In most towns and cities where electricity for light and power is
carried over long distances, it will be noticed that small iron boxes
are fastened to the poles at frequent intervals, usually wherever there
is a group of houses or buildings supplied with the current. Many boys
know that the boxes contain "transformers," but do not quite understand
exactly what their purpose is, and how they are constructed.
When it is desired to convey electrical energy to a distance, for the
purpose of producing either light or power, one of the chief problems to
be faced is, how to reduce to a minimum any possible waste or loss of
energy during its transmission. Furthermore, since wires and cables of
large size are very costly, it is desirable that they be as small as
possible and yet still be able to carry the current without undue
losses.
It has already been explained that wires offer resistance to an
electrical current, and that some of the energy is lost in passing
through a wire because of this resistance. Small wires possess more
resistance than large ones, and if small wires are to be used, in order
to save on the cost of the transmission line, the loss of energy will be
greater, necessitating some method of partially reducing or overcoming
this fault.
In order to explain clearly how the problem is solved, the electric
current may for the moment be compared to a stream of water flowing
through a pipe.
[Illustration: Fig. 175.—Comparison between Electric Current and Flow of
Water.]
The illustration shows two pipes, a small one and a large one, each
supposed to be connected to the same tank, so that the pressure in each
is equal, and it is clearly apparent that more water will flow out of
the large one than out of the small one. If ten gallons of water flow
out of the large pipe in one minute, it may be possible that the
comparative sizes of the pipes are such that only one gallon of water
will flow out of the small one in the same length of time.
But in case it should be necessary or desirable to get ten gallons of
water a minute out of a small pipe such as _B_, what could be done to
accomplish it?
The pressure could be increased. The water would then be able better to
overcome the resistance of the small pipe.
This is exactly what is done in the distribution of electric currents
for power and lighting. The pressure or potential is increased to a
value where it can overcome the resistance of the small wires.
But unfortunately it rarely happens that electrical power can be
utilized at high pressure for ordinary purposes. For instance, 110 volts
is usually the maximum pressure required by incandescent lamps, whereas
the pressure on the line wires issuing from the power-house is generally
2,200 volts or more.
Such a high voltage is hard to insulate, and would kill most people
coming into contact with the lines, and is otherwise dangerous.
Before the current enters a house, therefore, some apparatus is
necessary, which is capable of reducing this high pressure to a value
where it may be safely employed.
This is the duty performed by the "transformer" enclosed in the black
iron box fastened on the top of the electric light poles about the
streets.
If a transformer were to be defined it might be said to be a device for
changing the voltage and current of an _Alternating_ circuit in pressure
and amount.
The word, _alternating_, has been placed in italics because it is only
upon alternating currents that a transformer may be successfully
employed. Therein, also, lies the reason why alternating current is
supplied in some cases instead of direct current. It makes possible the
use of transformers for lowering the voltage at the point of service.
Many boys possessing electrical toys and apparatus operating upon direct
current only, have bemoaned the fact that the lighting system in their
town furnished alternating current. Very often in the case of small
cities or towns one power-house furnishes the current for several
communities and the energy has to be carried a considerable distance.
Alternating current is then usually employed.
[Illustration: Fig. 176.—Alternating Current System for Light and
Power.]
The illustration shows the general method of arranging such a system. A
large dynamo located at the power-house generates alternating current.
The alternating current passes into a "step-up" transformer which raises
the potential to 2,200 volts (approximately). It is then possible to use
much smaller line wires, and to transmit the energy with smaller loss
than if the current were sent out at the ordinary dynamo voltage. The
current passes over the wires at this high voltage, but wherever
connection is established with a house or other building, the "service"
wires which supply the house are not connected directly to the line
wires, but to a a "step-down" transformer which lowers the potential of
the current flowing into the house to about 110 volts.
In larger cities where the demand for current in a given area is much
greater than that in a small town, a somewhat different method of
distributing the energy is employed.
[Illustration: Fig. 177.—Motor Generator Set for changing Alternating
Current to Direct Current.]
The alternating current generated by the huge dynamos at the "central"
station is passed into a set of transformers which in some cases raise
the potential as high as five or six thousand volts. The current is then
sent out over cables or "feeders" to various "sub" stations, or
"converter" stations, located in various parts of the city. Here the
current is first sent through a set of step-down transformers which
reduce the potential to the approximate value originally generated by
the dynamos. It then passes into the "rotary converters" which change
the alternating current into direct current after which it is sent by
underground cables direct to the consumers in the neighborhood.
A transformer in its simplest form consists of two independent coils of
wire wound upon an iron ring. When an alternating current is passed
through one of the coils, known as the primary, it produces a magnetic
field which induces a current of electricity in the other, or secondary,
coil.
The potential or voltage of the current in the secondary is in nearly
the same ratio to the potential of the current passed into the primary
as the number of turns in the secondary is to the number of turns in the
primary.
[Illustration: Fig. 178.—Step-Up Transformer.]
Knowing this, it is very easy to arrange a transformer to "step" the
potential up or down as desired. The transformer in Figure 178
represents a "step-up" transformer having ten turns of wire on the
primary and twenty turns on the secondary. If an alternating current of
10 volts and 2 amperes is passed into the primary, the secondary winding
will double the potential, since it has twice as many turns as the
primary and the current delivered by the secondary will be approximately
20 volts and 1 ampere.
The action may be very easily reversed and a "step-down" transformer
arranged by placing twenty turns of wire on the primary and ten turns on
the secondary. If a current of 20 volts and 1 ampere is passed into the
primary, the secondary will deliver a current of only 10 volts and 2
amperes, since it contains only half as many turns.
A circular ring of iron wire wound with two coils would in many respects
be somewhat difficult to construct, and so the iron core is usually
built in the form of a hollow rectangle and formed of sheets of iron.
[Illustration: Fig. 179.—Step-Down Transformer.]
It is often desirable to have at hand an alternating current of low
voltage for experimental purposes. Such a current may be used for
operating induction coils, motors, lamps, toy railways, etc., and is
quite as satisfactory as direct current for many purposes, with the
possible exception of electro-plating and storage-battery charging, for
which it cannot be used.
When the supply is drawn from the 110-volt lighting circuit and passed
through a small "step-down" transformer, the alternating current is not
only cheaper but more convenient. A transformer of about 100 watts
capacity, capable of delivering a current of 10 volts and 10 amperes
from the secondary will not draw more than approximately one ampere from
the 110-volt circuit. This current is only equal to that consumed by two
ordinary 16-candle-power lamps or one of 32 candle-power, making it
possible to operate the transformer to its full capacity for about one
cent an hour. A further advantage is the fact that a "step-down"
transformer enables the small boy to use the lighting current for
operating electrical toys without danger of receiving a shock.
[Illustration: Fig. 180.—Core Dimensions.]
The transformer described in the following pages can be easily built by
any boy at all familiar with tools, and should make a valuable addition
to his electrical equipment, provided that the directions are carefully
followed and pains are taken to make the insulation perfect.
The capacity of the transformer is approximately 100 watts. The
dimensions and details of construction described and illustrated are
those of a transformer intended for use upon a lighting current of 110
volts and 60-cycles frequency. The frequency of most alternating current
systems is 25, 60, or 120 cycles. The most common frequency is 60.
Dimensions and particulars of transformers for 25 and 120 cycles will be
found in the form of a table farther on.
The frequency of your light circuit may be ascertained by inquiring of
the company supplying the power.
[Illustration: Fig. 181.—The Core, Assembled and Taped.]
The first part to be considered in the construction of a transformer is
the core. The core is made up of thin sheet-iron strips of the
dimensions shown in Figure 180. The iron may be secured from almost any
hardware store or plumbing shop by ordering "stove-pipe iron." Have the
iron cut into strips 1 1/4 inches wide and 24 inches long. Then, using a
pair of tinner’s shears, cut the long strips into pieces 3 inches and 4
3/4 inches long until you have enough to make a pile of each 2 1/2
inches high when they are stacked up neatly and compressed. The long
strips are used to form the "legs" of the core, and the short ones the
"yokes."
[Illustration: Fig. 182.—Transformer Leg.]
The strips are assembled according to the diagram shown in Figure 180.
The alternate ends overlap and form a hollow rectangle 4 1/4 x 6 inches.
The core should be pressed tightly together and the legs bound with
three or four layers of insulating tape preparatory to winding on the
primary. After the legs are bound, the yoke pieces may be pulled out,
leaving the legs intact.
Four fiber heads, 2 1/2 inches square and 1/8 of an inch thick, are made
as shown in Figure 183. A square hole 1 1/4 x 1 1/4 inches is cut in the
center. Two of these are placed on each of the assembled legs as shown
in Figure 184.
[Illustration: Fig. 183.—Fiber Head.]
The primary winding consists of one thousand turns of No. 20 B. & S.
gauge single-cotton-covered magnet wire. Five hundred turns are wound on
each leg of the transformer. The wire should be wound on very smoothly
and evenly with a layer of shellacked paper between each layer of wire.
The two legs should be connected in series. The terminals are protected
and insulated by covering with some insulating tape rolled up in the
form of a tube.
The secondary winding consists of one hundred turns of No. 10 B. & S.
gauge double-cotton-covered wire. Fifty turns are wound on each leg,
over the primary, several layers of paper being placed between the two.
[Illustration: Fig. 184.—Leg with Heads in Position for Winding.]
A "tap" is brought out at every ten turns. The taps are made by
soldering a narrow strip of sheet-copper to the wire at proper
intervals. Care must be taken to insulate each joint and tap with a
small strip of insulating tape so that there is no danger of a short
circuit being formed between adjacent turns.
After the winding is completed the transformer is ready for assembling.
The yoke pieces of the core should be slipped into position and the
whole carefully lined up. The transformer itself is now ready for
mounting.
[Illustration: Fig. 185.—How to make a Tap in the Primary by soldering a
Copper Strip to the Wire.]
The base-board measures 11 x 7 3/4 x 7/8 inches. It is shown in Figure
192.
The transformer rests upon two wooden strips, _A_ and _B_, 4 1/4 inches
long, 1 1/4 inches wide, and 3/4 of an inch high. The strips are nailed
to the base so that they will come under the ends of the core outside of
the fiber heads.
The transformer is held to the base by two tie-rods passing through a
strip, _C_, 6 inches long, one-half of an inch thick and three-quarters
of an inch wide. The strip rests on the ends of the core. The tie-rods
are fastened on the under side of the base by means of a nut and washer
on the ends. When the nuts are screwed up tightly, the cross-piece will
pull the transformer firmly down to the base.
[Illustration: Fig. 186.—The Transformer completely Wound and ready for
Assembling.]
The regulating switches, two in number, are mounted on the lower part of
the base. The contact points and the arm are cut out of sheet-brass,
one-eighth of an inch thick. It is unnecessary to go into the details of
their construction, because the dimensions are plainly shown in Figure
188.
The contacts are drilled out and countersunk so that they may be
fastened to the base with small flat-headed wood screws.
Each switch-arm is fitted with a small rubber knob to serve as a handle.
The arm works on a small piece of brass of exactly the same thickness as
the switch-points. Care must be taken that the points and this washer
are all exactly in line, so that the arm will make good contact with
each point. There are five points to each switch, as shown in Figure
190.
[Illustration: Fig. 187.—Wooden Strips for mounting the Transformer on
the Base.]
The switch, _D_, is arranged so that each step cuts in or out twenty
turns of the secondary, the first point being connected with the end of
the winding. The second point connects with the first tap, the third
contact with the second tap, the fourth contact with the third tap, and
the fifth contact with the fourth tap.
[Illustration: Fig. 188.—Details of the Switch Parts.]
The switch, _E_, is arranged so that each step cuts in or out five
turns. The contacts on this switch are numbered in the reverse
direction. The fifth contact of switch _D_, and the fifth contact of
switch _E_, are connected together. The fourth contact is connected to
the fifth tap, the third contact to the sixth tap, the second contact to
the seventh, and the first contact to the end of the winding.
This arrangement makes it possible to secure any voltage from one-half
to ten in one-half-volt steps from the secondary of the machine. Each
step on the switch, _D_, will give two volts, while those on _E_ will
each give one-half of a volt.
[Illustration: Fig. 189.—The Complete Switch.]
Two binding-posts (marked _P_ and _P_ in the drawing) mounted in the
upper corners of the base are connected to the terminals of the primary
winding. The two posts in the lower corners (marked _S_ and _S_ in the
drawing) are connected to the switch levers, and are the posts from
which the secondary or low voltage is obtained.
[Illustration: Fig. 190.—Diagram of Connections.]
The transformer may be connected to the 110-V. alternating current
circuit by means of an attachment plug and cord. One end of the cord is
placed in each of the primary binding-posts. The other end of the cord
is connected to the attachment plug so that the latter may be screwed
into any convenient electric-light socket.
[Illustration: Fig. 191.—Top View of the Transformer.]
The transformer must not be connected directly to the line. An
instrument such as this is not designed for continuous service and is
intended to be disconnected as soon as you are through using it.
[Illustration: Fig. 192.—Side View of the Transformer.]
It will be found a great convenience in operating many of the electrical
devices described, wherever direct current is not essential.
CHAPTER VIV WIRELESS TELEGRAPHY
Probably no branch of electrical science ever appealed more to the
imagination of the experimenter than that coming under the heading of
wireless telegraphy. Wherever you go, you are likely to see the
ear-marks of _amateur_ wireless telegraph stations in the aerials and
masts set up in trees and on house-tops. It is estimated that there are
nearly a quarter of a million such stations in the United States.
There is really no great mystery about this wonderful art which made
possible the instantaneous transmission of messages over immense
distances without any apparent physical connection save that of the
earth, air, or water.
Did you ever throw a stone in a pool of water? As soon as the stone
struck, little waves spread out from the spot in gradually enlarging
circles until they reached the shore or died away.
By throwing several stones in succession, with varying intervals of time
between them, it would be possible so to arrange a set of signals, that
they would convey a meaning to a second person standing on the opposite
shore of the pool.
Wireless telegraphy is based upon the principle of _creating and
detecting_ waves in a great _pool_ of ether.
Modern scientists suppose that all space is filled with an "imaginary"
substance called _ether_. The ether is invisible, odorless, and
practically weightless. This ether, however, bears no relation to the
anaesthetic of that name which is used in surgical operations.
It surrounds and penetrates all substances and all space.
[Illustration: Fig. 193.—Little Waves spread out from the Spot.]
It exists in a vacuum and in solid rocks. Since the ether does not make
itself apparent to any of our physical senses, some of these statements
may seem contradictory. Its definite existence cannot be proved except
by reasoning, but by accepting and imagining its reality, it is possible
to understand and explain many scientific puzzles.
A good instance is offered by the sun. Light and heat can be shown to
consist of extremely rapid vibrations. That fact can be proved. The sun
is over 90,000,000 miles away from our earth and yet light and heat come
streaming down to us through a space that is devoid even of air.
Something must exist as a medium to transmit these vibrations; it is the
ether.
Let us consider again the pool of water. The waves or ripples caused by
throwing in the stone are vibrations of the water. The distance between
two adjacent ripples is called the _wave length_.
The distances between two vibrations of light can also be _measured_.
They are so small, however, that they may be spoken of only in
_thousandths_ of an inch. The waves created in the ether by wireless
telegraph apparatus are the same as those of light except that their
length usually varies from 75 to 9,000 _feet_ instead of a fraction of a
thousandth of an inch.
[Illustration: Fig. 194.—A Simple Transmitter.]
*A Simple Transmitter* is illustrated in Figure 194. A telegraph key is
connected in series with a set of cells and the _primary_ of an
induction coil, which, it will be remembered, is simply a coil
consisting of a few turns of wire. This induces a high voltage in a
second coil consisting of a larger number of turns and called the
_secondary_.
The terminals of the secondary are led to a spark-gap—an arrangement
composed of two polished brass balls, separated by a small air-gap. One
of the balls, in turn, is connected to a metal plate buried in the
earth, and the other to a network of wires suspended high in the air and
insulated from all surrounding objects.
When the key at the transmitter is pressed, the battery current flows
through the primary of the induction coil and generates in the secondary
a current of very high voltage, 20,000 volts or more, which is able to
jump an air-gap in the shape of a spark at the secondary terminals. The
latter are connected to the earth and aerial, as explained above. The
high potential currents are therefore enabled to charge the aerial. The
charge in the aerial exerts a great tendency to pass into the ground,
but is prevented from doing so by the small air-gap between the
spark-balls until the charge becomes so great that the air-gap is
punctured and the charge passes across and flows down into the ground.
The passage of the charge is made evident by the spark between the two
spark-balls.
The electrical charges flowing up and down the aerial disturb the ether,
strike it a blow, as it were. The effect of the blow is to cause the
ether to vibrate and to send out waves in all directions. It may be
likened to the pond of water which is suddenly struck a blow by throwing
a stone into it, so that ripples are immediately sent out in widening
circles.
*These Waves in the Ether* are called electro-magnetic or _Hertzian_
waves, after their discoverer, Hertz. The distance over which they pass
is dependent upon the power of the transmitting station. The waves can
be made to correspond to the dots and dashes of the telegraphic code by
so pressing the key. If some means of detecting the waves is employed we
may readily see how it is possible to send wireless messages.
*The Action of the Receiving Station* is just the opposite of that of
the transmitter. When the waves pass out through the ether, some of them
strike the aerial of the receiving station and generate a charge of
electricity in it which tends to pass down into the earth. If the
transmitting and receiving stations are very close together and the
former is very powerful, it is possible to make a very small gap in the
receiving aerial across which the charge will jump in the shape of
sparks. Thus the action of the receptor simply takes place in a reversed
order from that of the transmitter.
If the stations are any considerable distance apart, it is impossible
for the currents induced in the receiving aerial to produce sparks, and
so some more sensitive means of detecting the waves from the transmitter
is necessary, preferably one which makes itself evident to the sense of
hearing.
The telephone receiver is an extremely sensitive instrument, and it only
requires a very weak current to operate it and produce a sound. The
currents or _oscillations_ generated in the aerial, however, are
alternating currents (see pages 97-99) of _high frequency_, that is,
they flow in one direction and then reverse and flow in the other
several thousand times a second. Such a current cannot be made to pass
through a telephone receiver, and in order to do so the nature of the
current must be changed by converting it into direct current flowing in
one direction only.
Certain minerals and crystals possess the remarkable ability to do this,
_silicon_, galena, and iron pyrites are among the best.
[Illustration: Fig. 195.—A Simple Receptor.]
The diagram in Figure 195 shows the arrangement of a simple receiving
outfit. The _detector_ consists of a sensitive mineral placed between
two contacts and connected so that the aerial currents must pass through
it on their way to the ground. A telephone receiver is connected to the
detector so that the _rectified currents_ (currents which have been
changed into direct current) pass into it and produce a sound. By
varying the periods during which the key is pressed at the transmitting
station, according to a prearranged code, the sounds in the receiver may
be made to assume an intelligible meaning.
HOW TO BUILD WIRELESS INSTRUMENTS
*The Aerial*
Every wireless station is provided with a system of wires elevated high
in the air, above all surrounding objects, the purpose of which is to
radiate or intercept the electromagnetic waves, accordingly as the
station is transmitting or receiving. This system of wires is, as
already has been stated, called the _aerial_ or _antenna_.
The arrangement of the aerial will greatly determine the efficiency and
range of the apparatus.
The aerial should be as long as it is reasonably possible to make it,
that is from 50 to 150 feet.
It will be necessary for most amateurs to put up their aerial in some
one certain place, regardless of what else may be in the vicinity, but
whenever possible the site selected should preferably be such that the
aerial will not be in the immediate neighborhood of any tall objects,
such as trees, smoke-stacks, telephone wires, etc., because such objects
will interfere with the aerial and noticeably decrease the range of the
station, both when transmitting and receiving.
Bare copper wire makes the best aerials. Aluminum wire is very often
used and on account of its light weight causes very little strain on the
poles or cross arms. Iron wire should never be used for an aerial, even
if galvanized or tinned, because it tends to choke the currents which
must flow up and down the aerial when the station is in operation.
[Illustration: Fig. 196.—Molded Aerial Insulator]
The aerial must be very carefully insulated from its supports and all
surrounding objects. The insulation must be strong enough to hold the
weight of the aerial and able to withstand any strain caused by storms.
Special aerial insulators made of molded insulating material and having
an iron ring imbedded in each end are the best.
[Illustration: Fig. 197.—A Porcelain Cleat will make a Good Insulator
for Small Aerials.]
Ordinary porcelain cleats may be used on small aerials where the strain
is light.
One insulator should be placed at each end of each wire close to the
spreader or spar.
Most aerials are made up of four wires. The wires should be placed as
far apart as possible.
There are several different forms of aerials, the principal ones of
which are shown in Figure 199. They are known as the grid, “V," inverted
“L,” and “T” types.
Most amateurs support their aerials from a pole placed on the top of the
house, in a tree, or erected in the yard. Many use two supports, since
such an aerial has many advantages. The facilities to be had for
supporting the aerial will largely determine which form to use.
[Illustration: Fig. 198.—Method of Arranging the Wires and Insulating
them from the Cross Arm or Spreader.]
The grid aerial has no particular advantages or disadvantages.
The “V” aerial receives waves much better when they come from a
direction opposite to that in which the free end points. The "free" end
of the aerial is the one not leading into the station.
The inverted “L” aerial possesses the same characteristics as the “V”
type.
The “T” aerial is the best “all around" and is to be recommended
whenever it is possible to put up an aerial of this sort.
Much of the detail of actually putting up an aerial or antenna must be
omitted, because each experimenter will usually meet different
conditions.
It should be remembered, however, that the success of the whole
undertaking will rest largely upon the construction of a proper aerial.
The most excellent instruments will not give very good results if
connected to a poor aerial, while, on the other hand, inferior
instruments will often give fair results when connected to a good
aerial.
[Illustration: Fig. 199.—Various Types of Aerials.]
The aerial should be at least thirty feet high.
The wire should not be smaller than No. 14 B. & S.
The masts which support the aerial should be of wood and provided with
pulleys so that the wires may be lowered any time it may be necessary.
The mast should be thoroughly braced with stays or guys so as to
counteract the strain of the aerial.
The aerial should not be hoisted up perfectly tight, but should be
allowed to hang somewhat loose, as it will then put less strain on the
ropes and poles that support it.
When an aerial is to be fastened in a tree, it is best to attach it to a
pole placed in the top of the tree, so that it will come well above any
possible interference from the branches.
The wires leading from the aerial to the instruments should be very
carefully insulated throughout their length. This part of the aerial is
called the "rat-tail" or lead-in.
The illustrations in Figure 199 show the proper place to attach the
“lead-in" form of aerial. The wires should gradually converge.
[Illustration: Fig. 200.—A Ground Clamp for Pipes.]
It is very important that a good ground connection be secured for
wireless instruments. A good ground is absolutely necessary for the
proper working of the apparatus. Amateur experimenters usually use the
water or gas-pipes for a ground, and fasten the wires by means of a
ground clamp such as shown in Figure 200. In the country, where such
pipes are not available, it is necessary to bury a sheet of copper,
three or four feet square, in a moist spot in the earth and connect a
wire to it.
*The Receiving Apparatus*
The receiving instruments form the most interesting part of a wireless
station and usually receive first attention from the amateurs. They are
the ears of the wireless station and are wondrously sensitive, yet are
very simple and easy of construction.
The instruments necessary for receiving are:
A Detector,
A Tuning Coil or a Loose Coupler,
A Fixed Condenser,
A Telephone Receiver.
Other devices, such as a test buzzer, variable condenser, etc., may be
added and will improve the outfit.
After the aerial has been properly erected, the first instrument
necessary to construct will be either a tuning coil or a loose coupler.
It is a good plan to make a tuning coil first, and a loose coupler after
you have had a little experience with your apparatus.
*A Tuning Coil* is a very simple arrangement making it possible to
receive messages from greater distances, and also somewhat to eliminate
any messages not desirable and to listen without confusion to the one
wanted.
A tuning coil consists of a single layer of wire wound upon a cylinder
and arranged so that connection may be had with any part of it by means
of sliding contacts.
The cylinder upon which the wire is wound is a cardboard tube six and
three-quarters inches long and two and seven-eighths inches in diameter
outside. It should be given two or three coats of shellac both inside
and out so that it is thoroughly impregnated, and then laid away until
dry. This treatment will prevent the wire from becoming loose after the
tube is wound, due to shrinkage of the cardboard.
[Illustration: Fig. 201.—Details of the Tuning Coil.]
After having become dry, the tube is wound with a single layer of No. 25
B. & S. gauge green silk or cotton-covered magnet wire. The wire must be
wound on very smoothly and tightly, stopping and starting one-quarter of
an inch back from each end. The ends of the wire are fastened by weaving
back and forth through two small holes punched in the cardboard tube
with a pin.
The winding should be given a single coat of clear varnish or white
shellac and allowed to dry.
The coil heads or end pieces are cut from one-half-inch wood according
to the plan and dimensions shown in the accompanying illustration.
The top corners are beveled and notched to receive the slider-rods. A
circular piece of wood two and five-eighths inches in diameter and
three-eighths of an inch thick is nailed to the inside of each of the
coil heads to support the ends of the cylinder.
The wooden parts should be stained mahogany or some other dark color and
finished with a coat of shellac or varnish.
The slider-rods are square brass 3-16 x 3-16 inches and seven and
three-quarters inches long. A small hole is bored near the ends of each,
one-quarter of an inch from the edge, to receive a round-headed brass
wood screw which holds the rod to the tuner end.
The sliders may be made according to the plan shown in Figure 201.
The slider is made from a small piece of brass tubing, three-sixteenths
of an inch square. An 8-32 flat-headed brass screw is soldered to one
face, in the center. A small strip of phosphor bronze sheet or spring
copper soldered to the bottom of the slider forms a contact for making
connection to the wire on the cylinder. A small "electrose" knob screwed
to the slider makes a neat and efficient handle.
Two sliders are required, one for each rod.
The tuning coil is assembled as shown in Figure 203. The cardboard tube
is held in place by several small brass nails driven through it into the
circular pieces on the coil heads.
A slider is placed on each of the slider-rods and the rods fastened in
the slots in the coil ends by a small round-headed brass screw, passing
through the holes bored near the ends for that purpose.
[Illustration: Fig. 202.—Side and End Views of the Tuning Coil.]
Two binding-posts are mounted on one of the coil ends. One should be
connected to each of the slider-rods. A third binding-post is placed
below in the center of the head and connected to one end of the wire
wound around the cylinder.
A small, narrow path along the coil, directly underneath each slider and
to which the copper strip can make contact, must be formed by scraping
the insulation off the wire with a sharp knife. The sliders should make
contact with each one of the wires as they pass over, and should slide
smoothly without damaging or disarranging any of the wires.
[Illustration: Fig. 203.—Complete Double-Slider Tuning Coil.]
When scraping the insulation, be very careful not to loosen the wires or
remove the insulation from between them, so that they are liable to
short-circuit between adjacent turns.
*A Loose Coupler* is a much more efficient tuning device than a
double-slider tuner, and sooner or later most amateur wireless operators
install one in their station.
The loose coupler shown in the figure given on the next page is a very
simple one and is both easy and inexpensive to build. Its simplicity is
a disadvantage in one respect, however. Owing to its construction, it is
impossible to move the slider on the secondary when the latter is inside
the primary. The reason that I have chosen this sort of loose coupler to
describe is to acquaint my young readers with the methods of making a
loose coupler.
The "Junior" loose coupler described farther on is a more elaborate
instrument of greater efficiency, but much harder to build.
[Illustration: Fig. 204.—A Simple Loose Coupler.]
The base of the loose coupler is of wood and measures twelve by four
inches. The head supporting the primary is of the same size as those
used on the "Junior" double-slide tuning coil just described. It may be
made in the same manner, and fitted with a circular block to support the
tube. The primary tube is of the same diameter as that on the tuning
coil but is only four inches long. It is fastened to the primary head
with glue and then secured with a number of small tacks. One or two
coats of shellac liberally applied will render it non-shrinkable, so
that the wire will not be apt to loosen after the loose coupler has been
in use a while.
The secondary is of the same length as the primary, but of smaller
diameter, so that it will easily slip inside. It also is treated with
shellac.
The primary should be wound with a single layer of No. 22
single-silk-covered magnet wire. The secondary is wound with No. 29
single-silk.
The head supporting the secondary is smaller than that used for the same
purpose on the primary. The round boss to which the tube is fastened,
however, is much thicker.
The secondary slides on a "guide-rod" supported at one end by passing
through the primary head and at the other by a brass upright. The
upright may also be made of wood.
If the secondary is "offset," that is, placed out of center slightly to
one side, it will leave room so that the secondary slider will possibly
pass inside of the primary without striking.
Both the primary and the secondary must be fitted with "sliders" to make
contact with the various turns of wire.
The method of constructing a slider has already been described.
The ends of the slider-rods are bent at right angles and fastened to the
coil heads by two small screws passing through holes bored near the
ends. A small narrow path must be scraped in the insulation under each
so that the slider will make contact with each turn. The secondary head
may be provided with a small electrose handle to facilitate sliding it
back and forth.
Two binding-posts are mounted on each of the coil heads.
One post on each is connected to the end of the coil farthest from the
head, and the other posts are each connected to the slider-rods.
Figure 220 shows how to connect the loose coupler in the receiving set.
In order to tune with a loose coupler, first adjust the slider on the
primary until the signals are the clearest. Then set the secondary
slider in the best place and move the secondary in and out of the
primary until the signals are clearest.
*How to Build the Junior Loose Coupler*
A loose coupler of the sort just described is simple and quite easily
constructed, but will not be found to work as well as one in which the
secondary may be varied by means of a switch while it is inside of the
primary.
The base of the instrument measures twelve by three and five-eighths
inches. The primary is composed of a single layer of No. 24 B. & S.
gauge single-silk-covered wire wound on a cardboard tube two and
three-quarter inches in diameter and three and three-quarter inches
long. The winding is laid on in a single layer and should comprise about
150 turns. After winding on tightly, it should be given a coat of clean
white shellac and allowed to dry. The shellac is for the purpose of
fastening the wire down tightly to the tube so that it will not loosen
when the slider is moved back and forth.
The primary is mounted between two heads, the details of which are shown
in Figure 205. One of the heads, _B_, has a flanged hole two and
three-quarter inches in diameter cut through the center so as to receive
the end of the tube and permit the secondary to pass inside.
[Illustration: Fig. 205.—Details of the Wooden Parts.]
The secondary winding is composed of a single layer of No. 28 B. & S.
gauge silk-covered wire and divided into six equal sections. The
secondary is supported by two circular wooden pieces, _C_ and _F_, and
slides back and forth on two guide-rods. The guide-rods should be brass.
Iron or steel rods running through the center of a loose coupler will
seriously weaken the signals, and such practice must by all means be
avoided.
[Illustration: Fig. 206.—Side View of the Loose Coupler.]
[Illustration: Fig. 207.—Top View of the Loose Coupler.]
The secondary sections are connected to six contacts and a switch-arm
mounted on the end of the secondary so that by turning the switch either
one, two, three, four, five, or six sections of the winding may be
connected.
[Illustration: Fig. 208.—End Views of the Loose Coupler.]
[Illustration: Fig. 209.—Complete Loose Coupler.]
The two binding-posts near the secondary end of the coupler are
connected to the terminals of the secondary winding by means of two
flexible wires. They have not been shown in several of the illustrations
because they would be liable to confuse the drawing.
The primary is provided with a slider moving back and forth over a
narrow path scraped through the insulation so that it may make contact
with each wire independently.
*Detectors*
Detectors are very simple devices and consist merely of an arrangement
for holding a small piece of certain minerals and making a contact
against the surface.
The crystal detector shown in Figure 210 is a very efficient form that
may be easily and quickly made. When finished, it will make a valuable
addition to almost any amateur experimenter’s wireless equipment.
[Illustration: Fig. 210.—A Crystal Detector.]
The bracket is bent out of a piece of strip brass about one-eighth of an
inch thick and five-eighths of an inch wide, according to the shape
shown in the illustration. The bracket is mounted on a circular wooden
base about three inches in diameter. The circular wooden blocks used by
electricians in putting up chandeliers, called “fixture blocks,” will
make a satisfactory base. An electrose knob of the typewriter type may
be purchased from any good dealer in wireless supplies. It should be
fitted with a threaded shank which will screw into a hole in the upper
part of the bracket.
The mineral is contained in a small brass cup mounted on the base below
the end of the knob.
Contact with the mineral in the cup is made by means of a fine wire
spring soldered to the end of the adjusting screw.
Moving the screw up or down will vary the pressure of the spring on the
mineral and permit the most sensitive adjustment to be secured. The
bracket is connected to one of the binding-posts and the cup to the
other.
[Illustration: Fig. 211.—Details of the Crystal Detector.]
The detector shown in Figure 212 is of the type often termed a
"cat-whisker," because of the long, fine wire resting on the mineral.
It consists of a small clip, formed by bending a strip of sheet-brass,
which grips a piece of galena.
[Illustration: A Double Slider Tuning Coil.]
[Illustration: A Junior Loose Coupler.]
[Illustration: Crystal Detectors.]
Galena may be obtained from any dealer in radio supplies. A piece of No.
30 phosphor bronze wire is soldered to the end of a short length of
brass rod supported by a binding post. The other end of the rod is
fitted with an electrose knob. This part of the detector is called the
"feeler."
[Illustration: Fig. 212 Details of the "Cat Whisker" Detector.]
The detector is fitted with binding posts and may be mounted upon any
suitable small base. The mineral clip is connected to one post and the
binding-post supporting the "feeler" to the other. The tension or
pressure of the end of the fine wire upon the mineral may be regulated
by twisting the electrose knob so as to twist the rod. The different
portions of the crystal may be "searched" for the most sensitive spot by
sliding the rod back and forth.
A somewhat similar form of cat-whisker detector is shown in Figure 213.
It is provided with a cup to hold the mineral in place of a clip.
The detector shown in Figure 214 is more elaborate than any of the
others described so far.
[Illustration: Fig. 213.—Another Form of the "Cat-Whisker" Detector.]
[Illustration: Fig. 214.—"Cat-Whisker" Detector.]
The base is a wooden block, three and one-half by one and three-quarters
inches by one-half inch. The binding-posts are of the type commonly used
on electrical instruments. One of the posts is pivoted so that it will
swing from side to side. A short piece of brass rod fitted with a rubber
or fiber knob passes through the wire hole in the post. A piece of No.
30 B. & S. gauge bronze wire is soldered to the end of the rod. A small
brass cup contains the mineral, which may be either _galena_, or
_silicon_. By twisting the post and sliding the rod back and forth, any
portions of the mineral surface may be selected.
*Fixed Condenser.*
The construction of the condenser is illustrated in Figure 205. Take
twenty-four sheets of thin typewriter paper, three by four inches, and
twenty-three sheets of tinfoil, two by four inches. Pile them up, using
first a sheet of paper then a sheet of tinfoil, then paper, and so on,
so that every two sheets of tinfoil are separated by a sheet of paper.
Each sheet of tinfoil must, however, project out beyond the edge of the
paper. Connect all the tinfoil projections on one end of the condenser
together and and attach a small wire. Connect all those on the opposite
side in a similar manner. Then fasten a couple of rubber bands around
the condenser to hold it together.
[Illustration: Fig. 215.—Building up a Fixed Condenser.]
[Illustration: Fig. 216.—A Fixed Condenser enclosed in a Brass Case made
from a Piece of Tubing fitted with Wooden Ends.]
If it is desired to give the condenser a finished appearance, it may be
placed in a brass tube fitted with two wooden or fiber ends. The ends
are provided with binding-posts to which the terminals of the condenser
are connected.
*Telephone Receivers* for use with wireless instruments must be
purchased. Their construction is such that they cannot be made by the
experimenter.
[Illustration: Fig. 217.—A Telephone Head Set.]
A seventy-five ohm, double-pole telephone receiver will do for stations
not wishing to receive farther than fifty miles.
In order to secure the best results from wireless instruments, it is
necessary to have receivers especially made for wireless. Each receiver
should have 1000 ohms resistance. Some boys may find it necessary to
purchase one receiver at a time. Two receivers, a double headband, and a
double cord, forming a complete head set as shown in Figure 217, should
be secured as soon as possible.
[Illustration: Fig. 218.—A Circuit showing how to connect a
Double-Slider Tuning Coil.]
*Connecting the Receiving Apparatus*
Figure 218 shows how to connect a double-slide tuner, a detector, a
fixed condenser and a pair of telephones to the aerial and ground. The
same instruments with a loose coupler in place of the double-slide tuner
are shown in Figure 219.
The diagrams in Figure 220 are the same circuits as those shown in
Figures 218 and 219, but show different instruments.
[Illustration: Fig. 219.—Circuit showing how to connect a Loose
Coupler.]
[Illustration: Fig. 220.—A Diagram showing how to connect some of the
Instruments described in this Chapter.]
After the instruments are connected, place a piece of galena or silicon
in the cup of the detector and bring the wire down on it. Then move the
sliders on the tuning coil or loose coupler and adjust the detector
until you can hear a message buzzing in the telephones. It may require a
little patience and practice, but if you persist you will soon learn how
to adjust the apparatus so as to receive the signals loudly and clearly
with very little trouble.
*The Transmitting Apparatus*
Spark coils have already been described in Chapter XII. They may be used
to transmit wireless messages simply by connecting to a spark-gap and a
key.
Spark coils which are especially made for wireless telegraphy will
usually send farther than an ordinary spark coil used for experimental
purposes.
[Illustration: Fig. 221.—A Wireless Spark Coil.]
A good one-inch coil costs from $4.50 to $5.00 and will send from three
to five miles if used with a fair aerial.
A spark coil requires considerable current for its successful operation
and will give the best results if operated on storage cells, dry cells,
or bichromate cells. If dry cells are used, it is a good plan to connect
them in series multiple as shown in Figure 69.
Spark-gaps may be made by mounting two double binding-posts on a wooden
base as shown in Figure 222.
Zinc possesses some peculiar property which makes it very efficient for
a spark-gap, and for this reason the electrodes of a spark-gap are
usually zinc.
[Illustration: Fig. 222.—Small Spark Gaps.]
The figure shows two different forms of electrodes. In one, they are
made of zinc rods and provided with “electrose” handles. In the other
gap, the zinc electrodes are in the shape of "tips" fitted on the ends
of two short brass rods.
A one-inch spark coil will give very good results by connecting the
spark-gap directly across the secondary of the coils. The aerial is
connected to one side of the gap and the ground to the other.
The transmitter may be "tuned" and the range sometimes increased by
using a condenser and a helix.
A condenser is most easily made by coating the inside and outside of a
test-tube with tinfoil so as to form a miniature Leyden jar. The end of
the tube is closed with a cork through which passes a brass rod
connecting to the inner coating of tinfoil.
[Illustration: Fig. 223.—Diagram showing how to connect a Simple
Transmitter.]
If such a condenser is connected directly across the spark-gap, the
spark will become very white and crackling.
Several tubes may be arranged in a rack as shown in Figure 225.
A helix consists of a spiral of brass ribbon set in a wooden frame. The
two strips composing the frame are each nine inches long. The spiral
consists of eight turns of brass ribbon, three-eighths of an inch wide,
set in saw-cuts made in the frame. A binding-post is connected to the
outside end of the ribbon.
Figure 228 shows how to connect a helix and a condenser to a coil and a
spark-gap.
The two clips are made by bending a strip of sheet brass and connecting
a piece of flexible wire to one end.
[Illustration: Fig. 224.—A Test-Tube Leyden Jar.]
In large stations, the best position for the clips is found by placing a
"hot-wire ammeter" in the aerial circuit and then moving the clips until
the meter shows the highest reading.
The young experimenter will have to tune his set by moving the helix
clips about until the best results are obtained in sending.
[Illustration: Fig. 225.—Eight Test-Tube Leyden Jars mounted in a Wooden
Rack.]
If the spark coil is a good one and capable of giving a good hot spark,
it may be possible to tell when the set is in proper tune by placing a
small miniature tungsten lamp in series with the aerial and changing the
clips, the condenser, and the length of the spark-gap until the lamp
lights the brightest.
An _oscillation transformer_ is sometimes used to replace an ordinary
helix when it is desirable to tune a station very closely so that its
messages shall not be liable to be confused with those of another
station when both are working at the same time.
[Illustration: Fig. 226.—A Helix and Clip.]
An oscillation transformer consists of two helixes arranged so that one
acts as a primary and the other as a secondary. An oscillation helix may
be made by making two sets of helix frames similar to that in Secondary
Figure 226.
[Illustration: Fig. 227.—An Oscillation Transformer.]
The primary should be provided with eight turns of brass ribbon and the
secondary with twelve. The primary supports a stiff brass rod upon which
the secondary is mounted. The secondary should slide up and down on the
rod but move very stiffly so that it will stay where it is put.
[Illustration: AN OSCILLATION HELIX.]
[Illustration: AN OSCILLATION CONDENSER.]
An ordinary double-throw, double-pole knife switch having a porcelain
base will make a very good aerial switch in a small station. It is used
to connect the aerial and ground to either the transmitting or receiving
apparatus at will. Such a switch is shown in Figure 230.
[Illustration: Fig 228.—Circuit showing how to connect a Helix and a
Condenser.]
The aerial should be connected to the post _A_ and the ground to _B_.
The posts _E_ and _F_ lead to the transmitter, and _C_ and _D_ to the
receptor, or vice-versa according to which is the more convenient from
the location of the apparatus on the table or operating bench.
A suitable table should be arranged to place the wireless instruments
upon so that they may be permanently connected together.
[Illustration: Fig 229.—Circuit showing how to connect an Oscillation
Transformer and a Condenser.]
The Continental Code is the one usually employed in wireless telegraphy.
It differs slightly from Morse as it contains no space letters. It will
be found easy to learn and somewhat easier to handle than Morse.
[Illustration: Fig 230.—An Aerial Switch.]
Two or three months’ steady practice with a chum should enable the young
experimenter to become a very fair wireless telegraph operator. Then by
listening for some of the high power wireless stations which send out
the press news to ships at sea during the evening it should be possible
to become very proficient. The press news is sent more slowly than
ordinary commercial wireless messages, and is therefore easy to read and
a good starting point for the beginner learning to read.
[Illustration: Fig 231.—A Complete Wiring Diagram for both the
Transmitter and the Receptor.]
[Illustration: Fig. 232.—The Continental Alphabet.]
*A Coherer Outfit*
*A Coherer Outfit* is usually capable of only receiving messages coming
from a distance of under one mile. In spite of this fact, however, it is
an exceedingly interesting apparatus to construct and experiment with,
and for this reason is found fully described below.
A coherer set will ring a bell or work a sounder for short distances and
therefore is the best sort of an arrangement for demonstrating the
workings of your wireless apparatus to your friends.
The first thing that you need for a coherer is a pair of double
binding-posts. Mount these about an inch and three-quarters apart on a
wooden base, six inches long and four inches wide as shown in Figure
233.
[Illustration: Fig. 233.—A Coherer and a Decoherer.]
Get a piece of glass tubing about an inch and one-half long and about
one-eighth of an inch inside diameter. You will also need some brass rod
which will just slide into the tube tightly. Cut off two pieces of the
brass rod each one and three-quarters inches long and slip these through
the upper holes in the binding-posts and into the glass tube as shown in
Figure 234. Before putting the second rod in place, however, you must
put some nickel and silver filings in the tube, so that when the rods
are pushed almost together, with only a distance of about one-sixteenth
of an inch between them, the filings will about half fill the space.
The filings must be very carefully prepared, and in order to make them,
first use a coarse-grained file on the edge of a five-cent piece. Do not
use the fine dust and powder, but only the fairly coarse filings. Mix a
few silver filings from a ten-cent piece with the nickel in such
proportion that the mixture is 90% nickel and 10% silver.
[Illustration: Fig. 234.—Details of the Coherer.]
You will have to experiment considerably to find out just the right
amount of filings to place in the tube, and how far apart to place the
brass rods or plugs.
Remove the gong from an old electric bell and mount the bell on the base
as shown in Figure 233. It should be in such a position that the bell
hammer will touch the coherer very lightly when the bell is ringing.
The two binding-posts, tube rods, and filings constitute the _coherer_.
The bell is the _decoherer_.
The next thing required in order to complete the apparatus is a relay.
You may use the relay described in Chapter X or build one according to
the plan shown in Figure 235. This relay consists of a single
electro-magnet mounted on a wooden base, two inches wide and four inches
long. The armature is a piece of soft iron rod one-quarter of an inch in
diameter and one-eighth of an inch long, riveted to the end of a thin
piece of spring brass, about No. 34 B. & S. gauge in thickness.
[Illustration: Fig. 235.—The Relay.]
The other end of the spring is fitted to a bracket and provided with a
thumbscrew to adjust the tension of the spring.
The under side of the armature and the upper side of the magnet core are
each fitted with a small silver contact.
The contacts should meet squarely when the armature is drawn down on to
the core by a current of electricity passing through the electro-magnet.
By turning the adjusting screw, the armature can be raised or lowered.
It should be adjusted so that it almost touches the core and is only
just far enough away to slip a piece of thick paper under.
The terminals of the magnet are connected to the two binding-posts on
the base marked _S_ and _S_. One of the binding-posts, _P_, is connected
to the brass upright, and the other is connected to the core of the
magnet.
Figure 236 shows how to connect up the outfit. It will require some very
nice adjusting before you will be able to get it to working properly.
[Illustration: Fig. 236.—The Complete Coherer Outfit.]
If you wish to use the outfit for demonstration purposes or for sending
messages for very short distances, as for instance across a room, you do
not need an aerial but merely a pair of "catch-wires."
The "catch-wires" are two pieces of stiff copper wire, about two feet
long, placed in the lower holes in the double binding-posts forming part
of the coherer.
In order to set the apparatus for operation, raise the adjusting screw
of the relay until the armature is quite far away from the core. Then
push the armature down against the contact on the core. The decoherer
should then immediately operate and begin to tap the coherer. Then turn
the thumbscrew until the armature is brought down to the core in such a
position that it is as close as it is possible to get it without ringing
the bell.
The transmitter should consist of a spark coil, battery, key, and a
spark-gap. The gap should be connected to the secondary of the coil and
adjusted so that the electrodes are only about one-eighth of an inch
apart. The key is placed in series with the primary of the coil and the
battery, so that pressing the key will send a stream of sparks across
the gap. Fit the spark-gap with two catch-wires similar to those on the
coherer and place the transmitter about four or five feet away from the
coherer outfit.
You are now likely to find that if you press the key of the transmitter,
the decoherer will ring. It is possible that it will continue to ring
after you have stopped pressing the key. If such is the case, it will be
necessary to turn the adjusting screw on the relay so as to move the
armature upward a short distance away from the core.
If the decoherer will not operate each time when you press the key, the
brass plugs in the coherer need adjusting. You must not be discouraged
if you have some difficulty in making the apparatus work at first. After
you learn how to adjust it properly, you will find that you can move the
transmitter quite a distance away from the coherer and it will still
operate very nicely.
After you manage that, you can place the apparatus in separate rooms and
find it possible to work it just the same, because ordinary walls will
not make any difference to wireless waves.
Bear in mind that the nearer the coherer plugs are to each other, the
more sensitive the coherer will be, but that if too close, the decoherer
will not be able to shake the filings properly and will not stop when
you stop pressing the key.
The operation of the apparatus depends upon the fact that when properly
adjusted the resistance of the filings between the two brass plugs is
too great to allow sufficient battery current to flow to attract the
armature of the relay. As soon as any wireless waves from the
transmitter strike the catch-wires of the coherer, they cause the
filings to cling together or cohere. When in this state, they have a low
resistance and permit the current to flow in the relay circuit and draw
down the armature. The armature closes the second circuit and sets the
decoherer into operation. The decoherer shakes the filings and causes
them to decohere or fall apart and so makes them ready again for the
next signal.
A coherer set of this sort may be used on an aerial and ground by
substituting the coherer for the detector, but otherwise following any
of the receiving circuits which have already been shown.
CHAPTER XV A WIRELESS TELEPHONE
Probably many readers of the "Boy Electrician" are amateur wireless
operators and have constructed their own apparatus with which they are
able to pick up commercial messages or communicate with other
experimenters in the neighborhood, but not many have ever built a
wireless telephone.
The device described in the following pages is easy to make and arrange,
and will serve for some very interesting experiments.
It is of no practical value as a commercial wireless telephone, because
the distance over which it will transmit speech is limited to from 250
to 300 feet. If you have a chum who lives across the street and within
the distance named above, it is possible for you to construct a simple
wireless telephone which will enable you to remain in your own rooms and
talk to each other without any connecting wires.
The instruments operate by magnetic induction. It has already been
explained how it is possible for the current in the primary of an
induction coil to induce a current in the secondary coil, even though
the two are not electrically connected. This type of wireless telephone
really consists of an induction coil in which the two windings are
widely separated.
Suppose that two coils of wire are connected as in Figure 237. The
illustration shows that one coil, _A_, is connected in series with a set
of batteries and a telegraph key. The terminals of the other coil, _B_,
are connected to a telephone receiver. The coils are placed parallel to
each other and a few inches apart. If the key is pressed so that the
battery current may flow through the coil, _A_, it will create a
magnetic field, and lines of force will be set up in the immediate
vicinity. The lines of force will pass through the coil, _B_, and induce
in it a current of electricity which will cause a sound like a click to
be heard in the telephone receiver.
[Illustration: Fig. 237.—A Simple Arrangement showing the Inductive
Action between two Coils.]
If a telephone transmitter is substituted for the key and words are
spoken into it, the current passing through the coil from the battery
will vary with each vibration of the voice and the words will be
distinctly repeated by the receiver connected to _B_.
This experiment may be tried by any boy with the equipment he probably
has already around his shop. Twenty-five to thirty turns of wire wound
around a cardboard tube five or six inches in diameter will serve as a
coil. Two such coils, an ordinary telephone transmitter, a telephone
receiver and a couple of dry cells are all that is required.
[Illustration: Fig. 238.—A Simple Wireless Telephone. Speech directed
into the Transmitter can be heard in the Receiver, although there is no
direct electrical connection between the two.]
The diagram in the accompanying illustration shows how the apparatus is
arranged. The coils may be used several inches apart and the voice will
be clearly heard in the receiver.
Such an outfit is, however, only experimental, and if it is desired to
make a practical set, the coils, etc., must be much larger in diameter
and contain a greater number of turns.
Larger coils are made by first drawing a circle four feet in diameter on
the floor of the "shop" or attic. Then drive a number of small nails
around the circumference, about four inches apart.
Procure two and one-half pounds of No. 20 B. & S. gauge cotton-covered
magnet wire and wind it around the circumference of the circle. The wire
should form at least sixty complete turns. About one foot should be left
at each end to establish connections with. After winding, the coil
should be tied about every six inches with a small piece of string so
that it will retain its shape and not come apart. The nails are then
pulled out so that the coil may be removed.
The coil may be used just as it is for experimental purposes, but if it
is intended for any amount of handling it is wise to procure a large
hoop such as girls use for rolling along the sidewalk, and make the coil
the same diameter as the hoop so that upon completion they may be firmly
bound together with some insulating tape. Two binding-posts may then be
fastened to the hoop and the terminals of the coil connected to them.
Two such coils are required for a complete wireless telephone system,
one to be located at each station.
It is also necessary to make a double-contact strap-key. Such a key is
easily built out of a few screws and some sheet-brass. The illustration
shows the various parts and construction so clearly that no detailed
explanation is necessary.
[Illustration: Fig. 239.—A Double-Contact Strap-Key. The Dotted Lines
show how the Binding-Posts are connected.]
The telephone transmitter and the telephone receiver required for this
experiment must be very sensitive, and it is hardly possible for the
young experimenter to build one which will be satisfactory. They can be
secured from a second-hand telephone or purchased at an electrical
supply house. The transmitter should be of the "long distance" type. An
80-ohm receiver will serve the purpose, but if you also have a wireless
station, use the same 1000-ohm receivers belonging to your wireless set
and you will secure very good results.
A battery capable of delivering about 10 volts and a good constant
current is required.
The apparatus is connected as shown in Figure 240.
When the key is pressed, the coil is connected to the battery and the
telephone transmitter. If words are then spoken into the transmitter
they will vary the amount of current flowing and the magnetic field
which is set up in the neighborhood of the coil will induce currents in
the coil at the other station, provided that it is not too far away, and
cause the words to be reproduced in the telephone receiver.
When the key is released it will connect with the upper contact and
place the telephone receiver in the circuit for receiving, so that your
chum at the other station can answer your message by pressing his key
and talking into his transmitter.
[Illustration: Fig. 240.—The Circuit of the Wireless Telephone. When the
Key is up, the Receiver is ready for Action. When the Key is pressed,
the Transmitter and Battery are thrown into the Circuit.]
The best plan is to mount each of the coils upon a tripod and experiment
by placing them close together at first and gradually moving them apart
until the maximum distance that the apparatus will work is discovered.
Be very careful to keep the two coils exactly parallel.
Much depends upon the battery. Be certain that it is capable of
delivering a good strong current. Do not hold the key down any longer
than is absolutely necessary, or the telephone transmitter will become
hot.
By making the coils six feet in diameter and placing from 200 to 400
turns of wire in each coil you can make a set which is capable of
transmitting speech 300 feet or more.
[Illustration: Fig. 241.—A Complete Wireless Telephone and Telegraph
Station for Amateurs. 1. The Telephone Coil. 2. The Telephone
Transmitter. 3. Double-Contact Strap-Key. 4. The Battery. 5. Spark Coil.
6. Key. 7. Spark-Gap. 8. Aerial Switch. 9. Loose Coupler. 10. Detector,
11. Fixed Condenser. 12. Code Chart. 13. Amateur License. 14. Aerial.
15. Telephone Receivers.]
The coil may be mounted on the wall of your shop in a position where it
will be parallel to one located in your chum’s house.
The success of a wireless telephone system of this sort lies in making
the coils of large diameter and many turns, in keeping the coils
parallel, using a sensitive transmitter and receiver, and in employing a
good strong battery. Storage cells are the best for the purpose.
CHAPTER XVI ELECTRIC MOTORS
The first American patentee and builder of an electric motor was Thomas
Davenport. The father of Davenport died when his son was only ten years
old. This resulted in the young inventor being apprenticed to the
blacksmith’s trade at the age of fourteen.
Some years later, after having thoroughly learned his trade, he married
a beautiful girl of seventeen, named Emily Goss, and settled in the town
of Brandon, Vermont, as an independent working blacksmith.
About this time Joseph Henry invented the electro-magnet. Davenport
heard of this wonderful "galvanic magnet" which it was rumored would
lift a blacksmith’s anvil. This was his undoing, for never again was he
to know peace of mind but was destined to always be a seeker after some
elusive scientific "will-o’-the-wisp." Although many times he needed
iron for his shop, the greater part of his money was spent in making
electro-magnets and batteries.
In those days insulated wire could not be purchased, and any one wishing
insulated wire had to buy bare wire and insulate it himself. It was then
supposed by scientists that silk was the only suitable material for
insulating wire and so Davenport’s brave young wife cut her silk wedding
gown into narrow strips and with them wound the coils of the first
electric motor.
Continuing his experiments in spite of almost insurmountable
difficulties and making many sacrifices which were equally shared by his
family, he was enabled to make a trip to Washington in 1835 for the
purpose of taking out a patent. His errand was fruitless, however, and
he was obliged to return home penniless.
Nothing daunted, he made the second and third trip and finally secured
his memorable patent, the first of the long line of electric-motor
patents that have made possible both the electric locomotive that hauls
its long train so swiftly and silently, and the whirring little fan
which stirs up a breeze during the hot and sultry days.
These are a few of the reasons why a modest country blacksmith, in turn
an inventor and an editor, through perseverance in struggling against
adversity and poverty succeeded in placing his name on the list which
will be deservedly immortal among the scientists and engineers of the
world.
*A Simple Electric Motor* can be made in fifteen minutes by following
the plan shown in Figure 242.
The armature is made by sticking a pin in each end of a long cork. The
pins should be as nearly central as it is possible to make them, so that
when the cork is revolved upon them it will not wabble. The pins form
the shaft or spindle of the motor. Then take about ten feet of fine
magnet wire (Nos. 28-32 B. & S. gauge) and wind it on as shown in the
illustration, winding an equal number of turns on each side of the two
pins.
[Illustration: Fig. 242.—A Simple Electric Motor which may be made in
Fifteen Minutes.]
When this is finished, fasten the wire securely to the cork by binding
it with thread.
Bend the two free ends (the starting and the finishing end) down at
right angles and parallel to the shaft so as to form two commutator
sections as shown in the upper left hand corner of Figure 242. Cut them
off so that they only project about three-eighths of an inch. Bare the
ends of the wire and clean them with a piece of fine emery paper or
sandpaper.
The bearings are made by driving two pins into a couple of corks so that
the pins cross each other as shown in the upper right-hand corner of
Figure 242.
They must not be at too sharp an angle, or when the armature is placed
in position, the friction of the shaft will be so great that it may not
revolve.
The motor is assembled by placing the armature in the bearings and then
mounting two bar magnets on either side of the armature. The magnets may
be laid on small blocks of wood and should be so close to the armature
that the latter just clears when it is spun around by hand. The north
pole of one magnet should be next to the armature and the south pole of
the other, opposite.
Connect two wires about one foot long and No. 26 B. & S. gauge in
diameter to a dry cell. Bare the ends of the wires for about an inch and
one half.
Take the ends of the two wires between the forefinger and thumb and bend
them out, so that when the armature is revolved they can be made just to
touch the ends of the wire on the armature, or the "commutator
sections," as they are marked in the drawing.
Give the armature a twist so as to start it spinning, and hold the long
wires in the hand so that they make contact with the commutator as it
revolves.
Very light pressure should be used. If you press too hard, you will
prevent the armature from revolving, while, on the other hand, if you do
not press hard enough, the wires will not make good contact.
The armature will run in only one direction, and so try both ways. If
you start it in the right direction and hold the wires properly, it will
continue to revolve at a high rate of speed.
If carefully made, this little motor will reward its maker by running
very nicely. Although it is of the utmost simplicity it demonstrates the
same fundamental principles which are employed in real electric motors.
*The Simplex Motor* is an interesting little toy which can be made in a
couple of hours, and when finished it will make an instructive model.
[Illustration: Fig. 243.—Details of the Armature of the Simplex Motor.]
As a motor itself, it is not very efficient, for the amount of iron used
in its construction is necessarily small. The advantage of this
particular type of motor and the method of making it is that it
demonstrates the actual principle and the method of application that is
used in larger machines.
The field of the motor is of the type known as the "simplex" while the
armature is the "Siemens H" or two-pole type. The field and the armature
are cut from ordinary tin-plated iron such as is used in the manufacture
of tin cans and cracker-boxes.
The simplest method of securing good flat material is to get some old
scrap from a plumbing shop. An old cocoa tin or baking-powder can may,
however, be cut up and flattened and will then serve the purpose almost
as well.
[Illustration: Fig. 244.—The Armature.]
*The Armature.* Two strips of tin, three-eighths of an inch by one and
one-half inches, are cut to form the armature. They are slightly longer
than will actually be necessary, but are cut to length after the finish
of the bending operations. Mark a line carefully across the center of
each strip. Then, taking care to keep the shape symmetrical so that both
pieces are exactly alike, bend them into the shape shown in Figure 243.
The small bend in the center is most easily made by bending the strip
over a knitting-needle and then bending it back to the required extent.
[Illustration: Fig. 245.—The Field.]
A piece of knitting-needle one and one-half inches long is required for
the shaft. Bind the two halves of the armature together in the position
shown in Figure 249. Bind them with a piece of iron wire and solder them
together. The wire should be removed after they are soldered.
[Illustration: Fig. 246.—The Field and Commutator.]
*The Field Magnet* is made by first cutting out a strip of tin one-half
by four and then bending it into the shape shown in Figure 245.
The easiest way of doing this with accuracy is to cut out a piece of
wood as a form, and bend the tin over the form. The dimensions shown in
Figure 245 should be used as a guide for the form.
[Illustration: Fig. 247.—The Bearings.]
Two small holes should be bored in the feet of the field magnet to
receive No. 3 wood screws, which fasten the field to the base.
*The Bearings* are shown in detail in Figure 247. They are easily made
by cutting from sheet-tin. Two small washers, serving as collars, should
be soldered to the shaft as shown in Figure 243.
*The Commutator Core* is formed by cutting a strip of paper
five-sixteenths of an inch wide and about five inches long. It should be
given a coat of shellac on one side and allowed to get sticky. The strip
is then wrapped around the shaft until its diameter is three-sixteenths
of an inch.
*The Base* is cut from any ordinary piece of wood and is in the form of
a block about two by one and one-half by one-half inch.
[Illustration: Fig. 248.—The Complete Motor.]
*Assembling the Motor.* The parts must be carefully prepared for winding
by covering with paper. Cut a strip of paper one-half inch wide and one
and one-eighth of an inch long and give it a coat of shellac on one
side. As soon as it becomes sticky, wrap it around the top bar of the
field magnet. The armature is insulated in exactly the same way, taking
care that the paper covers the entire flat portion.
The field and armature are now ready for winding. It is necessary to
take proper precautions to prevent the first turn from slipping out of
place.
This is accomplished by looping a small piece of tape or cord over it.
The next two turns are then taken over the ends of the loop so as to
embed them. Wind on three layers of wire and when in the middle of the
fourth layer embed the ends of another loop, which may be used at the
end of the fourth layer to fasten the end so that it will not unwind.
After the winding is finished, give it a coat of shellac.
The winding of the armature is somewhat more difficult.
The wire used for winding both the armature and the field should be No.
25 or No. 26 B. & S. gauge double-cotton-covered.
In order to wind the armature, cut off about five feet of wire and
double it back to find the center. Then place the wire diagonally across
the center of the armature so that there is an equal length on both
sides. Place a piece of paper under the wire at the crossing point to
insulate it. Then, using one end of the wire, wind four layers on half
of the armature. Tie the end down with a piece of thread and wind on the
other half.
The ends of the wire are cut and scraped to form the commutator
segments. Figure 246 shows how this is done.
Bend the wires as shown so that they will fit closely to the paper core.
Bind them tightly into position with some silk thread. Use care so that
the two wires do not touch each other. Cut the free ends of the wires
off close to the core.
When finished, the relative positions of the armature and the commutator
should be as shown in Figure 248.
The brushes are made by flattening a piece of wire by a few light hammer
blows.
The brushes are fastened under a small clamp formed by a strip of tin
held down at each end with a wood screw. They can be adjusted to the
best advantage only under actual working conditions when the current is
passing through the motor. One or two dry cells should be sufficient to
operate the motor.
[Illustration: Fig. 249.—Details of the Motor.]
One end of the field winding is connected to one of the brushes. The
other brush and the other end of the field form the terminals to which
the battery is connected.
The motor, being of the two-pole armature type, must be started when the
current is turned on by giving it a twist with the fingers.
*A Larger Motor* may be built in somewhat the same manner as the one
just described by cutting armature and field out of sheet tin. It will
be more substantial if it is built up out of laminations and not bent
into shape, as in the case of the other.
Lay out an armature disk and a field lamination on a sheet of tin in
accordance with the dimensions and pattern shown in Figure 249. These
pieces are used as patterns for laying out the rest of the laminations.
[Illustration: Fig. 250.—Complete Motor.]
Place them on some thin sheet-iron and trace the outline with a
sharp-pointed needle. Then cut a sufficient number of pieces of each
pattern to form a pile three-quarters of an inch thick.
Four laminations for the field should be cut with extensions shown by
the dotted lines. They are bent out at right angles for mounting the
motor and holding it upright.
Assemble the armature and field by piling the pieces on top of each
other and truing them up. Enough laminations should be used to form a
pile three-quarters of an inch thick when piled up and clamped tightly.
File off any burrs and rough edges and then bind the laminations
together with some string to hold them until wound.
Wrap a couple of layers of paper around those portions of the armature
and field which are liable to come into contact with the iron. Five or
six layers of No. 18 B. & S. gauge double-cotton-covered magnet wire are
sufficient to form the field coil.
The armature is wound with three or four layers of wire of the same
size.
The commutator is made out of a circular piece of hard wood or fiber,
fitted with segments cut out of thin sheet-copper. The segments may be
fastened to the core with thick shellac or some melted sealing-wax. The
ends may be bound down tightly by wrapping with silk thread.
The brushes are cut out of thin sheet-copper similar to that used for
the commutator segments.
The bearings are strips of heavy sheet-brass bent into the shape shown.
They are mounted by passing a nail through the holes in the ends and
through the holes, A and B, in the field and then riveting the ends
over.
Assemble the motor as shown in Figure 255. If desirable, a small pulley
may be fitted to the shaft and the motor used to run small mechanical
toys. If it is properly constructed, two or three dry cells will furnish
sufficient current to run the motor at high speed.
CHAPTER XVII DYNAMOS
There is perhaps no other electrical device entering into the young
experimenter’s domain requiring the careful workmanship and tool
facilities that the dynamo does. In order to construct a practical
working dynamo it would be necessary to have at hand a lathe for turning
the castings.
Rather than describe a machine which comparatively few of my readers
would be able to build, I have explained below how it is possible to so
alter an old telephone magneto that it may be made to serve as a small
dynamo. Telephone magnetos, also sometimes called hand generators, are
used in many telephone systems to supply the current which rings the
telephone bell at the other end. The magneto is placed in a small box on
the telephone, only the handle being exposed. In order to make a call
the handle is given several brisk turns before raising the receiver.
When the handle is turned the moving parts of the generator revolve and
produce a current of electricity which goes forth over the line and
rings the bell at the other end.
[Illustration: Fig. 251—A Telephone Magneto.]
Telephone magnetos are gradually being discarded in all the large
telephone systems, a method known as "central energy," in which the
current for ringing bells is supplied from the central office, taking
their place. For that reason, there are a great many telephone magnetos
to be found in second-hand shops and at electrical houses, where they
can be purchased for a fractional part of the original cost. Fifty cents
will buy a first-class second-hand telephone magneto. The author saw a
pile of telephones as large as a haystack, each telephone containing a
magneto, in the back yard of a second-hand shop, and the owner would
have been glad to sell the complete instruments for fifty cents each.
Before explaining how to reconstruct such a machine, it is best to
impress upon the reader that a careful study of the principles of the
dynamo is well worth the time spent.
Almost any book on physics or electricity, or even the encyclopedia,
will be found to contain a description of this wonderful machine that
supplies the power for running the trolley cars, electric lights, etc.,
in fact all of the electricity in use to-day with the exception of that
generated by batteries for telegraph and telephone lines.
It will be remembered that if a bar magnet is suddenly plunged into a
hollow coil of wire, a momentary electric current will be generated in
the coil. The current is easily detected by means of an instrument
called a galvanometer. The space in the vicinity of a magnet is filled
with a peculiar invisible force called magnetism. The magnetism flows
along a certain path, passing through the magnet itself and then
spreading out in curved lines. If a sheet of paper is laid over a magnet
and a few iron filings are sprinkled on the paper, they will follow the
magnetic lines of force.
When the magnet is plunged into the hollow coil, the lines of force flow
through the turns of wire, or are said to cut them. Whenever lines of
force cut a coil of wire and they are in motion, electricity is
produced. It does not matter whether the coil is slipped over the magnet
or the magnet is plunged into the coil, a current will be produced as
long as they are in motion. As soon as the magnet or the coil stops
moving the current stops.
By arranging a coil of wire between the poles of a horse-shoe magnet so
that it can be made to revolve, the motion can be made continuous and
the current of electricity maintained.
Figure 252 shows such an arrangement. Some means of connection with the
coil of wire must be established so that the current can be led off. If
two metal rings are connected to the ends of the coil, connection can be
made by little strips of metal called brushes rubbing against the rings.
This scheme is the principle of the telephone magneto and the basis of
all dynamos.
[Illustration: Fig. 252.—The Principle of the Alternator and the
Direct-Current Dynamo.]
In the telephone magneto, more than one horseshoe magnet is usually
provided. The coil of wire revolves between the poles of the magnets.
The coil is wound around an iron frame and together they are called the
armature. The end of the armature shaft is fitted with a small spur gear
meshing with a larger gear bearing a crank, so that when the crank is
turned the motion is multiplied and the armature is caused to revolve
rapidly. One end of the coil or armature winding is connected to a small
brass pin. This pin connects with a second pin set in the end of the
shaft in an insulating brush of hard rubber. The other terminal of the
coil is connected to the armature itself. Thus connection can be had to
the coil by connecting a wire to the frame of the machine and to the
insulated pin.
[Illustration: Fig. 253.—Details of the Armature, Commutator, and
Brushes.]