butt-ends of wood
and the sides, against which they impinge, must be thoroughly coated
with the asphaltum or compound. Put together the four sides first and
then make the bottom fast, placing the screws two inches apart and
countersinking the wood, so that the screw-heads will lie flush, as
shown in Fig. 16. The box should be large enough to allow about one inch
of space all around the pile, and deep enough for the solution to cover
the plates and two inches of space above it to the top edge of the cell.
The complete storage-battery will then appear as shown in Fig. 17.
The electrolyte is composed of sulphuric acid and water in the
proportion of one ounce of acid to four of water, making a five-part
solution. This should be mixed in an earthen or glass jar, and the acid
poured slowly into the water, the latter being stirred while the acid is
added. When the solution cools (for adding acid to water creates heat),
add about two ounces of bicarbonate of soda, and mix the solution
thoroughly.
When the pile is in place within the box (having first removed the
string which bound the plates together) pour the electrolyte slowly
into the cell, taking care that none of it spatters, for it will eat
clothing or anything else that it touches. Before placing the pile, or
electrolyte, in the box, it should be thoroughly tested for leaks by
allowing water to stand in it for several days. Indeed, you should be
very generous with the asphaltum, or compound, when coating the angles
and points inside the box; for if the acid solution gets at the screws
it will corrode them and the box will soon leak and fall apart. As a
precaution against the acid working over the top of the box, the upper
edge, for an inch or two, should be coated with paraffine over the
asphaltum or acid-proof coating.
A cell constructed in this way should accumulate about two volts and one
hundred ampere-hours, and will run a one-sixteenth horse-power motor.
The expense of making these plates is about twenty-five cents each, and,
including the cell and coating materials, each storage-battery will cost
approximately two dollars. The lasting qualities of the battery depend
on the use or abuse it is put to; but with ordinary care it should last
from three to five years.
When the battery ceases to accumulate properly the pile should be
removed, and, after washing it thoroughly, the bars should be cut away
and new positive plates made and installed. The positive plates are the
ones that deteriorate and need replacing; the negatives are almost
everlasting, and with proper usage will live for fifteen or twenty
years.
Directly the electrolyte is in the cell, connect the poles of your
primary cells so as to begin the accumulation of current. Never exhaust
the charge of electricity from your storage-cell, and never leave it
uncharged when the electrolyte is in, or the plates will be ruined. A
battery consisting of from five to twenty bluestone cells will be the
best with which to charge this accumulator; and if more than one cell is
desired, any number of them can be made and coupled up in series. Take
care, when connecting the wires from the primary cells, to see that the
positive wire is connected with the positive plates and the negative
with the lead bar joining the yellow plates. If by accident you should
make a misconnection, bubbles will rise from the electrolyte. This is
not right, so reverse the wires and the accumulation of current will
then take place without agitation in the cell.
Dry-cells and Batteries
Dry-cells are extensively used nowadays, since their cleanliness, high
efficiency, and low internal resistance make them preferable to the
Leclanché and other open-circuit batteries for bells, annunciators, and
other light work. In the dry-cell, the electrolyte, instead of being a
liquid, is a gelatinous or semi-solid mass, which will not run nor slop
over. When the capping of pitch or tar is in place, the cell may be
placed in any position, with full assurance that the electrolyte will
not become displaced nor run out. Dry-cells may be made of almost any
size for convenience of handling, but those commonly used vary from one
to four inches in diameter, and from four to fifteen inches high. For
bells and general electric work, a cell two inches and a half in
diameter and seven inches high will be found a convenient size to make
and handle.
The component parts of a dry-cell are the cell itself (which is made of
zinc and acts as the positive pole), the carbon, the electrolyte or
active excitant element, and the pitch or tar cap to hold the
electrolyte and carbon in place.
From a tinsmith obtain some pieces of sheet zinc, and roll them into
cylindrical form as shown in Fig. 18 A. The sheets should measure seven
by eight inches, and when formed the edges are to be lapped and
soldered.
[Illustration: FIG. 18]
From a smaller piece of zinc cut round bottoms, fit them in the
cylinders and solder securely in place, taking care to close up all
seams or joints to prevent the escape of the electrolyte.
From a supply-house obtain battery-carbons, one inch and a half wide by
half or three-eighths of an inch thick and eight inches long. These
should be provided with a thumb-screw or small bolt and nut at the top
so as to make wire connections with the carbon. A strip of zinc should
be soldered to the outside upper edge of the zinc cup to which wire
attachments may be made with thumb-screws or small bolts and nuts. When
the parts are ready to assemble, make a wooden mould or form a trifle
larger than the carbon. This is intended to act as a temporary plunger,
and is inserted, at first, in place of the carbon plate. This wooden
plunger should be smooth, and given a coat of shellac to prevent it
from absorbing any moisture.
Insert the plunger in the zinc cup and support it so that it will be at
least half an inch above the bottom and centred at the middle of the
cup. The electrolyte is then placed in the cup, and, when it has set a
little, the wooden plunger is removed and the carbon inserted in its
place.
The electrolyte is composed as follows:
Ammonium chloride 1 part
Zinc chloride 1 part
Plaster of Paris 3 parts
Flour ¾ part
Water 2 parts
Mix these together and place the compound within the zinc cups, so that
the mass settles down and packs closely about the plunger. The space
left unfilled about the carbon should be filled with a mixture composed
as follows:
Ammonium chloride 1 part
Zinc chloride 1 part
Manganese binoxide 1 part
Granulated carbon 1 part
Flour 1 part
Plaster of Paris 3 parts
Water 2 parts
These proportions may be measured in a tin cup, a table-spoon, or any
other small receptacle. Note that the measurement by parts is always by
bulk and not by weight.
Do not fill the zinc cup to the top, but leave an inch of space, so that
half an inch of sealing material may be added. See that the inside top
edge of the zinc cup is clean; then melt some tar or pitch and pour it
over the top of the electrolyte, so that it binds the zinc cup and
carbon into a solid form. Drive an awl down through the capping material
when it is nearly dry, and leave the holes open for the escapement of
gases.
Give the outer surface of the zinc cells a coat of asphaltum varnish,
and wrap several thicknesses of heavy paper about them to prevent
contact and short-circuiting. Protect the bottoms in a similar manner,
and as a result you will have a cell that will appear as shown in Fig.
18 B. A battery of cells powerful enough for any light work can be made
by connecting the cells in series, each having an electro-motive force
of one and a half volts, with an internal resistance of less than
one-third of an ohm.
Chapter III
PUSH-BUTTONS AND SWITCHES
Push-buttons
Push-buttons and switches are a necessity in every home where electric
bells, lights, or fans are used, for with them connections are made or
broken. The telegraph-key and the commutators on a motor and dynamo are
only improved forms of the push-button, and this simple little device is
really an indispensable part of any electrical equipment.
The simplest form of push-button is a bent piece of tin or thin
sheet-metal screwed fast to a small block of wood, as shown in Fig. 1.
Under the screw-head one end of a wire is caught, and the other wire end
is secured by a washer and a screw driven into the block directly under
the projecting end of the strip of metal. By pressing a finger on the
tin it is brought into contact with the screw-head under it, and the
circuit is closed; on releasing it, the tin flies up and the circuit is
opened again.
An enclosed push-button is shown in Fig. 2. It is made of the cover or
body of a wooden box, a spool-end, and several other small parts. A
round piece of thin wood is cut to fit inside the box and so form the
base for the button. On this the spring strip is attached with screws,
and the wire ends are made fast, as shown in Fig. 3. The wires are
carried through the bottom of the base and along grooves to the edge,
and thence to their final destination. The end of a spool is cut off and
glued to the top of the box, as shown in Fig. 2, and a hole is made in
the box to correspond in size with that in the spool. Through this
aperture the button (cut from a wooden dowel or shaped out with a knife)
passes, so that the end projects about a quarter of an inch beyond the
spool. To prevent the button from falling out, a small steel nail should
be driven across the inner end, or a washer may be tacked to the end of
the stick, as shown in Fig. 4.
[Illustration: FIG. 1]
[Illustration: FIG. 2]
[Illustration: FIG. 3]
[Illustration: FIG. 4]
The button is mounted by screwing the base fast to the door or window
casing, it being understood that the wires have been first arranged in
place. The button is then set in the hole and the cap is placed over
the base, covering it completely. By means of small screws, passed
through the rim of the box and into the edge of the base, the cap is
held in place. A coat of paint or varnish will finish the wood-work
nicely, and this home-made button should then answer every requirement.
Switches and Cut-outs
In electrical equipment and experimental work, switches and cut-outs
will be found necessary, particularly so for telegraph and telephone
lines. Care should be taken to construct them in a strong and durable
fashion, for they will probably be subjected to considerable wear and
tear.
A simple switch (Fig. 5) is made from a base-block of wood three inches
long, two wide, and half an inch in thickness, together with some small
metal parts. It has but one contact-point, and that is the brass-headed
tack (T in Fig. 5) driven through the binding-post, the latter being a
small plate of brass, copper, or even tin screwed to the base-block. The
end of a wire is caught under the screw-head before it is driven down. A
similar binding-post is arranged at the lower side of the block, and the
movable arm is attached to it with a screw. Between the arm and the
post-plate there should be a small copper washer, to make it work more
easily. The arm is cut from a thin piece of hard sheet brass or copper
(tin or zinc will also answer very well), and at the loose end the half
of a small spool is attached, with a brass screw and washer, to serve as
a handle. The end of the screw that passes through a hole in the arm is
riveted to the under side to hold it securely in place. This
arrangement is shown in Fig. 6.
[Illustration: FIG. 5]
[Illustration: FIG. 6]
[Illustration: FIG. 7]
[Illustration: FIG. 8]
[Illustration: FIG. 9]
The under edges of the arm may be slightly bevelled with a file, so that
it will slip up easily on the oval head of the brass tack. The drawing
shows an open switch; when the circuit is closed the arm rests on the
tack-head. By means of small screws this switch-board may be fastened to
a table or to any part of the wood-work in a house.
In Fig. 7 a complex switch is shown. This is the principle of the
shunt-box switch, of the resistance-coil, and also of the commutators of
a motor. A motorman’s controller on a trolley-car is a good example of
the shunt, and, with it and the resistance-coils, the car can be
started, stopped, or run at any speed, according to the current that is
admitted to the motor.
The complex switch is made in the same manner as described for the
single switch, except that any number of binding-posts may be employed,
arranged on a radial plan, so that the end of the arm will rest on any
tack-head at will. Bells in various parts of the house may be rung by
this switch, or it may be coupled with a series of resistance-coils to
control any amount of current.
The simple cut-out (Fig. 8) is constructed in the same manner as the
simple switch, except that there are two points of contact instead of
one. This is the principle of the telephone and telegraph instrument
wiring, so that a bell or sounder may be rung from a distance. The arm
is then thrown over and the bell cut out, allowing the “phone” or key to
be brought into use. In lifting the transmitter from the hook on a
telephone, a cut-out is operated and the bell circuit is thrown out of
action. It is in operation again directly the transmitter is returned to
the hook. The switch cut-out (Fig. 9) is inactive when the arm is in the
position shown in the illustration; but when it is thrown over (as shown
by the dotted line) it connects the poles at opposite ends of the board.
It may be thrown over in both directions, and is a useful switch for
many purposes.
For strong currents the lever-switch, that rests on a brass tack-head,
will not be suitable, as the switch-bar must be held firmly in place to
make a perfect connection. Strong currents throw weak switches open,
causing an open or broken circuit.
A single pole-switch, to carry a current up to one hundred and
twenty-five volts and twenty-five amperes, is shown in Fig. 10. This
consists of a base-block, a bar which is attached to the vertical ears
of a binding-post, and a clutch that will hold the bar when it is
pressed down between the ears.
The base-block should be made from some non-conducting material, such as
soapstone, marble, or slate. If a piece of soapstone can be procured,
that will be just the thing, since it is easily worked into the proper
shape and size. Soapstone may be sawed and smoothed with a file; it is
easily bored into with a gimlet-bit, and it is one of the best
non-conducting substances. The base for this switch is six inches long,
two inches wide, and as thick as the soapstone happens to be--say
three-quarters of an inch. The top edge may be bevelled for the sake of
appearance or left square.
Two pieces of heavy sheet copper or brass are to be cut as shown at A in
Fig. 11. The ears are half an inch wide, and the total height of the
strip is two inches and a half, while the part with two holes in it side
by side is one inch and a quarter long, including the half-inch width of
the vertical strip. With round and flat-nosed pliers bend the long ears
into shape, so as to form a keeper for the bar which is then to be
riveted in place. Omit the holes at the ends of the long ears in the
other plate; then bend it into shape to form a clutch that will hold the
bar when it is pressed down between the ears. These binding-posts should
be made fast to the base-block with brass machine-screws and nuts, which
will fit in countersunk holes in the bottom of the soapstone. If
hard-wood is used for the base, ordinary brass wood-screws will answer
very well.
The connection-bar is cut from metal the same thickness as that employed
for the binding-posts and clutches; it should be shaped so as to appear
as shown at B in Fig. 11. A handle should be driven on the slim end, and
where the lower edge enters between the ears of the clutch, the corners
of the bar should be rounded with a file. Countersunk screw-holes are
bored in the base, so that it can be made fast to the wood-work.
[Illustration: FIG. 10]
[Illustration: FIG. 11]
[Illustration: FIG. 12]
A double pole-switch is shown in Fig. 12, and in general construction it
is similar to the single pole-switch described above. The binding posts
and bars are cut and bent from the patterns A and B in Fig. 11; but in
this case the long, slim ends of the bars are omitted. A short turn is
made at the handle end of each bar and a hard-wood block is placed
between the bar-ends and held in position with screws driven through
holes made in the bars and into the ends of the block. A handle is made
fast to the middle of the block with a long and slim wood-screw; or a
steel-wire nail may be passed through the handle and block, a burr
slipped over the end opposite the head, and the small end riveted fast.
When the binding-posts (to which the ends of the bars are attached) are
screwed onto the base, be sure and see that the bars are parallel and
the same distance apart at both ends. In like manner, when the cleat
binding-posts are made fast, see that they are directly in line with the
bars, so that the yoke will drop into the spaces between the ears
without having to be pulled to one side or the other. This is a very
useful switch for strong currents, and may be placed close to a dynamo,
so that the current in both wires may be cut out at once.
Table-jack Switches
A table-jack switch is a most convenient piece of apparatus where
several lines of bells, alarms, or telephone circuits are to be switched
on and off.
The single table-jack switch, shown in Fig. 13, is made of a hard-wood
block three-quarters of an inch thick, five inches wide, and seven
inches long. It is to be smoothed and varnished, or given several coats
of shellac. At the four corners small holes are made to receive slim
screws, and at one end of the block five short metal plates are screwed
fast, with the heads of the screws countersunk, so that they will be
flush with the top of the plates. These small plates should be half an
inch wide and one inch long, and may be of brass, copper, or tin. But if
they are of tin the plates are made of a longer strip tacked to the
board and then bent over, as shown at A in Fig. 14. They will therefore
form short springs, the upper parts of which will rest against the long
spring-arms. From spring brass or copper five arms are to be cut and
shaped, as shown in Fig. 13. Holes are made at one end of each, and
others again two inches from these, through which to pass screws.
Screw-eyes are passed through copper washers and the end holes in the
strips, and then screwed into the wood plate. These will act as
binding-posts, while the second line of screws will hold the plates down
to the base. The arms should be bent, so that when the screws are driven
down the lower edge will press on the small plates under them.
The outlet wires are attached to the binding-posts at the head of the
block, and the plug (A in Fig. 13) is inserted between the arm and plate
at the foot, so that contact and connection are made. This plug is a
small plate of metal to which the end of a flexible wire is made fast.
It should be of copper or brass, but for light work a strip of tin may
be bent over with the wire caught between the plates and a copper tack
passed through the sides and riveted, as shown at B in Fig. 14.
A double jack-switch (Fig. 15) is made on the same general plan as the
single, but it has no binding-posts. A block of the same size is used,
and two rows of short plates are made fast at each end. The arms are
made with two screw-holes near the middle, as shown in Fig. 15, and
through these holes screws are driven to hold the arms down to the base.
Several plugs are used for each end, so that the in and out lines can be
shifted, and from one to four lines used at a time.
[Illustration: FIG. 13
FIG. 14
FIG. 15
FIG. 16
TABLE-JACK SWITCHES]
A convenient slip-switch for single or double line work is shown in Fig.
16. This consists of a small wooden base, on which a brass arm and
handle are screwed fast and connected with a binding-post (A in Fig.
16). A slip-plate is made from a piece of sheet-brass and bent so as to
form a pocket into which the arm will fit. This pocket piece is
connected with the binding-post B. When the switch is thrown out the
circuit is broken, unless a contact-point, C, is provided, from the
under side of which a wire leads out to a second circuit. When the
switch is in place, as shown in Fig. 16, the circuit is closed through A
and B; but when the arm is thrown out the circuit through A and B is
broken and that through A and C is closed.
Binding-posts and Connectors
To make quick connections between wires and other parts of electrical
apparatus, binding-posts are the most convenient device, since the turn
of a screw binds or releases a wire instantly. Binding-posts may be made
in many forms, but the simple ones that a boy will need can be made from
screw-eyes, burrs, stove-bolts, and nuts, together with thin strips of
metal and nails.
Five simple posts are shown in Fig. 17. A is made from a screw and two
burrs, B from a screw-eye and two burrs, and C from a thin plate of
metal and two screws, with oval or round heads. This last, however is
more of a connector than a binding-post. The ends of the wires to be
connected should be caught under the screw-heads or between the burrs
before the screws are driven down.
In D a simple arrangement of a stove-bolt and two nuts is shown. The
under bolt is screwed down tightly against the wood, and under the head
a wire is made fast, so that another wire may be caught under the upper
nut. If a small thumb-nut can be had in place of the plain nut, it will
be easier to bind the upper wire. In Fig. 17 E a thin strip of metal may
be folded over, and at the loose ends a hole should be punched through
which a screw-eye will pass. The metal is held to a wood base with a
screw, under the head of which a wire is caught. The second wire end is
slipped between the metal plates, and a turn of the screw-eye will bind
and hold it securely.
[Illustration: FIG. 17]
[Illustration: FIG. 18]
Connectors are employed to unite the ends of wires temporarily, and are
made in many forms. A simple and useful one is made from a piece of
spiral spring fastened to a block of wood by two staples, as shown at
Fig. 18 A. The ends of the wires are pressed down into the coils of the
spring and are held with sufficient security for temporary use. Another
connector is made from a block of wood, a strip of thin metal, and two
screw-eyes (Fig. 18 B). The metal is bent around the ends of the block,
and through holes made in the ends of the strip screw-eyes are driven
into the block. When the ends of wires are slipped under the metal, a
turn of the eyes will hold them fast, as shown at Fig. 18 B.
A short bolt threaded at each end and provided with four nuts will also
act as a connector. The inner nuts are screwed on tightly and the outer
ones are loose, so that when wires are placed between them they may be
tightened with the fingers, as shown at C in Fig. 18. These are a few
simple forms of connectors; the ingenious boy can devise many others to
suit his needs and ideas.
Lightning-arresters and Fuse-blocks
All lines of exposed wire that run from out-doors into the house should
be provided at both ends with lightning-arresters, particularly if they
are telephone or telegraph lines, burglar alarms, or messenger
call-boxes. In many instances where unprotected telephone lines have
been the plaything of lightning, painful accidents have happened, and it
is only the part of prudence to provide against them. It is better to
have an arrester at both ends of a line, and as the cost is
insignificant it is hardly worth considering as against its feature of
safety.
Lightning-arresters may be constructed in many ways and of different
materials; the ones here shown and described are easily made and
efficient. The principle of all arresters is simply a fuse which burns
out whenever the wire is carrying a greater amount of current than is
required for the proper working of the apparatus, thereby arresting the
current and protecting the instruments from destruction.
Induction-coils, relays, fine windings on armatures, or a magnet helix
are quickly destroyed if a too powerful current is permitted to pass
through them, and it is therefore advisable to protect them. When a fuse
burns out under a trolley-car, or in the shunt-box of a motor-car or
engine, it is because a greater amount of current is trying to pass in
than the motor will safely stand. When a fuse “blows out,” the apparatus
or motor is put out of commission until the fuse is replaced, but the
delicate mechanism and the fine wiring on the field-magnets or armatures
are saved.
The simplest form of single pole-fuse is a fine piece of lead wire held
between two binding-posts, as shown at A in Fig. 19. The lead wire may
be of any length; but for small instruments, where a moderate current is
employed and where there is a possibility of lightning travelling on the
wire, the fuse should be from two to three inches long. For inside work,
however, where it is to be used simply as a safety, the wire may be
shorter and finer.
To make the lightning-arrester shown in Fig. 19, cut out a hard-wood
block five inches long, an inch wide, and half an inch thick. Give this
several coats of shellac; then place a piece of mica, or asbestos paper,
over the top of the block, and make it fast with thick shellac to act as
a glue. From small pieces of copper or brass cut two plates one-half by
one inch, and drill holes in them to take screws and screw-eyes. Place
copper burrs under the screw-eyes for connectors, and drive two brass
screws half-way down in the block through the holes at the inner ends of
the binding-post plates. See that these screws fit snugly in the holes
in the plates so that contact is perfect. If the holes are too large and
the screws fit loosely, two copper burrs will have to be used and the
screws driven in, so that the heads bind the burrs on the ends of the
fuse-wire. From an electrician, or supply-house, purchase a few inches
of fine lead fuse-wire--say Nos. 20, 22, or 24--and twist the ends of a
length around the screws, as shown in the drawing. Perfect contact
should be had between the lead wire and the screws; by way of
precaution, a bit of solder will dispel all doubt. Just touch the point
with a little soldering solution; then apply a soldering-iron having a
drop or two of solder on the end.
Perfect connection is absolutely necessary for telephone, telegraph, or
annunciator work, and where there is a lightning-arrester and the line
is not working well, the trouble may often lie in the poor contact of
lead and brass or copper, or possibly in using wire that is too fine.
Lead is a very poor conductor, and a fine wire would act as a check. For
a test, first insert a piece of copper wire to see that the line is
working properly; then use lead wire of sufficient size to carry the
current as well as the copper did. The action of metals and wire, as
current retarders, will be explained in the chapter on resistance and
resistance-coils.
For general commercial use the base-blocks of all lightning-arresters
should be made of porcelain, slate, or some of the composition
non-conductors, such as moulded mica, silex and shellac, or fibre. As
these are not always available, wood, with a covering of mica, will
answer every purpose and can be readily adapted for use.
The apparatus pictured in Fig. 19 is known as a single-pole
lightning-arrester, and is the simplest form of this kind of electrical
paraphernalia. In Fig. 20 a double-pole arrester is shown. This is
constructed in the same manner as described for the single one. The
block is five inches long, two inches wide, and half or five-eighths of
an inch thick. A countersunk hole is made in the middle of all the
lightning-arrester blocks through which a screw can be passed to hold
the apparatus fast in any desired location.
In Fig. 21 another form of fuse is shown. It is made from a piece of
mica three-quarters of an inch wide and four inches long, two pieces of
thin sheet-copper, and a piece of lead fuse-wire. The copper is
three-quarters of an inch wide, and one piece of it is bent in the form
of a [V], as shown at A in Fig. 21. One end of the mica strip is dropped
in the [V], and with a pair of pliers the [V] is closed up by pinching
it at the bottom. To further insure its staying in place, the top and
end, or open edges, should be soldered. Punch a small hole through the
copper ends, at the inside edge, slip the ends of the fuse-wire in them,
and touch the union with a drop of solder to insure perfect contact.
With shears and file cut a [U] from the side of one copper band and from
the end of the other; these will allow the copper ends to pass under the
heads of screws, thus avoiding the necessity of removing the entire
screw from the block in order to set the fuse in place.
[Illustration: FIG. 19
FIG. 20
FIG. 21
FIG. 22
FIG. 23
FIG. 24
FIG. 25
FIG. 26
LIGHTNING-ARRESTERS AND FUSE-BLOCKS]
The block on which this fuse is held is shown in Fig. 22, and is made in
a similar manner to the one shown in Fig. 19, except that the metal
plates are a trifle longer and are bent up, as shown in the drawing.
Thus the mica fuse-plate will be elevated above the block. If the brass
or copper used for the binding-post plates is too thin to stand the
pressure of the screws when the fuse ends are screwed fast, put a few
burrs on the screws below the plates; then the pressure of the screws
cannot bend down the extending ears of the plates and make an imperfect
contact.
Another form of fuse-block is shown in Fig. 23. The same sort of a fuse
is employed as shown in Fig. 21, but without the [U] cuts at the ends.
The clutches are made by binding brass or copper plates, as shown in the
drawing; they should then be screwed fast to a base-block five inches
long, one inch and a half wide, and five-eighths of an inch thick. The
opening between them should just admit the copper ends of the fuse, and
pressure should be used to force the fuse in place so that the contact
will be perfect.
Still another form of fuse is shown in Fig. 24. This last may more
properly be called a non-sparking fuse, for the lead wire is encased in
a glass tube, and when it fuses no sparks fly and no small pieces of
melted metal can get away from the inside of the tube. The plug is made
from a piece of glass tube half an inch in diameter, two metal caps, and
a short piece of lead wire. The metal caps are of thin sheet-copper, and
are caught at the edges with solder. One end of the lead fuse-wire is
passed through a hole in the end of a cap and soldered, as shown at A in
Fig. 24. The wire is then passed through the tube and the cap placed
over one end of it. This is repeated at the other end and the wire
soldered fast. As a result, you will have a glass tube with metal caps
held on the ends of the tube, by means of the thin lead wire which runs
through the middle of the tube. The base-block to which this fuse-plug
is attached is of wood one inch and a half wide, five or six inches
long, and five-eighths of an inch thick. Two metal straps are made and
screwed fast to the block, and the circuit wires are attached under the
copper burrs and held down by the screw-eyes.
To place or replace a fuse-plug, unscrew the eyes and raise each strap
slightly, so that the copper cap ends will pass under them. A turn or
two of the eyes will clamp the plug in position and at the same time
bind the circuit wires.
A spring lightning-arrester is shown in Fig. 25; it is simply a modified
form of that shown and described in Fig. 19. The base-block is five by
one-and-a-quarter by five-eighths of an inch, and is properly protected
by a sheet of mica or asbestos. The two metal plates are cut for the
binding-posts and screwed in place with screws, burrs, and screw-eyes.
From spring-brass wire bend a hook and slip one end of it under the
screw-head at the left side of the block. From a longer piece of wire
make two or three turns around a piece of wood half an inch in diameter;
then form a hook at one end and a turn at the other, so that it can be
made fast under the screw-head of the binding-post. When at rest, the
spring-hook should stand in an upright position, but when sprung and
tied it occupies the position shown in the drawing. The spring-hook is
to be bent down so that the two hooks are brought within an inch of each
other. They are held in this position with a piece of lead fuse-wire.
This last is given a turn about the hooks and one or two turns about
itself, close to each hook, to prevent the spring from tearing itself
away. When the wire is fused by a current the spring-hook flies up and
away from possible contact with the short hook attached to the opposite
binding-post. This is the construction for a single-pole-spring
lightning-arrester; a double one is made in a similar manner, and the
parts mounted on a wider block, as shown in Fig. 20.
For doubtful currents, where there is no means of knowing how strong
they are, a combined fuse and single-pole switch is shown in Fig. 26.
This is nothing more than a combination of the apparatus shown in Fig.
21, and the single-pole switch (Fig. 10). The base block is seven inches
long and two inches wide. Or it may be made half an inch wider if it is
to be bevelled at the top, as shown in the drawing. It should be
three-quarters of an inch thick and provided with two countersunk holes
for screws that will hold it in place on a ledge or against a casement.
The little angles to hold the copper-ended mica fuse-plate are described
for the apparatus pictured in Fig. 21. If it is desired that one of the
ends should be provided with burrs and a screw-eye, the little plate of
brass should be an inch long and an inch wide, with a half-inch-square
piece snipped from one corner, as shown at A in Fig. 26. It is provided
with two holes, and then bent on the dotted line, so that the part with
the holes will lie on the block and the ear will stand in a vertical
position. A reverse-plate made on this pattern will act as one side of
the switch-bar clutch at the opposite end of the block. For the metal
clutch and keeper at the middle of the block the metal plate (before it
is bent) will appear as shown at B in Fig. 26. The long plate with two
holes lies on the base, while the first ear (or the one without the
hole) forms part of the clutch for the fuse end, the ear with the hole
acting as one side of the bar-lever strap. An opposite plate to this
forms the other side of the clutch and strap, and the two plates are
screwed side by side, so that the fuse-plate will be held securely when
pushed into place.
For the switch-bar use a piece of hard copper or brass four inches long,
half an inch wide, and about one-eighth of an inch thick, or the same
thickness as the copper straps at the ends of the mica fuse-plate. Bore
a hole at one end of this bar, and with a copper rivet attach it between
the two upright ears at the middle of the block. With a file cut away
the two edges at the other end of the bar for a distance of an inch, so
that the bar will have an end as shown at C in Fig. 26. Drive a small
file-handle on this end and give it a coat or two of shellac; then bevel
the lower edges of the bar with a file where it enters between the
blades of the clutch. The circuit wires are made fast at both ends of
the block, and the current travels through the binding-posts, the lead
fuse-wire on the mica plate D, and the switch-bar. If the current is too
strong, then when the switch-bar is pushed into the clutch the
safety-fuse will burn out and save the apparatus; or it will arrest a
flash of lightning.
Chapter IV
MAGNETS AND INDUCTION-COILS
Simple and Horseshoe Magnets
Every boy has a horseshoe magnet among his collection of useful odds and
ends, and whether it is a large or small one its working principle is
the same. If large enough it will lift a jack-knife, nails, or solid
weights, such as a small flat-iron or an iron padlock. A horseshoe
magnet is made of highly tempered steel and magnetized so that one end
is a north pole and the other a south pole. In more scientific language
these poles are known as, respectively, positive and negative. Once
magnetized the instrument retains that property indefinitely, unless the
power is drawn from it by exposure to intense heat, and even then, by
successive heating and cooling, the magnetism may be partially restored.
An electro-magnet may be made of any scrap of soft iron, from a piece of
ordinary telegraph-wire to a gigantic iron shaft. When a current of
electricity passes through a wire a magnetic “field” is produced around
the wire, and if the latter is insulated with a covering and coiled
about a soft iron object, such as a nail, a bolt, or a rod, that object
becomes a magnet so long as a current of electricity is passing through
the coils of wire or helix. A coil of wire in the form of a spiral
spring has a stronger field than a straight wire carrying the same
current, for each turn or convolution adds its magnetic field to that of
the other turns.
A simple form of electro-magnet is made by winding several layers of No.
20 insulated copper wire around a stout nail or a carriage-bolt; by
connecting the ends to a battery of sufficient power, some very heavy
objects may be lifted. A single magnet, like the one shown in Fig. 1, is
made with a piece of soft iron rod six inches long and half an inch in
diameter, the ends of a large spool sawed off and worked on the rod, and
half a pound of No. 20 insulated copper wire. The spool-ends are
arranged as shown in Fig. 2. An end of the wire is passed through a hole
in one flange when you begin to wind the coils, and when finished, the
other end is passed through a hole at the outer rim of the same flange.
This magnet may be held in the hands when in use; or a hand-magnet may
be constructed of a longer piece of iron on one end of which a handle is
driven and held in place with a nut and washer, as shown in Fig. 3. The
wires from the coil pass through holes made in the handle and come out
at the butt end, where they may be attached by connectors to the
pole-wires of a battery. To protect the outer insulated coil of wire
from chafing and a possible short-circuit, it would be well to wrap
several thicknesses of stout paper around the coil and glue it fast; or
a leather cover will answer as well.
[Illustration: FIG. 1
FIG. 2
FIG. 3
FIG. 4
FIG. 5
FIG. 6
FIG. 7
SIMPLE AND HORSESHOE MAGNETS]
A more powerful magnet may be made from a stout bolt, two nuts, and a
wooden base, with about three-quarters of a pound of No. 18 insulated
copper wire to wind about the body of the bolt. A block of wood an inch
thick, four inches wide, and six inches long is provided with a hole at
the middle for the bolt to pass through. A larger hole is made at the
under side of the block so that a nut can be easily turned in it. A
three-quarter-inch machine-bolt, with a square head, and seven inches
long, is set in the block, head up, as shown in Fig. 4; and composition
or thin wooden disks or washers are placed on the bolt to hold the coils
of wire in place. The ends of the wire pass out through the bottom
washer and are made fast to binding-posts on the block, and to these
latter the battery-poles are made fast when the magnet is in use. Coils
of wire may be wound on an ordinary spool, and the hole in the middle
may be filled with lengths of soft iron wire. When a current is passing
around the spool the wires become highly magnetic, but lose the
magnetism directly the current ceases.
Horseshoe electro-magnets are made by winding coils on the ends of
[U]-shaped pieces of soft iron, but the winding must be done so that the
current will pass around them in opposite directions, otherwise you
would have two negatives instead of a negative and positive. For a small
horseshoe magnet a stout iron staple may be used, but for the larger
magnets it would be best to have a blacksmith bend a piece of round iron
in the desired shape.
A powerful horseshoe magnet may be made from a piece of tire-iron bent
as shown in Fig. 5 A; when wound with No. 18 wire it will appear like
Fig. 5 B. A volt or two of current passing through the coils will render
this magnet powerful enough to lift several pounds.
For bells, telegraph-sounders, and other electrical equipment requiring
the horseshoe or double magnet, several kinds may be used, but the
simplest is constructed from two carriage or machine bolts and a yoke of
soft iron, as shown in Fig. 6. The yoke is five-eighths of an inch in
width, two inches and a half long, and provided with two
three-eighths-inch holes, one inch and a half apart from centre to
centre. Two-inch carriage or machine bolts are used, and they should be
three-eighths of an inch in diameter. The nuts are turned on the thread
far enough to admit the yoke, and then another nut is applied to hold it
in place and bind the three pieces into one compact mass. Wooden
spool-ends or composition washers are placed on the bolts to hold the
ends of the wire coils in place, and the winding may be done on each
bolt separately and locked to the yoke after the winding is completed.
Double cotton-insulated No. 20 or 22 copper wire should be used for the
coils.
It is a tedious and bothersome job to wind a coil by hand, and if
possible a winder should be employed for this purpose. Several varieties
of winders are on the market, but a simple one for ordinary spools may
be made from a stick held in an upright piece of wood with staples. This
idea is pictured in Fig. 7, where the round stick is shown cut with two
grooves into which the staples fit. One end of the stick is made with a
square shoulder, so that a handle and crank can be fitted to it. A few
wraps of wire are taken around the crank to prevent it from splitting,
and it is held to the round stick with a slim steel nail. The opposite
end of the round stick is shaved off so that it will fit snugly in the
hole of a spool; if it should be too small for some spools, a few turns
of cord around the small end will make it bind. The block to which the
shaft and crank is attached may be held in a vise or screwed to the edge
of a table.
Induction-coils
A simple induction or shocking coil may be made of a two-and-one-half by
five-sixteenths-inch bolt, a thin wooden spool, and two sizes of
insulated copper wire. An induction-coil is a peculiar and wonderful
apparatus; it figures largely in electrical experimenting and is a part
of every complete equipment.
A piece of curtain-pole may be used for the spool. First bore a
five-sixteenths-inch hole through the wood to receive the bolt; then in
a lathe turn it down into a spool with less than one-eighth of an inch
of wood about the hole and with flanges about one-eighth of an inch in
thickness. Smooth the spool with sand-paper, while it is still in the
lathe, and give it a thin coat or two of shellac.
Slip the spool on the winder (Fig. 7) and wind on three layers of No. 24
cotton-insulated copper wire, taking care to wrap the coils evenly and
close. Bring six inches of the ends out at either end of the spool
through small holes pierced in the flanges; then wrap several
thicknesses of brown paper around the coil. A current passing around
this three-layer coil will magnetize the bolt. This is the primary coil
and the one through which the battery current will pass.
A secondary coil is now made over the primary one with eleven or
thirteen layers of No. 30 insulated copper wire. It will take some time
to carefully put on these layers, and when doing so mark down each layer
so as to keep an accurate count, for there must be the right number of
layers to make the coil act properly. No. 30 wire is quite fine, and if
the layers are not inclined to lie smooth, make a wrap or two of brown
paper between each three layers. Bring six inches of each end of the
wire out from the flanges of the spool, and to protect the outer coil
wrap paper about the coils and attach it fast with thread or paraffine.
Slip the bolt through the hole and screw the nut on the threaded end.
Cut out a wooden block four inches long, three inches wide, and
three-quarters of an inch thick, and with two thin metal straps and
screws attach the coil to the middle of the block, as shown in Fig. 8.
Make four binding-posts and screw them fast at the corners, and to A and
B of Fig. 8 attach the ends of the heavy wire from the primary coil, and
to C and D of Fig. 8 the ends of the fine wire from the secondary coils.
The induction-coil is now ready for any use to which it may be put, and
by mounting it on the block with the delicate wire ends attached to the
binding-posts, it is in less danger of damage than if the wire ends were
left loose for rough-and-ready connections.
In order to get a shock from this coil it will be necessary to have a
pair of handles and a current interrupter. The handles may be made from
two pieces of tin rolled into the form of cylinders to which wires are
soldered. Or, better yet, use pieces of thin brass tubing an inch in
diameter. The buzzer shown in Fig. 9 may be employed for a current
interrupter, and a bichromate battery will furnish the current.
In order to make the connections, the wires from the handles are
attached to the binding-posts C and D in Fig. 8--that is, to the wires
of the secondary coil. One spool of the battery is connected with A of
Fig. 8 and the other with A of Fig. 9. A wire connects C of Fig. 9 with
B of Fig. 8, and the circuit is closed. The buzzer now begins to
vibrate, and any one holding the handles will receive a shock the
intensity of which depends on the strength of the batteries. A switch
should be introduced somewhere in the circuit, so that it may be opened
or closed at will; a good place for it is between a battery-pole and the
binding-post A in Fig. 8.
If the shock is too intense it may be weakened by drawing the carbon and
zinc poles partly out of the bichromate solution; or a regulator may be
made of a large glass tube and a glass preserving-jar filled with water.
If the tube cannot be had, an Argand gas-burner chimney will answer as
well.
Solder a wire to the edge of a small tin or copper disk, as shown in
Fig. 10, on which the chimney rests at the bottom of the jar, and
another wire to a tin box-cover with some small holes punched in its
top, this latter being suspended within the chimney. This second wire is
passed out through a cork at the top of the chimney made of a disk of
cardboard and a piece of wood. One wire is connected with A of Fig. 8
and the other with a battery-pole. This apparatus acts the same as a
resistance-coil, and by raising or lowering the box-cover the current is
increased or diminished. The closer the cover comes to the disk the
stronger the current, as there is less water for the electricity to pass
through and therefore less resistance; while if the cover touches the
disk the current flows as freely as if there were no regulator and the
wires ran directly to the cell.
An apparatus comprising a coil, an interrupter, or armature, and a
switch may be set on one block, and the arrangement of the several parts
is clearly shown in the drawing of the complete galvano-faradic
apparatus (Fig. 11). The block should be six inches long, four inches
wide, and seven-eighths of an inch in thickness.
[Illustration: FIG. 8]
[Illustration: FIG. 9]
[Illustration: FIG. 10]
[Illustration: FIG. 11]
[Illustration: FIG. 12]
The coil is made as described for Fig. 8, the spool being three inches
long and one inch and a quarter in diameter. A carriage-bolt three
inches and a half long and five-sixteenths of an inch in diameter, with
a bevelled head, is made fast in the spool, and this coil is strapped to
the block with two metal bands and screws. Two binding-posts (A and B of
Fig. 11) are arranged at the upper corners, and to these the ends of the
secondary coil wires are attached. Two more binding-posts (C and D of
Fig. 11) are arranged at the lower side and provided with a switch to
open and close the circuit. One of the primary coil wires is made fast
to C, and the other one to a block which contains the set-screw that
bears against the vibrating armature. Its arrangement and the wire
connection is explained in Fig. 9 B.
An armature of thin brass or tin is made and attached to a block (E in
Fig. 11). At the loose end that is opposite the bolt-head several wraps
of tin are made and soldered fast, or a small block of soft iron may be
riveted to the armature. It must be of iron or tin, however, so as to be
attracted by the electro-magnetized bolt-head. This arrangement may be
seen in Fig. 12. Attach a thick piece of paper over the bolt-head, so
that the lug at the end of the armature will not adhere to it through
residual magnetism.
In regular galvano-faradic machines the current regulator is formed of a
hollow cylinder which is drawn from the core of the coil; but in this
simple machine the water-jar regulator may be connected between a pole
of the battery and the binding-posts (D or E of Fig. 11). The wires of
the handles are attached to posts (A and B of Fig. 11), and when all the
wires are in place and the current turned on by means of the switch, the
vibrator begins to work and the shocking-current is felt through the
handles. By means of the regulating-screw that bears on the armature,
the number of vibrations may be increased or diminished, but for faradic
purposes the vibrations should be as quick as possible. Much amusement
may be had with this apparatus, and a large number of people may be
given a shock by getting them to join hands when standing or sitting in
a circle.
An Electric Buzzer
This piece of apparatus is, in theory, nothing more than the electric
bell, and might properly be included in Chapter V., on Annunciators and
Bells. But since it is the logical development of principles just laid
down, it has been thought best to give it its present position.
The electric buzzer is constructed on the principle of the
telegraph-sounder, but instead of making a single click or stroke the
current is made to act on the armature and keep up a continuous motion
so long as the electricity passes through the helix of the cores, the
armature, and the contact-points of the apparatus.
A buzzer has the same movement as an electric bell with the ringing
apparatus removed. For offices, houses, and quiet calls it is often
preferred to the loud ringing of a bell.
The electric buzzer shown in Fig. 13 is easy to make; it is operated by
the aid of a cell and a push-button. Cut a base-block three inches and a
half wide, five inches long, and three-quarters of an inch thick, and
mount a horseshoe magnet made of bolts and a yoke and coils about at the
middle of it, as shown in Fig. 9. The magnet is held to the base by a
flat wooden cleat and a screw passed down through a hole in the cleat
and into the base, between the coils. An armature of soft iron, two
inches long and half an inch wide, is riveted to a piece of
spring-brass, as shown in Fig. 14 A, and the end is bent so that it will
fit around the corner of a block to which it is held fast with two
screws. This armature is mounted so that there is a space one-sixteenth
of an inch wide between it and the bolt-heads, as you can see in Fig. 9.
The brass is bent out slightly and runs parallel with the armature for
one inch and a quarter. Against this the end of the screw mounted in
block B Fig. 9 rests.
[Illustration: FIG. 13]
[Illustration: FIG. 14]
The block B is a small piece of hard-wood screwed fast to the side of
the base to hold the set-screw and also the wire that comes from the
outside of the upper coil. A small hole is made in the edge of the block
and the wire passed in, so that the end rests in the screw-hole as shown
by the dotted line. When the screw is placed in the hole and turned, it
comes into contact with the wire and makes a connection. This block and
its attachment is shown in Fig. 14 B.
On the base, near the armature-block, a binding-post is made fast, and
the current, passing in through the wire A in Fig. 9, goes through the
coils and around to the screw B, then through the armature to the block,
and out through the wire C. In its circuit the bolts are magnetized, and
they draw the armature, but the instant they do so the loose
spring-brass end is pulled away from the screw-point B and the circuit
is broken, the bolts cease to be magnetized, and the armature flies back
under the influence of the spring-brass neck at D. The loose brass end,
on touching the screw-point, conducts the current through the coils
again, with a continual vibrating action, so long as the electric
current is passing in at A and out at C. The greater the volume of
current the greater the number of vibrations, and to properly regulate
the contact the set-screw B must be adjusted at the right point. Paste
pieces of heavy paper over the heads of the bolts to overcome residual
magnetism.
A single electric bell is made the same as a buzzer, but continuing on
from the end of the armature a wire or rod is mounted with a ball at the
end which strikes the bell as the current causes the armature to
vibrate. The bell-block may be made longer, and a bell from an old clock
or a bicycle should be mounted at the proper place on a wooden dowel
driven into the base. A screw passes through the hole at the middle of
the bell and into the top of the dowel. The ball at the end of the rod
may be made of brass with a hole in it, and a drop of solder will hold
it in place. Or it may be made of wire wound round the end and soldered
into a compact mass.
A Large Induction-coil
As has been said, the induction-coil is one of the mysterious phenomena
of electrical science. While its practical value is known and recognized
in all branches of voltaic electricity for use in transforming currents,
its actual workings have never been clearly explained.
The construction of a small induction-coil was explained in the
description of a shocker or medical battery. For bigger equipments,
wireless telegraphy and other uses, a large induction-coil will be
necessary, and the following illustrations and descriptions should
enable the young electrician to construct an apparatus that will be both
simple and efficient in its working.
For the tube (in which to wind the primary coils) obtain a piece of red
fibre-tubing, one inch inside diameter and not more than one-eighth of
an inch in thickness. The length should be ten inches. If fibre cannot
be had use a paste-board tube.
From white-wood, half an inch in thickness, saw two blocks four inches
square and in the centre of each cut a hole so that the tube will pass
through it and fit snugly. Some shellac and a few slim brass escutcheon
pins will hold the blocks in place, as shown at Fig. 15. The wood blocks
and fibre or paper tube should be treated to several successive coats of
shellac to give them a good finish and prevent the absorption of
moisture. Four binding-posts, with wood screw-ends, are to be made fast
at the top edges of the end-blocks, as shown at Fig. 15. Holes bored in
the blocks near the foot of the binding-posts will admit the ends of
the coil-wires so that contact can be made. The ends of the
conductor-wires should then be placed in the holes in the binding-posts
and held in place with the thumb-screws.
[Illustration: FIG. 15]
[Illustration: FIG. 16]
The primary coil is made by winding four layers of No. 20 insulated
copper wire on the tube and between the end-blocks, as shown at Fig. 16.
Each layer must be wound evenly, and the strands should lie close to
each other. When the first layer is on give it a coat of shellac; then
wrap a piece of thin paper about it and give that also a coat of
shellac. When the second layer is on repeat the operation of shellacking
and paper-coating, and continue with the third layer. When the fourth
layer is on give the coil a double wrap of paper and two or three coats
of shellac to thoroughly insulate it and keep out all moisture. The
winding may be done by hand, but it is much easier to do it on a winder
or reel, which can be operated to revolve the core, the wire unwinding
from its original spool as it is wound on the tube.
A convenient winder may be made on a base-board which can be clamped to
a table or bench. The board is twelve inches long, eight or ten inches
wide, and seven-eighths of an inch thick. Two uprights, three inches
wide, ten inches long, and three-quarters of an inch thick, are screwed
and glued to the ends of the base-board. A notch is cut in the top of
the end-boards, into which the spindle or shaft can rest; and at the top
of the end-pieces two small plates of wood or metal are screwed down to
hold the spindle in place when the tube and ends are being revolved. A
small hole, bored in each upright end two inches above the top of the
base-board, will admit a rod on which a spool of wire can revolve, as
shown at Fig. 17.
Two plugs of wood, shaped like corks, are made to fit in the ends of the
fibre-tube. A hole is bored through each one so that a wire or rod
spindle will pass through them and fit tightly. One end of the rod is
bent and provided with a small wooden handle, by means of which the core
may be revolved.
This winding-rack makes it easy to handle the core-tube while putting on
the layers of wire, and it holds the tube securely while the wraps of
paper and shellac are applied.
The secondary coil is laid over the primary, and should be of Nos. 30 to
36 insulated copper wire. The finer the wire the higher the resistance
and the longer the spark, but nothing heavier than No. 30 should be
used.
Begin by making one end of the wire fast to a binding-post; then turn
the core-tube with one hand, holding the wire in the other. Take care
not to bind the wire nor stretch it, but wind it on smoothly and evenly,
like the coils of thread on a new spool of cotton or silk. Be very
careful to avoid kinks, breaks, or uninsulated places in the wire.
Should the wire become broken, give the coil a coat of shellac to bind
the wound strands; then make a fine twisted point and cover it with the
silk or cotton covering, with a coat of shellac to hold it in place, and
proceed with the winding. Between each layer of wire place a thin sheet
of paper and coat it with paraffine, or shellac, to make a perfect
insulation; then proceed with the next layer.
With a battery and small bell test the wire layers occasionally to see
that everything is all right, and that there are no breaks or short
circuits. This is very necessary to avoid making mistakes, and,
considering the time and care spent in winding the coils, it would be a
great disappointment if the coil were defective.
[Illustration: FIG. 17]
[Illustration: FIG. 18]
About one pound and a half of wire should constitute the secondary coil,
and, if possible, it is best to have it in one continuous strand,
without splices.
Over the last coil, after the winding is completed, several thicknesses
of paper should be laid and well coated with shellac between each wrap.
This is a protector to insure the fine wire strands from damage. To
improve the appearance of the coil a wrap of thin black or colored
leather may be glued fast, with the seam or point at the under side.
The ends of the wires forming the primary coil should be made fast to
the binding-posts at one end, while those of the secondary coil should
be attached to the posts at the other end.
For the core, obtain some soft iron wire, about No. 18, and cut a number
of lengths. Straighten these short wires and fill the tube with them,
packing it closely, so that the wires will remain in place under a
mutual pressure. It is better to make a core of a number of rods or
wires rather than to have it of one solid piece of soft iron.
Now, from hard-wood, cut a base three-quarters of an inch thick, five or
six inches wide, and twelve inches long. Attach the coil to the base by
means of screws passed up through the board and into the lower edges of
the end-blocks. The wood is to be stained and given several successive
coats of shellac.
Now connect the wires of a battery to the binding-posts in contact with
the primary coil, and attach two separate wires to the secondary coil
binding-posts. Bring these ends near to each other, and a spark will
leap across from one end to the other, its size or “fatness” depending
on the strength of the battery. The completed apparatus is shown at Fig.
18.
In producing a long spark a condenser is an important factor; it is used
in series with an induction-coil. There are several forms of
condensers, but perhaps the simplest and most efficient is the Fizeau
condenser, which is made up of layers of tin-foil with paraffined paper
as separators.
From a florist’s supply-house purchase one hundred and fifty sheets of
tin-foil seven by nine inches, or sheets that will cut to that size
without waste; also ten or twelve extra sheets for strips. At a paper
supply-house obtain some clear, thin, tough paper about the thickness of
good writing-paper. Be careful to reject any sheets that are perforated
or have any fine holes in them. The sheets should be eight by ten
inches, or half an inch larger all around than those of the tin-foil.
The paper must be thoroughly soaked in hot paraffine to make it
moisture-proof and a perfect non-conductor. This is done by placing
about two hundred sheets on the bottom of a clean tin tray, or
photographic developing-dish of porcelain. Don’t use glass or rubber.
After placing some lumps of paraffine on the paper, put the tray in an
oven so as to dissolve the paraffine and thoroughly soak the paper.
Open the oven door and, with a pin, raise up the sheets one at a time,
and draw them out of the liquid paraffine. As soon as it comes in
contact with cool air the paraffine solidifies and the sheet of paper
becomes stiffened. Select each sheet with care, so that those employed
for the condenser are free from holes or imperfect places.
From pine or white-wood, a quarter of an inch in thickness, cut two
boards, eight by ten inches, and give them several good coats of
shellac.
To build up the condenser, lay one board on a table and on it place two
sheets of paraffined paper. On this lay a sheet of tin-foil, arranging
it so that half an inch of paper will be visible around the margin. From
the odd sheets of tin-foil cut some strips, one inch in width and three
inches long. Place one of these strips at the left end of the first
sheet of foil, as shown at Fig. 19. Over this lay a sheet of the
paraffined paper, then another sheet of the foil. Now on this second
sheet of foil lay the short strip to the right end, and so proceed until
all the foil and paper is in place, arranging each alternate short strip
at the opposite end. Care must be taken to observe this order if the
condenser is to be of any use.
[Illustration: FIG. 19]
[Illustration: FIG. 20]
When the last piece of foil is laid on, with its short strip above it,
add two or three thicknesses of paper, and then the other board. With
four screw-clamps, one at each corner, press together the mass of foil,
paper, and boards as closely as possible, then bind the boards about
with adhesive tape, or stout twine, and release the clamps. Attach all
the projecting ends of foil at one side by means of a binding-post, and
those at the other end with another binding-post. The complete condenser
will then appear as shown in Fig. 20.
When in operation one wire leading from the secondary coil should be
connected with a binding-post of the condenser, so that it is in
series.
The object of the condenser is to increase the efficiency of induction,
and it should be made in proportion to the size of the induction-coil
with which it is to be employed.
Circuit-Interrupters
When an induction-coil is to be employed as a shocker (and there is no
vibrating armature arranged in connection with the core), a
circuit-interrupter must be employed to get the effect of the
pulsations, as given out by the secondary coil when a current is passing
through the primary.
There are various forms of circuit-breakers that may be made for this
purpose, but for really efficient service the type shown in Fig. 21 is
perhaps the best that can be devised.
This interrupter consists of a metal cog-wheel with saw-teeth, a pinion
or axle, and a handle. Also a base-block, with uprights to support it,
and a piece of spring-brass wire, arranged so as to bear against the
wheel. When the wheel is revolved the spring-wire will be driven out by
each tooth; and when released it flies back to the wheel, striking the
bevelled edge of a tooth at each trip.
Two binding-posts, arranged on the block, will provide means of
connecting in-and-out wires. With a coat or two of shellac on the
wood-work and black asphaltum varnish on all surfaces of the metal that
are not used for contact, this circuit-interrupter will be ready for any
use in connection with an induction-coil.
The base-block is of pine, white-wood, or cypress, seven-eighths of an
inch thick, three inches wide, and five inches long. The uprights, which
support the wheel, are half an inch thick and one inch wide. The wheel
is three inches in diameter and is made of brass one-sixteenth of an
inch thick. The design of the wheel should be laid out with a compass
and marked with lead-pencil or a sharp-pointed awl, which will leave a
mark clear enough to be seen when sawing and filing the teeth and open
places.
[Illustration: FIG. 21]
[Illustration: FIG. 22]
A true plan is shown at Fig. 21 A. Through the middle of the wheel a
small hole is bored to receive the axle of brass which is to be soldered
in place. When the wheel is set up, a metal crank and wooden handle
should be soldered fast to one end of the axle. A piece of spring-brass
wire is fastened to the block, with a staple, and the lower end bent so
that the screw in one binding-post will hold it in place. The upper end
of the wire is bent in the form of an [L]. From the other binding-post,
through the block and up one support, a wire is passed, the end of which
comes into contact with the axle. The current, passing in through one
binding-post, is carried through this wire to the axle, then to the
wheel, and so on out through the spring-wire and remaining binding-post.
When in action the circuit is constantly being broken, as the
spring-wire jumps from the end of one tooth back to the face of the next
tooth. The pulsations are increased or diminished by the fast or slow
speed of the wheel, as regulated by the hand motion. The strength of the
current is regulated by the force of the battery and should be
controlled by a water resistance, as described for the medical battery,
or shocking-coil.
The interrupter, shown in Fig. 22, is built up on a block six inches
square and seven-eighths of an inch thick.
A circle is cut from sheet-lead and laid on the face of the block,
through which pins, or steel-wire nails, are driven. The lead circle is
five inches in diameter and half an inch in width, making the inside
diameter four inches.
The pins or nails are driven a quarter of an inch apart, and should be
properly and accurately separated, so that an even make-and-break will
be the result.
It is not necessary to bore holes in the lead, but the pins or nails
should be driven clear through it, so that perfect contact can be had by
the metal parts coming together. Otherwise the apparatus would be
useless.
Over the circle of pins a brass bridge is erected, so that the
cross-strips will clear the heads of the pins. A hole is bored at the
middle of the bridge so that the revolving axle will pass through it.
The axle is made from a piece of stout wire, or light rod, and near the
foot of it, and about half an inch above the base-board, a disk of metal
is soldered fast. A piece of spring-brass wire is attached to this disk,
so that when the axle is turned the end of the wire trips from pin to
pin, thus making and breaking the circuit. The upper part of the axle
is bent and provided with a small wooden or porcelain knob.
One wire from the secondary coil is caught under a screw that holds one
end of the brass bridge to the base; and the other to a screw, which may
be placed at one corner of the block, and from which a short wire leads
to the lead ring. Binding-posts may be arranged to serve the same
purpose, and, of course, they are much better than the screws, because
they can be easily operated by the fingers and do not require a
screw-driver every time the interrupter is placed in series with an
induction-coil. An interrupter on this same order may be made from a
straight strip of lead with the pins driven through the middle of it.
One wire from the secondary coil is made fast to the lead plate, and the
end of the other wire is passed along the pins, thus making and breaking
the circuit in a primitive manner.
Chapter V
ANNUNCIATORS AND BELLS
A Drum Sounder
A unique electric sounder that is sure to attract attention is in the
shape of an electric-bell apparatus, with a drum sounder in place of a
bell, or knockerless buzzer. Fig. 1.
The outfit is mounted on a block four inches and a half wide and seven
inches long. The cores and yoke are made as described for the electric
buzzer (chapter iv.) and are wound with No. 22 cotton-insulated wire.
The magnet is then strapped fast to the block by means of a hard-wood
plate having a screw passed down through it; and between the coils and
into the block an armature is made and mounted on a metal plate, which
in turn is screwed to the block. Another block, with a contact-point, is
arranged to interrupt the armature, and the series is connected as shown
in the drawing Fig. 1.
The end of the wire projecting above the armature is provided with a
hard-wood knocker which operates upon the head of the drum. The drum is
made from a small tin can, having one or two small holes punched in the
bottom. Over the top a thin membrane, such as a bladder or a piece of
sheep-skin or cat-skin, is drawn and lashed fast with several wraps of
wire, having the ends twisted together securely. The membrane must be
wet when drawn over the can end, and great care should be taken to get
it tight and even. Then, when it dries, it will stretch and draw, like a
drumhead, and hold its elastic, resonant surface so long as it does not
become moistened or wet.
This drum is arranged in the proper position and lashed fast with wires
passed over the box and down through holes in the block; where, after
several turns, the ends may be securely twisted together. In place of
the drum a small wooden box may be lashed fast with its open end against
the block, so as to form a hollow enclosure. The raps of the knocker
against its sides will give forth a resonant xylophone tone.
An Annunciator
A simple annunciator may be made from a core, a helix, and some brass
strips. A soft iron core, made of a piece of three-eighth-inch round
iron and threaded at one end, is converted into a magnet by having a
spool and wire coil arranged to enclose it. This in turn is screwed into
a strip of brass bored and threaded to receive it. Fig. 2.
This brass strip is shaped as shown at Fig. 3 A, and the ears are bent
to serve their several purposes. The lowest ears are turned out and the
lower part of the plate is bent forward so as to form the hinge on which
the drop-bar turns. The drop-bar is only a strip of metal turned up at
one end, on which a numeral or letter can be attached; while at the
other the metal should be bent over so as to form a core into which a
pin or wire may be passed and the extending ends bent down, after being
caught through the holes in the ears. Above the magnet the strip is bent
forward and the top or end ears bent up, so as to form the hinge on
which the armature swings. Holes are made in the long ears, through
which screws pass to hold the annunciator fast to the box or wood-work.
The armature is made from a strip of brass and is shaped like B in Fig.
3. The two ears at the top are bent down and fit within those at the top
of the first strip. A screw or wire passed through the holes in the ears
will complete the hinge. The strip is bent down so as to fall in front
of the magnet, and to its inner side a button or disk of sheet-iron is
riveted fast, so as to form an attraction-plate to be drawn against the
magnet when the current is passing around it. The lower part of the
armature is bent in hook fashion so as to hold up the drop-bar.
A slot is cut in the drop-bar through which the hooked end will project.
A short spring is arranged at the top of the annunciator so as to keep
the bar and the hook in place when not in action. The current passing
around the soft iron core magnetizes it and draws the iron button on the
armature towards it. This action immediately releases the hook from
under the edge of the metal at the forward end of the slot, and the bar
drops, bringing the figure down and into plain sight. It is necessary,
of course, to mount this annunciator so that the bar will not drop down
too far. This may be done by having a platform or wire run along under a
series of the drops, so that they will rest on it.
[Illustration: FIG. 1
FIG. 2
FIG. 3
FIG. 4
FIG. 5
FIG. 6
ANNUNCIATORS AND BELLS]
A Double Electric Bell
For loud ringing, and to get the benefit of both the forward and
backward stroke of the knocker, a double bell, similar to the one shown
in Fig. 4, may be constructed upon the general principle of the
single-stroke buzzer already described (chapter iv.).
Two soft iron cores are made, as described for the other bells, but
instead of being yoked together with iron, so that the three parts will
form a horseshoe magnet, the yoke is of brass or copper. Each core will
then be an independent magnet.
The spools are wound with No. 22 insulated wire and the ends left free,
so that the coils are not connected together. If the drawing is examined
closely you will see that the armature swings on a pivot at the base of
the knocker-bar. When the bell is not in action the knocker might rest
naturally against one bell or the other; or it might stand in the centre
and not touch a contact-point, were it not for the small spring which
draws it to the left. Directly the current is run through the coils it
alternately magnetizes first one and then the other. This action is due
to the making and breaking of the circuit by the spring on the armature.
It first comes into contact with one point, and then is drawn away from
it to come into contact with the other. Fig. 4 shows the knocker-bar at
rest between both bells and the armature unattracted by either magnet.
This position is purposely given so as to indicate the balance of the
armature and the spaces between it and the cores and also the
contact-points above it.
The small, light wire spring shown in the drawing draws the knocker to
one side; therefore, when at rest, one circuit is closed. Otherwise the
bell would not act when the current is run through the parts--in fact,
the current could not run through at all, if one or the other contact
were not made.
The magnets are held fast to a base with a long screw and a small plate
of wood laid across them as shown in Fig. 4. The armature is a piece of
soft iron one-eighth of an inch thick, half an inch wide, and three
inches long. This has a spring-brass piece attached to it as shown at A
A in Fig. 5. Small holes are bored through the strip and the iron, and
escutcheon pins are passed through and riveted. A small hole is made
down through the middle of the iron plate and a pin is driven into it,
so that a quarter of an inch projects at both sides.
Another hole is made through the side of the plate for the knocker-bar.
Then the armature is set in place so that there is a space of one-eighth
of an inch between it and the magnet ends. The armature is held in place
at the top by a bent metal strip (B B in Fig. 5). This is screwed fast
to the base and the bottom is countersunk into the wood.
Two contact-points (C C in Fig. 5) are arranged so that when a magnet
draws the armature down it brings the opposite end of the armature
spring into contact with a point.
The wiring is at the under side of the base and is shown in Fig. 6. The
current enters binding-post A, and passes around coil and magnet No. 1
by entering at B and leaving at C; from thence to D, entering the
armature spring at E, when the small spring has drawn the knocker-bar
over to the left. The current passes along the armature and out at F;
then along to binding-post G, and so on around through battery K and
push-button L, thus completing the circuit. Directly that this is done
the magnet draws the spring end of the armature away from contact-point
D and up against contact-point J, so that the circuit is broken through
coil No. 1 and is sent through coil No. 2. This immediately magnetizes
core No. 2 and draws the armature down to it, breaking its contact with
J and re-establishing it with D. The rapid alternate making and breaking
of the circuit, and the rapid and strong motion of the armature in its
seesaw action, causes the knocker to rap the bells soundly each time it
travels from right to left and back again.
Two bells of similar size, or two drums or wooden boxes, may be employed
for this double striker, and the more current that is run through the
coils the more power and a corresponding increase of noise.
An Electric Horn
One of the most useful pieces of apparatus where a loud noise is
required (such as in a motor-boat or an automobile) is the electric
horn.
It is a rearranged principle of the telephone, for instead of sound
entering or striking against the membrane or tympanum to be transmitted
elsewhere, the disturbance takes place within the horn and the sound is
emitted.
Everybody has been close to a telephone when others have been using it,
and has heard noises, rasping sounds, and even the voice of the speaker
at the other end of the line. If a cornet were played at the other end
of the line it could be distinctly heard through the receiver by many
persons in the room, since its vibrations are loud enough to set up a
forcible succession of sound-waves.
The same principle operates in the electric horn, but instead of being
agitated at a long distance it is done within the enclosure, and a very
much louder vibration is consequently produced.
It is quite as easy to make an electric horn as to construct a bell, but
care must be exercised to have the parts fit accurately and the wiring
properly done. If the drawings are studied and the description closely
followed, there is no reason why a horn cannot be made that will demand
any one’s attention from some distance away.
The complete horn is shown in the illustration Fig. 7, and as it is made
of wood it is easily attached with screws to a boat or a motor-car.
From white-wood, half an inch thick, cut two blocks three inches and a
quarter square. In one of them (the rear one) bore a hole at the centre,
of such size that a piece of three-eighth-inch gas-pipe can be screwed
into it. In the other one make a hole two inches in diameter, so that
the cover of a small tin can will fit into it. Outside this hole, and on
one side of the block, cut the wood away and down for one-eighth of an
inch, forming a rabbet, as shown at A in Fig. 7. This will be the back
of the front block.
[Illustration: FIG. 7
AN ELECTRIC HORN]
Have a gas or steam fitter cut a piece of two-inch iron pipe one inch
and three-quarters long. This will measure a trifle over two inches and
a quarter, outside diameter, and will form the cylinder or cover for the
mechanism. The piece of pipe should fit snugly in the front board, and
at the rear one the wood should be cut away so as to let it in an eighth
of an inch, as shown in the sectional plan of Fig. 7.
Obtain a piece of three-eighth-inch gas-pipe, threaded at one end. Cut
it with a hack-saw, and file the blunt end so that it will measure one
inch and seven-eighths long, as shown at C in Fig. 7. This is to be
screwed into the front of the rear block so that it will project one
inch and a half.
Make a spool to fit the pipe, as shown at B in Fig. 7, or use two wooden
button-moulds attached to the pipe with shellac or glue. Between them
wind on the coils of No. 22 wire to form the helix.
Cut a hole in the tin-can cover, as shown at D in Fig. 7, and have a
tinsmith solder a small funnel to it (for the horn, or bell, as it is
called), cutting away the lower part of the funnel so that the hole in
it will correspond in size with that in the can cover.
This joint can be made at home by fitting the funnel in the hole and
then turning back the edge, as shown in the sectional drawing at E in
Fig. 7. Then, with a spirit-lamp, some soldering solution, and solder,
make a good joint.
Small holes are to be made at the corners of the blocks, through which
stove-bolts two inches and a half long will fit to bind the front, back,
and cylinder together.
Select a good, clean, and flat piece of tin and cut a disk two inches
and a quarter in diameter, and through the middle make a small hole. Cut
two pieces of iron about the size and thickness of a cent, and bore a
small hole through the centre of each. Obtain a piece of stout brass
wire, or thin rod, and file one end of it as shown at G in Fig. 7, so
that the small end will fit in the holes made in the iron buttons. Place
one button on either side of the tin disk, and pass the wire through;
then clamp it in a vise and rivet the top of the rod so that you will
have a disk with a button at each side of the centre and all attached
firmly to a brass rod, as shown at F in Fig. 7. The total length of this
rod should be two inches and a half, and the lower end is to be threaded
and provided with two small brass nuts. A piece of spring-brass
three-eighths or half an inch wide is made fast to a small block at the
back of the horn, as shown at H in Fig. 7, and at its opposite end a
contact-piece of metal, bent at an angle, is screwed fast. Around the
back of the back block a wooden frame is attached to protect the rear
mechanism of the horn.
The parts are now ready to assemble. First see that the metal angle
contact-point is in place with the long brass strip resting on it, and
that this in turn is properly fastened to the block on the side opposite
the contact-point, as shown at H in Fig. 7. There should be a small hole
through the middle of the brass strip directly in line with the middle
of the hole in the gas-pipe. Place this back-board down on the table so
that it will lie in a position as indicated in the sectional plan of
Fig. 7. The gas-pipe is then to be screwed onto the plate. Over this the
spool with its layers of wire is to be slipped and made fast, and the
cylinder of iron is then placed in position. Over this the disk F is
laid, so that the brass rod extends down through the pipe and brass
strip; then the nut is screwed on to hold it in place. Next comes the
front block, with its horn or bell, and the entire mass is locked
together by means of the four bolts at the corners.
The wiring is simple. One inlet being through block I, the current
passes through strip J to contact-point K; then through the coil and out
at wire L. The inlet and outlet wires are connected to a battery and to
a push-button or switch, so that the horn can be operated. The proper
adjustment of this horn depends on the nuts at the foot of the brass
rod. They must be screwed on tight enough to draw the strip J so that it
rests on the contact-point K.
The current, passing in at I, through J, K, the coil, and out at L,
magnetizes the piece of pipe and draws the iron buttons or disks
attached to the tin disk. But so soon as it does so it breaks the
contact between J and K, and the buttons fly back into place, having
been drawn there by the rigidity of the tin disk to which they are
attached. Again the current is closed and the magnet draws the iron
buttons. The brass rod moves but a very slight distance up and
down--enough, however, to make and break the contact between J and K. As
a result of this rapid movement and the consequent snapping of the tin
disk, a loud noise is emitted through the bell, which can be heard a
long distance and closely resembles a long blast blown on a fish-horn.
Burglar-alarms
A unique burglar-alarm trap may be made from a plate of wood, five by
six inches and half an inch thick, a movable lever, and a brass strip
having the ends turned out. These are arranged as shown in Fig. 8. The
brass strip is fastened to the plate with screws, and the ends extend
out for half an inch from the board. The lever is made from a strip of
brass, and the upper part is bent out so as to clear the strip and
screws that are under it. A hole is made at the lower end of the lever,
through which a brass ring and the end of a spring may be fastened. The
opposite end of the spring is attached to a screw, and a wire carried
from it to a binding-post, A. Another wire connects the back plate with
binding-post B. A string or piece of fine picture-wire is made fast to
the ring and carried to any part of a room.
To set the trap, make the block fast in any convenient place, such as
the door-casing or the surbase, and carry the string out from the trap
and fasten the end of it. Any one running against it in the dark will
draw the lever over to the right side and connect the circuit.
[Illustration: FIG. 8]
[Illustration: FIG. 9]
[Illustration: FIG. 10]
When setting the trap, have the string adjusted so that the lever is in
a vertical position, as shown in the drawing of Fig. 8. When the string
is disturbed it will pull the top of the lever over to the right side;
but if the string is broken by the person running against it, the spring
attached to the bottom of the lever draws it over to the right side with
a snap, and the top of the lever goes to the left side, when the circuit
is closed and the alarm given.
This trap is connected the same as a push-button, one wire leading to
the bell, the other to the battery; then the battery and bell are
connected together so that when the circuit is closed the bell will ring
until some one throws a switch open to break it.
Another form of circuit-closer is shown in the door-trap (Fig. 9). This
is a wooden block that rests on the floor close to the bottom of a door,
and is held in place by means of four sharp-pointed nails driven down
through the corners of the block. The points should project a quarter of
an inch or more, according to whether the block is on a hard floor or on
a carpet. The front edge of the block is bevelled so that the bottom of
a door that fits closely to the floor will pass over it.
The block is five by seven inches, and three-quarters of an inch thick.
At the left side a strip of metal (A) is held close to the block with
straps or wide staples driven over it, but not so close but that it can
move freely back and forth. To the front end a round piece of wood is
made fast. This is the bumper against which the door will strike when
opened. At the middle of the strip a screw is riveted fast; or it may be
a machine-screw let into a threaded hole in the metal. At the right side
of the block another strip of metal (B) is attached, but this is held
fast with a screw at the middle and a screw-eye and washer at the rear
end to act as a binding-post. The front end of this strip is turned up
so as to form a stop; then a movable lever (C) mounted over both strips,
with one end bent up, is attached to the block with a screw. A slot is
cut at one end so that the screw in the movable strip (A) will move
freely in it, and near the other end a small hole is made to receive the
end of a spiral spring (D). To set the trap, the block is placed on the
floor and the wires from battery and bell are made fast to the
binding-posts. The spring D keeps the lever C away from the strip-end B,
while at the same time it throws the strip A forward. When the door is
opened it shoves the bumper and strip A back through the staples, while
the screw operates lever C and causes its loose end to come into contact
with the end B, thereby closing the circuit and ringing the bell or
buzzer. When the door is closed again the spring draws lever C away from
B, and the circuit is opened.
The block acts as an obstruction as well as an alarm, for the pins will
hold in the floor and the little block will stand its ground. A simple
form of contact for doors is shown at Fig. 10. This is simply two strips
of spring-brass bent as shown, and screwed fast on either side the crack
of a door, at the hinge side, so that when the door is opened one piece
of metal bears on the other and the circuit is closed. This is to be
operated in connection with a switch, so that the circuit may be opened
in the daytime when the door is in use.
Signals and Alarms
There are many different kinds of electric call-signals used in and
about the house; among these are some that a boy can readily make--for
example, the ordinary call-buttons and the signals between house and
stable or other out-buildings.
A portable call-bell, or alarm, is one of the most convenient things in
any home. It may be temporarily rigged up from one room to another, or
from one floor to the next, the small, flexible wire being run over the
tops of door-casings, where it is held by slim nails or pins driven into
the wood-work.
The main terminal of this portable outfit consists of a wooden box that
will hold a large dry-cell, and to the side of which an electric bell or
buzzer may be attached. Binding-posts are arranged at another side, to
which the ends of the flexible wire-cord can be made fast, and a cover
fitted to the box to hide the battery and wiring. The complete outfit,
except the flexible wire-cord and push-button, will appear as shown in
Fig. 11. No definite size can be laid down for the construction of this
box, as dry-cells vary in size and shape, some being long and thin,
while others are short and fat. By removing the cover and looking into
the box, it will appear as shown in Fig. 12. The carbon is connected
with one binding-post and the zinc to one of the poles of the bell. The
other bell-pole is connected with the remaining binding-post, and it
requires but a switch or push-button to close the circuit between the
two binding-posts. This is done by the long line of flexible wire-cord,
which may be of the silk or cotton covered kind, having the two strands
twisted together as is customary with flexible electric-light wire. A
pear-shaped push-button may be connected at the end of the line, and
this may be arranged at the head of a bed or on a chair placed
conveniently near an invalid’s couch.
This same apparatus is a very convenient thing for a lecturer where a
stereopticon is used. A buzzer takes the place of the bell, which would
be too loud in a hall or lecture-room, and the cord, passing from the
apparatus close to the operator, is hung over the lecturer’s stand, or
the button held by him in the hand, to be pressed whenever he desires
the pictures changed.
[Illustration: FIG. 11]
[Illustration: FIG. 12]
[Illustration: FIG. 13]
This apparatus can be used also in connection with an alarm-clock, where
the winding-key is exposed at the back, as it is in most of the
nickel-cased clocks that are operated by a spring and which have to be
wound each day. For this purpose obtain a piece of hard rubber or fibre,
one-sixteenth of an inch thick, an inch long, and half an inch wide. A
piece of stout card-board or a thin piece of hard-wood soaked in hot
paraffine will answer just as well, if the fibre or rubber cannot be
had. Bore a small hole at the two upper corners and one at the middle
near the lower edge. Obtain three garter-clips, with springs, and rivet
one of them fast to the little plate of non-conducting material. Cut two
lengths of old brass watch-chain, four inches long, or obtain eight
inches of chain at a hardware-store, and divide it in half. Attach a
garter-clip to one end of each piece, and make the other end fast in the
holes at the corners of the small plate as shown in Fig. 13. This will
be the connector and will close the circuit when the alarm goes off.
When the clock is wound and the alarm-spring is tight, catch one
binding-post with a clip at the end of a chain and the other post with
the remaining clip. Place the clock near the box and grasp the alarm-key
with the clip on the plate. When the alarm goes off the bell on the
clock will begin to ring, and when the key has made one revolution it
twists the two pieces of chain together, closes the circuit, and the
electric bell rings until some one unfastens one of the clips on the
binding-posts and breaks the circuit. The great advantage in this
double-alarm outfit is that it keeps the bell ringing until the
attention of the sleeper is attracted. The bell on the clock will stop
ringing directly the spring is unwound or run down; but in so doing it
twists the chain and sets the electric mechanism in motion, to run until
it is stopped, or until the battery polarizes or is exhausted.
A Dining-table Call
One of the most convenient of house electric-calls is that between the
dining-room and the butler’s pantry or the kitchen, its purpose being to
summon the waitress without the necessity of ringing a bell at the
table, or calling.
There are various forms of push-buttons for this purpose--some embedded
in the floor, others hanging from the centre light, and others again
where the wire runs up from under the table, and the pear-shaped push
rests on the cloth within easy reach. These last are good enough in
their way, but are inconvenient, unsightly, and quite liable to get out
of order.
In order to use the floor-push the table must stand in exactly the right
place; with the drop-string from a chandelier the cord is continually
getting in the way; and as for the portable push that comes from under
the table, one must be forever hunting for the button every time the
table is set. And yet it is quite possible to avoid all these troubles
and construct an apparatus that is always in order and always available,
wherever the table may be placed. A visitor at a certain house noticed
that the number of the family present at a meal was apt to vary largely,
necessitating frequent lengthenings and shortenings of the table. And
yet the waitress invariably appeared just at the right time, and whether
one end or the other of the table was to be served, she was always on
the spot where she was needed. The visitor tried to study it out, but
was finally obliged to ask for an explanation of the mystery. The boy of
the house smiled and intimated that he was responsible for this
domestic miracle; later on, when dinner was over, he removed the centre
leaves from the table and displayed the simple apparatus that he had
constructed and which had worked for several years without adjustment or
repairs.
The illustration (Fig. 14) represents the frame of a dining-table with
the middle cross-bar made fast to the side-rails, so as to support the
mechanism. At both ends, and inside the rail, push-buttons are arranged
and wires carried from them to binding-posts close at hand, as may be
seen at the left side. The cross-bar at the middle of the table supports
a large spool on which the flexible cord is wound, and kept taut by
means of a clock-spring. This spool takes up the slack between the ends
of the table when it is lengthened or shortened, while the smaller one
opposite it keeps taut the feed-wires that come up through the floor. A
short distance from the floor the wire is provided with a connector, so
that when the rug is removed the feed-wires may be disconnected and
slipped down.
The large spool can be had at any dry-goods store where braids or fancy
cords are kept. It should be about four inches long, three inches in
diameter, and with sides thick enough to enable screws to be driven into
it without fear of splitting the wood. An old clock-spring is attached
at one side of the spool, while at the other two circular bands of brass
are made fast, one within the other. An axle of stout wire should be
driven through the spool; but if the hole is too large, wooden plugs may
be glued in at each end so that a front view of the spool will appear as
shown at A. The metal bands are cut with shears from sheet-brass, and
are attached with fine steel nails, the heads of which are driven in
flush with the wood. A hole is made in the side of the spool, close
beside each band, so that the ends of wires may be brought through them
and attached to the bands. This arrangement is illustrated at B, and at
C the opposite end is shown, with its clock-spring, one end of which is
made fast to the side of the spool and the other to the cross-rail. A
small round piece of wood is slipped over the axle, at the spring side,
and projects a quarter of an inch beyond the spring. This is to keep the
spring away from the arm that stands out on that side to hold the spool
in place.
[Illustration: FIG. 14
A DINING-TABLE CALL]
About half an inch of space is left between the spool and the arm at the
opposite side, so that the spring contact-strips may be made fast to the
arm and still have room to act. A view looking down on the spool and
springs is shown at D, and E illustrates the arrangement of the circular
strips and the spring contact-strips. If the table is to remain
permanently in the same position, only one spool will be required, for
the floor wires can come up and connect directly with the
contact-strips. But if the table is to be moved about, a tension-spool,
independent of the push-button wires, is necessary so that the position
of the table may be changed without unwinding the large spool and
dropping the cords down to the floor. The smaller spool is made and
mounted in the same manner, and should be placed close to the large one.
But a lighter spring will operate it. One end of a double wire-cord is
made fast to binding-posts, mounted on a yoke of hard rubber or fibre,
so that the terminals will be kept apart, as shown at F. The other ends
are passed through the holes at one side of the small spool and soldered
fast to the circular strips, or a small screw may be passed down
through the hole, binding the wire and touching the edge of one strip.
Take care that it does not touch the other strip. The cord is then wound
on the spool, and it is slipped in place so that the loose end of the
spring is caught and held over a nail or screw-head. Turn the spool over
several times to partially wind the spring; then attach the terminals to
the wires that come up from the floor and the tension of the spring will
draw the wires taut. The two contact-strips of brass, that rest against
the brass circles, have insulated wires leading out from them in order
to connect them with the corresponding wires leading from the strips of
the larger spool.
A simple way to mount the spools is shown at A in Fig. 15. A notch is
cut in the face of the blocks large enough to admit the axle; then a
face-plate is screwed over the end of the block to hold the axle in
place. This arrangement makes it easy to remove the spool, in case of
necessity, without detaching the arms from the cross-rail.
[Illustration: FIG. 15]
Two sets of wires are wound on the large spool, one leading to the
right-hand and the other to the left-hand push-button on the
table-rails. The ends of the wires are arranged so that one leading from
both directions is made fast to one circular strip on the spool, the
other two being attached to the remaining band. This is more clearly
shown at B in Fig. 15, where the ends are visible as they are twisted
together and pass through their respective holes. The spool is then
turned over, and six or eight feet of wire wound on from each side. The
spring is coiled up and caught on the nail or screw, and the ends of the
wires are made fast to the binding-posts near the push-buttons. The
wires from both push-buttons are then in connection with the circular
bands, which in turn are connected to the bands on the smaller spool,
and lead the current down through the floor wires. By pushing the button
at either end the circuit is closed and the buzzer or bell is rung in
the kitchen or pantry.
Arranged in this manner, the wires are kept off the floor, no matter
where the table is moved, and it can be drawn open as wide as may be
found necessary to put in leaves. When closed again, the spring causes
the large spool to revolve and wind up the wire.
Chapter VI
CURRENT-DETECTORS AND GALVANOMETERS
A current-detector is a necessary part of the amateur electrician’s
equipment; technically, this piece of apparatus is called a
galvanoscope.
When a wire or a number of them are brought near a magnetic needle or a
small compass, the needle will be deflected from its north and south
line and will point east and west, or west and east, according to the
direction in which the current is passing through the wires. All wires
that are conducting electricity have a magnetic field, and when brought
near the magnetized needle of a compass they have the power to act on it
the same as another and stronger magnet would.
The action of detectors depends upon two things--first, the magnetized
needle that, when properly balanced, will point north and south; and,
secondly, a current of electricity passing through a wire or wires held
above the needle, or both above and below it. This is more clearly shown
in Fig. 1, where a compass is resting on a wire connected to a battery.
The wire also passes over the top of the compass, which doubles the
electro-magnetic field.
When the compass (with the needle pointing north) is resting on the
wire that is attached to the zinc pole of a battery, and when the end of
the wire that passes back over the top of the compass is attached to the
carbon pole, the needle will fly around and point to the east. When the
wires are reversed, the needle will point to the west. Thus the
combination of a battery or other source of electric current, a magnetic
needle, and a coil of wire properly arranged, make an instrument that
will detect electric currents and may be correctly called a
current-detector. The pressure of more or less current is determined by
instruments wound with wire of different sizes; the finer the wire the
more sensitive the instrument, and consequently the more delicate. A
very weak current can only be detected with a delicate and sensitive
instrument. The coarser the wire and the larger the instrument, the
better it will be for testing strong currents that would perhaps burn
out the fine wire of the more delicate apparatus.
This instrument, when placed between a source of electricity and a piece
of apparatus, such as a bell, a motor, or lamp, does not weaken the
current, for it requires no waste of electricity to operate the magnetic
needle. Consequently, when a very weak current is being used for any
tests, it is well to place a detector between the battery and the
apparatus to show that the current is actually passing through the wire.
A simple detector is made by winding fifteen or twenty feet of
cotton-insulated copper wire No. 26 or 28 around the lower end of a
drinking-glass. Leave six inches of each end loose; then after slipping
the coil from the glass, tie the wires with thread at least four times
around the circle, so as to bind the wires together. Press two sides of
the hoop together so as to flatten it; then with paraffine attach the
coil to a square block of wood, as shown in Fig. 2.
[Illustration: FIG. 1]
[Illustration: FIG. 2]
[Illustration: FIG. 3]
[Illustration: FIG. 4]
[Illustration: FIG. 5]
From a thin clock-spring, not more than three-eighths of an inch wide,
cut a piece two inches and a half long, and with a stout pair of
tin-shears cut the ends so as to point them, as shown in Fig. 3 A. With
two pair of pliers bend a hump at the middle of the strip on the dotted
lines shown in A, so that a side-view will appear like B in Fig. 3. Turn
this strip over on a hard-wood block or a piece of lead, and with a
stout steel-wire nail and a hammer dent the inverted [V] at the middle
so that it will rest on the top of a needle-point without falling off.
With three little pieces of wood make a bridge and attach it to the
wooden base over the paraffine that holds the wire-coil, and drive a
needle down in the middle of it, taking care that it does not go through
the back and touch the wires underneath. On this needle hang the strip
of steel spring, and, if it does not properly balance, trim it with the
shears or a hard file until it is adjusted properly. Rub this piece of
steel over the pole ends of a large horseshoe magnet, or place it within
the helix of a large coil and turn a powerful current through the coil.
This will magnetize the strip of steel, which will then become a
magnetic needle and hold the magnetism. Attach two binding-posts to
corners of the block, and make the loose ends of the coil-wires fast to
them. You now have a current-detector, or galvanoscope, as shown in Fig.
4. Turn the block so that the needle points to north and south and
parallel to the strands of wire.
When the conductors from the poles of a battery or dynamo are made fast
to the binding-posts, the needle will fly around to a position at right
angles to that which it first occupied, as shown by the dotted line A A
in Fig. 4. When the connection is broken the needle will turn around
again and point to north and south, since the magnetic field about the
wire ceases and disappears the instant the circuit is broken.
This is one of the strange and unknown phenomena of electricity, for
while the current exerts a force that deflects the needle, it does not
destroy its magnetism. On the breaking of the contact, no matter how
long it may have held the needle away from its true course, it again
points to north, and its magnetism is not affected.
Another simple current-detector is shown in Fig. 5. A piece of
broomstick is sawed in half and both pieces are made fast to a block
which is mounted on a base of wood three-quarters of an inch in
thickness. The vertical block should measure five inches long, three
inches high, and five-eighths of an inch thick. The half-circular pieces
of wood are mounted so that the flat surfaces are three inches apart and
the lower edges are one inch above the base-block. These may be held in
place with glue and screws driven through the back of the vertical block
and into the ends of the projecting half-circular pieces. The base-block
is six inches long and four inches wide, and the vertical block is
mounted on it one inch from an edge. The pieces of broomstick are two
inches long, and at the front ends a thin bar of brass or copper is
screwed fast to hold them apart and in proper position, as shown at A in
Fig. 5. To improve the appearance of this mounting, all the wood-work
may be stained and shellacked before the metal parts are attached.
With No. 26, 28, or 30 cotton-insulated wire make from fifteen to twenty
wraps about the middle of the half-circular pieces of wood and carry the
ends down through small holes in the base-block and thence through
grooves cut at the under side of the block to the front corners, where
they are to be made fast to binding-posts. A needle is to be set in the
base-block midway between the two pieces of half-circular wood and
through the strands of wire. Great care must be taken that the needle
does not touch any bare wires, and as a precautionary measure it would
be well to wrap the needle with a piece of insulating-tape where it
passes through the strands of wire. Now place on the top of it a
magnetized piece of steel, as described for the detector shown in Fig.
4. As it may not always be convenient to turn the instrument so that the
needle points north, a small bar of magnetized steel or a stout needle
that has been magnetized with a horseshoe magnet or a helix, may be laid
across the half-circular wood pieces, so that it is parallel with the
top layer of wires--in fact, it should rest on top of them.
By means of this needle, or bar, the magnetic piece of steel balanced on
the vertical needle between the upper and lower strands of insulated
wire may be held in one position no matter which way the block is
turned. When the current passes in through one binding-post and out
through the other (having thus travelled through the coil on the
half-circular blocks) the needle is deflected and points out at the
brass bar and back at the upright block.
When making any of these pieces of apparatus, where delicately balanced
magnetic needles are employed, all parts of the mounting blocks or other
sections must be put together with glue and brass nails or screws. It
will not do to use steel or iron nails, screw-eyes, or washers, nor
pieces of sheet-iron, tin, or steel, for they will exert their influence
on the vital parts of the apparatus and so destroy their usefulness.
This is not so important when making buzzers, bells, motor-induction
coils, or similar things, but in delicate instruments, where magnetic
needles or electro-magnets are used for recording, measuring, or
detecting, iron and steel parts should be carefully avoided, except
where their use is expressly indicated.
An Astatic Current-detector
Astatic current-detectors and galvanometers are those having two
magnetic needles arranged with the poles in opposed directions.
The ordinary magnetic or compass needle points to the North, and in
order to deflect it a strong magnetic field must be created near it. For
strong electric currents the ordinary single-needle current-detector
meets all requirements, but for weak currents it will be necessary to
arrange a pair of needles, one above the other, with their poles in
opposite directions, and placed within or near one or two coils of fine
wire. This apparatus will be affected by the weakest of currents, and
will indicate their presence unerringly.
The word “astatic” means having no magnetic directive tendency. If the
needles of this astatic pair are separated and pivoted each will point
to North and South, after the ordinary fashion. For all astatic
instruments we must employ two magnetic needles in parallel, either side
by side or one above another, as shown in Fig. 6, with the N and S poles
arranged as indicated. This combination is usually called Nobili’s pair.
If both needles are of equal length and magnetic strength, they will be
astatic, for the power of one counterbalances that of the other. As a
consequent neither points to North.
A compound needle of this form requires but a very feeble current to
turn it one way or the other, and this is the theory upon which all
astatic instruments are constructed.
A simple astatic current-detector may be made from a single coil of
fine insulated wire, a pair of magnetic needles, and a support from
which to suspend them, together with a base-block.
For the base-block obtain a piece of white-wood, pine, or cypress, four
inches square and three-quarters of an inch thick. Sand-paper it smooth,
and then give it two or three coats of shellac. From a strip of copper
or brass (do not use tin or iron) make a bridge, in the form of an
inverted [V], seven inches high, using metal one-sixteenth of an inch
thick and half an inch wide. This bridge is to be screwed to the outside
of the block, as shown at Fig. 7, so that it will be rigid and firm. A
small hole is drilled through the top of the bridge to admit a screw-eye
for the tension.
Make a coil of No. 30 insulated wire, using ten or fifteen feet, and
wind it about the base of a drinking-glass to shape it; then remove it
and tie the coil, in several places, with cotton or silk thread, so as
to hold the strands together. Shape it in the form of an ellipse and
make it fast to the middle of the base-board with small brass or copper
straps, and copper tacks or brass screws. Be very careful not to use
iron, steel, or tin about this instrument, as the presence of these
metals would deflect the needles and make them useless.
Separate the strands at the top of the coil so that one of the needles
may be slipped through to occupy a position in the middle of the coil.
Ordinary large compass needles may be employed for this apparatus, or
magnetized pieces of highly tempered steel piano-wire will answer just
as well.
[Illustration: FIG. 6]
[Illustration: FIG. 7]
[Illustration: FIG. 8]
A short piece of brass, copper, or wood will act as the carrier-bar for
the needles. These should be pushed through holes made in the bar, and
held in place with a drop of shellac or melted paraffine. A small hole
is drilled at the top of the bar, or a small eye can be attached,
through which to pass the end of a thread. The upper end of the thread
is tied in a screw-eye, the screw part of which passes up through the
hole in the bridge and into a wooden button or knob, which can be turned
to shorten or lengthen the thread, and so raise or lower the needles.
The lower needle must be pivoted at an equal distance between the upper
and lower parts of the coil.
Two binding-posts are arranged at the corners of the base, and the ends
of the coil wires are attached under the screw-heads. The in-and-out
wires are to be made fast under the copper washers on the screw-eyes.
Owing to the astatic qualities of the needles, the base-block does not
have to be turned so that the coil may face North and South, as in the
current-detector. When the slightest current of electricity passes
through the coil it instantly affects the needles, turning them to the
right or left according to the way in which the current is running
through the coil.
An Astatic Galvanometer
The sensitiveness of an astatic detector may be increased by the added
strength of the coil-field for a given current.
There are two ways of accomplishing this result. The number of turns of
wire may be increased in the coil, or two coils may be used, placed side
by side. The latter method is the more satisfactory, since then the coil
does not have to be opened at the top to admit the lower needle, the
latter being dropped down between the coils. This apparatus is shown in
the illustration of an astatic galvanometer, Fig. 8. The general
arrangement of needles, bridge, and coils, is the same as described for
the astatic current-detector.
Each coil is made separately of ten feet of No. 30 insulated copper
wire, wound about the base of a drinking-glass to shape it; then
pressed into elliptical shape, and fastened to a base-block with a brass
or copper strip, and held down with small brass screws.
The base-block should be four inches square, with the corners sawed off.
Smooth the block with sand-paper, and then give it several good coats of
shellac.
The bridge is made from brass one-sixteenth of an inch thick and half an
inch wide. The coils of wire are arranged about half an inch apart, and
at both ends a small separator-block is placed between the coils, and
then bound with silk or cotton thread. A circular indicator disk of
bristol-board should be cut out and marked and attached to the top of
the coils with a few drops of sealing-wax or paraffine; then the needles
are suspended so as to hang properly, one above the card, the other
between the coils.
Three binding-posts are placed at one end of the block, and to them the
end wires of the coils are led and attached. To the first binding-post
(at the left) the strand of wire leading to the first coil is attached.
It leads in and is coiled as the hands move on a clock, from left to
right. The leading-out wire from the coil is made fast to the middle
post. The leading-in wire to the second coil is also made fast to the
middle post. The coil wires should have the turns in the same direction
as the first coil; then the last wire is attached to the right-hand
post.
When making connections for a strong current, use an end and middle
post. This arrangement will operate but one coil. For very weak currents
make the leading in and out wires fast to the end-posts. This latter
plan is more clearly shown in the diagram, Fig. 9. A and B represent
the coils, C, D, and E the binding-posts. The current, entering at C,
passes through the coil A (as the hands move about the dial of a clock)
and out at D, where connection is made with the wire leading in to coil
B. The current passes through this coil in the same direction as the
clock hands move, and out to post E. Be careful to arrange the wiring
and connections after this exact manner, otherwise the instrument will
not be of any use.
The adjustment at the top of the bridge may be made with an inverted
screw-eye and a small cork into which the eye can be screwed, thus
raising or lowering the needles to the proper position. Be sure to have
the needles in parallel when at rest.
As the needles and coils are very sensitive it would be well to cover
the instrument with an inverted glass jar. A bluestone or gravity
battery jar will answer very well, and after the wires are connected to
the binding-posts the glass may be placed over the entire apparatus.
A Tangent Galvanometer
For testing the various degrees of intensity of a current a tangent
galvanometer is usually employed. In this apparatus the increased
strength is indicated by the index-pointer as it plays over a scale or
graduated circle.
A simple tangent galvanometer may be made from a flat hoop of wood-fibre
or brass, mounted on a base by means of two uprights, together with the
necessary compass needle, an index-card, insulated wire, and
binding-posts for the electrical connections. This piece of apparatus
is shown in Fig. 10. It is built on a base-block six by seven inches
and three-quarters of an inch thick. The block should be of selected
wood, and after it is made smooth it should be given several coats of
shellac.
Two upright pieces of wood, five inches long, half an inch thick, and
one inch in width, are screwed fast to the rear edges of the base-block
to support the hoop on which the insulated wire is wound. Be careful not
to use any iron or steel in the construction of this or any other
recording instrument, except where it is expressly stated. Screws,
nails, staples, or any bits of anchoring wire should be of copper or
brass. String, thread, or silk may be used, especially where coils of
wire are to be bound or fastened to hoops or base-blocks. The balance of
the indicating needle is so delicate, and the sensitiveness of the coils
is so easily affected, that nothing about or near the instruments should
be of iron or steel.
The hoop may be made of very thin hickory wood, steamed and bent so as
to form a ring six inches outside diameter and one inch wide. It is even
possible to construct a satisfactory hoop from a ribbon of brown paper,
rolled and lapped, the several thicknesses being glued as the turns are
made.
If a metal hoop is to be used, solder the ends of a thin, hard ribbon of
brass, copper, or zinc. This strip should be provided with holes, set in
pairs about four inches apart, all around the hoop, and where the hoop
is to be attached to the uprights two holes should be made close to the
margins through which brass screws may pass.
Across the middle of the hoop a strip of wood six inches long, an inch
wide, and a quarter of an inch thick is made fast. On this the graduated
card is placed, and at the centre the balanced magnetic needle is
arranged on a pivot.
After the cross-stick is in place, wind five turns of No. 24 insulated
copper wire about the hoop, keeping it as nearly in the centre as
possible. One end of the wire (the beginning) is to be attached to the
first binding-post on the front of the base, and the other end to the
second post. The wire should be wound round the hoop in the same
direction as the clock hands travel about a dial.
Another coil, composed of ten turns of wire, is made over the first one,
the beginning end being attached to the middle binding-post and the last
end to the third post. This arrangement is shown in Fig. 11, D and E
representing the coils, while A, B, and C are the binding-posts. The
current enters at A, passes through coil D, and out at post B. The next
passage is in at B, through E, and out at C. A current passing in at A
will travel to B, thence through E, and out at C. If the leading-in wire
is made fast to A, and the out wire to C, the current will travel
through the entire coil.
Under this plan one or both coils may be used (the short or long one as
desired) by making connections with the first and second binding-posts,
the second and third, or the first and third, as the strength of the
current will warrant.
Strong currents will deflect the needle when travelling through a short
coil, but the weaker the current the more coils it will have to pass
through to properly deflect the needle and indicating pointer.
[Illustration: FIG. 10
FIG. 11
FIG. 12
FIG. 13
FIG. 14
FIG. 15
TANGENT GALVANOMETERS]
When the coils are all on, the hoop should be attached to the uprights
with small brass screws driven through holes in the hoop and into the
wood. The wire is bound to the hoop by means of threads or silk passed
through each pair of holes in the hoop, and then tied fast. Fine
insulated wire may be used in place of the thread, but care should be
taken that the insulation is in perfect shape on both the binding and
coil wires; otherwise a short-circuit will quickly destroy the value of
the coils.
The hoop should not touch the base-block, but should clear it by a
quarter or half an inch. Make the coil ends fast (as described for the
astatic galvanometer and illustrated at Fig. 9) by means of
binding-posts. The wires need not be carried over the top of the block,
but may run through holes under the hoop and along grooves cut in the
under side of the block and leading to the foot of the binding-posts.
The graduated card should be made from a piece of stout bristol-board or
heavy card-board having a smooth, hard surface. It is laid out with a
pencil or pen compass, as shown at Fig. 12, and should be three inches
in diameter. The card is placed on the wood strip or ledge, so that the
zero marks will be at the front and rear, or at right angles to the hoop
and coils of wire. The compass needle, when at rest, should lie parallel
with the coils, so that the current will deflect the needle and send the
indicator around to one side or the other of zero, according to the
direction in which the current is passing through the coils.
This is more clearly shown at Fig. 13. The circle represents the outside
diameter of the card; the dark cross-piece, the magnetic needle; and the
pointed indicator, a stiff paper, or very thin brass or copper strip,
cut and attached to the needle with shellac or paraffine.
When at rest the magnetic needle should be parallel to the coils. To
insure this the instrument must be moved so that the lines of wire
forming the coil will run North and South. Otherwise the N-seeking end
of the magnetic shaft will point to North, irrespective of the position
occupied by the wire coil.
The magnetic needle may be made as described for the compass (see
chapter iv., Magnets and Induction Coils). It should be arranged to rest
on a brass pivot pressed down into the cross-piece of wood.
The indicator-needle may be cut from stiff paper, thin sheet-fibre, or
very thin cold-rolled brass or copper, the latter being commonly known
as hard or spring-brass. Only one pointer is really necessary--that
pointing to the front. But the weight of the material would have a
tendency to upset the magnetic needle, and therefore it is better to
carry an equally long tail or end, on the opposite side, to properly
balance the needle.
A very weak current, passing in through the first post and out at the
third, will cause the indicator to be deflected considerably, or so that
it will point from 40° to 60° on either side of the zero point,
according to the direction in which the current is running through the
coils.
When not in use the magnetic needle should be removed from the pivot,
and placed in a box or other safe place, where it will not become
damaged.
A differently arranged tangent galvanometer is shown at Fig. 14. As the
line of binding-posts would indicate, there are several coils of wire
about the circle or hoop.
This galvanometer can be used for either strong or weak currents, since
it is wound with both coarse and fine insulated wire. An upright plate
of wood, seven inches wide and eight inches high, supports the hoop and
compass. The top corners are sawed off, and four inches above the bottom
a straight cut is made across the plate, five inches wide and arched in
a half-circle five inches in diameter. A shelf of wood a quarter of an
inch thick, three inches wide, and five inches long is made, and
attached as a ledge in this arched opening, so that a compass three
inches in diameter may rest upon it.
The shelf should be arranged so that it will hold the compass in the
middle of the circle instead of at one side. The turns of wire will then
be in line with the magnetic needle when the latter is at rest. A
base-block seven inches long, three inches wide, and seven-eighths of an
inch thick is cut and attached to the upright plate by driving screws
through the bottom of the plate and into the rear edge of the base. The
corners are to be cut from the front of the base, and ten small holes
are to be bored half an inch out from the upright and about a quarter of
an inch apart. These are for the end wires that will extend down from
the coils, and from thence to the binding-post holes. Grooves may be cut
in the under side of the base-block for the wires to rest, in, as shown
at Fig. 15, which is a view of the inverted base.
A hoop is made of brass, six inches in diameter and an inch wide. It is
held to the upright plate with copper wire passed through a small hole,
bored at the inner edge of the band, and back through two small holes
bored in the plate, the ends being twisted together at the back of the
plate. A wire at the top, bottom, and both sides will be sufficient to
hold it securely in place.
The first coil of wire is made of No. 18 insulated, and the beginning
end is made fast to the binding-post at the left. The wire is carried up
through the first hole under the hoop, and after three turns have been
made the end is carried down through the second hole and made fast to
the foot of the second binding-post.
The second coil is of No. 24 insulated copper wire. The beginning end is
made fast to the second binding-post, carried up through the third hole,
given five turns about the hoop, drawn down through the fourth hole, and
attached to the third binding-post.
The third coil is of the same size wire but has ten turns. The fourth
coil has twenty turns, and the fifth, of No. 30 insulated wire, has
thirty turns, the last end being attached to the post at the right. In
all the coils there should be a total of sixty-eight turns, or about one
hundred and five feet of wire.
For strong currents the in-and-out wires may be attached to posts Nos. 1
and 2 at the left, and for weaker currents to Nos. 2 and 3. For still
weaker currents, use Nos. 3 and 4, and so on. To detect the very weakest
currents, attach the in-and-out wires to the first and last post, and
let the current travel through all the coils or the entire length of the
wire wound about the hoop.
The magnetic needle is made in the same manner as described for Fig. 10,
and the pointer is attached in a similar fashion. But instead of being
mounted on a pivot over a card, and so exposed to the open air and
possible draughts, the delicate mechanism is arranged within a brass
hoop, which is made fast to the ledge. The graduated card is at the
bottom of the hoop, or box formed by it, and to protect the needle and
prevent it from being displaced it should be covered with glass. This
can be done by making a split ring of spring-brass wire and pressing it
down inside the hoop. Over this a round piece of glass is placed, and
another hoop is pressed in above it to hold the glass in position. If
the rings are carefully made and of stout wire they will stay in place;
otherwise a drop of melted sealing-wax or paraffine will be necessary to
keep them where they are wanted.
The glass should be arranged close enough to the needle to prevent it
from jumping or being shaken off the supporting pin, but not so close as
to prevent its moving easily.
Part II