ELECTRICAL RESISTANCE
The science of controlling forces is so well understood and figured out
that it becomes a simple mechanical proposition to adapt the various
types of controllers to any form of power that may be employed. The
tremendous force stored within the mechanism of a great transatlantic
liner is governed by the twist of a man’s wrist. The locomotive that
will pull a hundred cars loaded with coal, representing a weight of
thousands of tons, is started and stopped by a short lever that is drawn
in one direction or the other by a man’s hand. Great forces of all kinds
are quite as easily controlled as the supply of gas through a jet--by
simply turning the key that lets out so much as may be required, no
matter what the pressure is back of the flow.
This same principle applies to electricity, but the means of governing
it is vastly different from the methods employed for other forces.
Electricity is an unknown and unseen force, coming from apparently
nowhere and returning to its undiscovered country immediately upon the
completion of its work. The flow of steam, water, liquid air, gas, and
compressed air through pipes is governed by a throttle or cock, which
allows as much or as little to pass as may be required; and if the
joints, unions, and couplings in the pipes are not absolutely tight
there will be a leakage. Electricity is controlled by resistance in its
passage through solid wires, rods, or bars, and cannot be confined
within a given space like water, nor held in tanks or pipes as a vapor
or gas. It is invisible, colorless, odorless, and occupies no apparent
space that can be measured; it is the most powerful and terrible and yet
docile force known to man, doing his bidding at all times when properly
governed and regulated. In some respects, electricity can be compared to
water stored in a tank--for instance, if you have a tank of water
containing fifty gallons at an elevation of twenty-five feet, and a pipe
leading down from it, the pressure of the water at the outlet of the
pipe will be a given number of pounds. Now if the tank were double the
size the pressure at the outlet of the pipe would be proportionately
greater. Now if you have a battery made up of a number of cells they
will develop a given number of volts, and if the number of the cells be
doubled the voltage will be correspondingly increased. Or if you have a
dynamo giving a certain number of volts, that number may be increased by
doubling the size.
The water contained within the tank represents its pressure at the
outlet of the pipe. The current in volts, generated in a battery or
dynamo, represents its pressure on an outlet or conductor wire; and both
represent the force behind their respective conductors. The valve, or
faucet, at the end of the pipe plus the friction in the pipe would
represent the resistance to the flow of water, while the
resistance-coils or other mediums plus the size of the wire, or
conductor and switch, would regulate the flow of electric current. The
flow of water in a pipe under certain pressure would represent its
gallons per minute or hour, while with electricity its flow in a wire or
other conductor would represent its amperage. It is to govern the flow
of current that resisting mediums are employed.
The resistance of electric current is measured in ohms, and it is with
this phase that we are interested in this chapter. If there is only a
small resistance put in the path of a current, then it requires but a
small pressure or voltage to send it through the wires or circuit. This
is easily understood by the boy who has experimented with small
incandescent lamps in which short pieces of carbon-filament are
contained. It requires the pressure of a few volts only to send the
current through the carbon; but for the large carbon-filaments,
measuring ten or twelve inches in length, from one hundred to five
hundred volts may be necessary. The ordinary house lamps require one
hundred and ten volts and half an ampere to give sixteen candle-power.
It is easily understood, then, that it requires a high pressure or
voltage to force the current through the resisting carbon-filament, or
across the space from one carbon to the other in the arc-lamps used for
street lighting. The shorter and larger the conducting wires the less
the resistance, and consequently the lower the voltage or pressure
necessary to force it. Contrariwise the longer and finer the conducting
wares, the greater the resistance. As copper is the best commercial
conductor of electric currents, it is in universal use, and in it the
minimum of resistance is offered to the current. Iron wire is a poorer
conductor, and is not used for high voltage (such as trolleys or
transmission of power), but is confined to telegraph and telephone lines
and low-pressure work. German-silver wire, one of the poorest
conductors, is not used for lines at all, but is employed solely as a
resisting medium for controlling current.
Ohm’s Law
This is the fundamental formula expressing the relations between
current, electro-motive force, and resistance in an active electric
circuit. It may be expressed in several ways with the same result, as
follows:
1. The current strength is equal to the E. M. F. (electro-motive force)
divided by the resistance.
2. The E. M. F. (electro-motive force) is equal to the current strength
multiplied by the resistance.
3. The resistance is equal to the E. M. F. (electro-motive force)
divided by the current strength.
All these are different forms of the same statement; and when figuring
electrical data, C stands for current, E for electro-motive force, and R
for resistance.
Resistance-coils and Rheostats
The method by which electricity is controlled is resistance. No matter
how great the voltage of a current, nor its volume in amperes, it can be
brought down from the deadly force of the electric trolley-current to
the mild degree needed to run a small fan-motor, an electric bell, or a
miniature lamp. This is accomplished by means of resisting mediums,
such as fluids or wires, which hold back the current, and allow only
the small quantity to pass that may be required to operate the
apparatus.
The jump from the high voltage of the trolley-current to the low one
required for the electric bell, a lamp, or a small motor, is frequently
made in traction-work, but in this case regular transformers are used.
For the small apparatus, that may have its current supplied from a
battery, or a small dynamo driven by a water-motor, the forms of
resistance-coils and rheostats described on the following pages should
meet every requirement.
The standard unit of resistance is called an ohm, so named after Dr. G.
S. Ohm, a German electrician, whose theory on the subject is accepted as
the basis on which to calculate all electrical resistance. The legal ohm
is the resistance of a mercury column one square millimetre in
cross-sectional area and one hundred and six centimetres in length, and
at a temperature of 0° Centigrade or 32° Fahrenheit, or the
freezing-point for water. The conductivity of metals is dependent
greatly on their temperature, a hot wire being a much better conductor
than a cold one. Since counter-electro-motive force sometimes gives a
spurious resistance, the ohmic resistance is the true standard by which
all current is gauged.
In technical mechanism and close readings the ohmic resistance counts
for a great deal, but in the simple apparatus that a boy can make the
German-silver resistance coils and the liquid resistance will answer
every purpose.
To give a clearer idea of the principle of the rheostats, a short
description of the mercurial column will first be presented. During the
early part of the last century wires were not used as a resisting medium
for electric currents. In their place, glass tubes, filled with mercury
sealed at one end and corked at the other, were arranged in rows and
supported in a wooden rack.
[Illustration: _=Fig. 1=_]
Wires led out from the top and bottom of each tube, and were brought
down to metal buttons arranged in a row along the front edge of the
base-plate, as shown in the illustration of a mercurial rheostat (Fig.
1). Each tube represented a certain resistance--one or more ohms, as
required. The outlet wire was attached to the button at one end of the
row, and the inlet could be moved along from button to button, until the
required amount of current was obtained.
The mercurial rheostat was an expensive, cumbersome, and treacherous
thing to handle; it was liable to break, and its weight often prohibited
its use in places where the more convenient and easily handled
German-silver rheostats are now in universal employment. Overheating the
mercury in the columns caused it to expand, and sometimes, before the
switch could be thrown open, an end would be forced out and the mercury
would climb over the edge of the glass columns.
All metals have a certain amount of resistance for electric currents,
and some have more than others. German-silver, for instance--a metal
made of a mixture of other metals with about eighteen per cent. of
nickel (see Appendix)--is considered to be the best commercial
resistance medium, while pure copper is regarded as the best commercial
conductor. Unalloyed copper is universally employed for electric
conductors of high voltage; but for telegraph and telephone work,
galvanized iron wire is still used extensively.
The finer the wire, the higher is its resistance, and the more resistant
the metal, the greater are the number of ohms to a given length. To nine
feet and nine inches of No. 30 copper wire there is one ohm resistance,
while to No. 24--which is six sizes coarser--there is one ohm to
thirty-nine feet and one inch. In many cases it is necessary to use the
coarser wire and greater length, as the current would superheat or burn
the fine wire, while the coarser would conduct it safely.
For very high voltage and amperage--such as used in traction cars, in
power stations, and in manufacturing plants--castings of German-silver
are employed and linked in series. They are more easily handled than the
coils of wire, and a greater number of them can be accommodated in a
small space.
[Illustration: _=Fig. 2=_]
For light currents in experimental work, where batteries are employed,
obtain a pound or two of bare German-silver wire, from Nos. 24 to 30,
and wind the strands on a round piece of stick attached to a winder (see
Magnets and Induction-Coils, chapter iv.). Make several of these coils,
two or three inches long, with the wire wound closely and evenly. When
pulled apart the coils will appear as shown in Fig. 2 A, and will
resemble a spiral spring. This can be made fast over a porcelain knob
and the ends caught down, as shown at B in Fig. 2, or it may be drawn
over a round stick, a porcelain tube, or a lug made of plaster of Paris
and dextrine (three parts of the former to one of the latter), as shown
at C in Fig. 2, and the ends securely bound with a strand or two of
wire, twisted tight to keep the ends from slipping.
The lugs may be made in a mold, using as a pattern a piece of
broom-handle--shellacked and oiled to prevent the plaster from adhering
to it. Obtain a small square and deep box, and drop some of the wet
mixture down in the bottom; on this place the broomstick, small end down
(it should be slightly tapered), and around it pour in the wet plaster
mixture. While it is setting, turn the stick with the thumb and fingers,
so as to shape the hole perfectly then draw it out, and a true mold will
be the result. When dry enough, pour some shellac down into the mold and
revolve it, so that the shellac will be evenly distributed, and let it
harden for a day. Then saw off the end of the mold, so that it will be
open at both ends.
In order to make the lugs, pour in the plaster mixture, taking care to
oil the mold before each pouring, so that the lug can be drawn out when
the mixture has set. If it sticks, tap the small end gently to start it.
For coils where there is little or no heat, ordinary pieces of
broom-handle, or round sticks having a coat or two of shellac, will
answer very well; but where the current heats the core, it must be of
some material that will not char.
Another method of making resistance-coils is to measure off a length of
wire; then double it, and with a small staple attach the loop end at one
end of the (wooden) core. Pay out the two strands of wire an equal
distance apart with the thumb and fingers, and with the other hand
twist the core. At the other end of the spool catch the loose ends of
the wire under small staples, taking great care not to let the staples
touch or even be driven close together. This arrangement is shown at D
in Fig. 2, and for a field resistance-board any number of these coils
may be made.
[Illustration: _=Fig. 3=_]
In Fig. 3 the mode of connecting coils is shown. The dots represent
contact-points to which the movable arm can be shifted. The wires at the
bottom of coil, Nos. 1 and 2, are connected together, while those at the
top of No. 2 and 3 are joined, and so on to the end. The leading-in
current is connected at pole H and so on to J, while the leading-out
wire is made fast to pole I. The switch-arm is moved on the first dot,
or contact-point, and the current passes up wire A, down coil No. 1, up
coil No. 2, down No. 3, up No. 4, and so on to No. 6, and down wire G
and out at I. Supposing that this offers too much resistance, the
switch-arm is moved up one point. This cuts out coil No. 1, as the
current passes up wire B, through coil No. 2, down No. 3, and so on, and
out through G and pole I. Another move of the switch and coil No. 2 is
cut out, the current passing up wire C, down coil No. 3, up No. 4, and
so on, and out at I. Each move of the switch cuts out one coil,
lessening the resistance; but when moved to the last contact-point the
current flows without resistance--in at H, through the switch-arm, and
out at I.
The plan of arranging the coils suggested at Fig. 2 B is shown in Fig.
4, where four of the coils are arranged in series over porcelain knobs,
and the lower ends made fast to the base-board with small staples. Small
pieces of brass are used for the switch contact-plates; those are
provided with one plain and one countersunk hole for a flat and round
headed screw.
The screw-heads are arranged in a semicircular fashion, so that the
switch-arm, attached at one end to the screw J, will touch each plate as
it is moved forward or backward.
[Illustration: _=Fig. 4=_
_=Fig. 5=_
TWO SIMPLE FORMS OF RHEOSTATS]
The current passing in at binding-post A travels to J and B, the latter
being the resting-plate for the switch-arm. A move of the arm to C sends
the current up over the first coil and down; then over the second,
third, and fourth coils, and out at G; through plate H (which is the
rest at the right side), and out at I.
A move of the switch-arm to D cuts out the first coil; a move to E, the
first and second coils; and so on until the last plate is reached, when
the current will pass without resistance in at A, through J, and out at
I.
A simple arrangement for a resistance-coil is shown in Fig. 5. This
consists of a set of small metal plates in which two holes are made, one
for a screw, the other for a screw-eye (see Binding-posts, chapter
iii.). Two lines of steel-wire nails are driven along a board, and
German-silver wire is drawn around them in zig-zag fashion, beginning at
the left and going towards the right side of the board. One end of wire
is made fast under the screw-head on plate A. The strand is carried out
around the first nail on the lower row and over the first one on the
upper row, then down, up, down until six nails have been turned. The
wire is then carried down to the screw in plate B, given two turns, and
carried up again and over the nail on the top row, repeating the
direction of zigzag No. 1, until six of them are made. The end of the
wire is then made fast to plate G, and all the screws are driven in to
hold the plates and wire securely.
The inlet wire is attached to A, the outlet to G, and any degree of
resistance can be had by moving the inlet wire to the various plates
along the line, cutting out sections Nos. 1 to 6 as desired.
For heavier wire the arrangement as shown in Fig. 6 should be
satisfactory.
[Illustration: _=Fig. 6=_]
[Illustration: _=Fig. 7=_]
A frame twelve by fifteen inches is constructed of wood three-quarters
of an inch thick and one inch and a quarter wide, having the ends
securely fastened with glue and screws. Spirals are wound of
German-silver wire (any size from No. 16 to 22), and drawn apart. The
ends are caught to the frame with small staples, and each alternate
coil-end is joined, as shown in Fig. 6. The leading-out wires to the
contact-points on the switch should be of insulated copper, and are to
run down the sides of the frame, and so to the switch-board. To clearly
illustrate, however, the plan of wiring, the drawing is made so as to
show the leads from the coil-ends to the switch. Care should be taken to
study this drawing well, so as not to make an error in connecting a
wrong end to a contact-point, thereby causing a short circuit. When
properly connected the current passes in at A and out at I; but if wires
are improperly connected, the current will jump when the switch-arm
reaches the misconnected contact.
The switch is an important part of every rheostat, and should be
carefully and accurately made. One of the simplest and most practical
switches is constructed from a short, flat bar of brass or copper having
a knob attached at one end and a hole provided at the other through
which a screw may pass (see Switches, chapter iii.). The contact-points
are made from one or two copper washers, with the holes countersunk so
that a machine screw of brass, with a flat head, will fit the hole
snugly. The top of the head will then be flush with the top of the
washer, as shown at Fig. 7 A. The bolt is passed down through a piece of
board, then slate or soapstone, and caught with a washer and nut, as
shown at Fig. 7 B. A loop of wire is passed about the bolt end, then
another nut is screwed tightly over it to hold it in place, as well as
to lock the first nut. The binding-posts that hold the inlet and outlet
wires may be made of bolts and nuts also, as shown at Fig. 7 B; but the
bolt must be passed through the switchboard so that the head is at the
rear and the ends project out to receive the nuts.
A very compact and simple rheostat and switch is shown in Fig. 8. It is
composed of a base-board, eight blocks of hard-wood, and a top strip
used as a binder to lock the upper ends of the blocks together. The
hard-wood blocks are three-quarters of an inch thick, one inch and a
half wide, and four inches long. A small hole is made near each end of
the block and through one of them an end of the wire is passed. The wire
is then wound round the block, taking care to lay it on evenly, and with
about one-eighth of an inch of space between each strand. When the
opposite hole is reached, pass the end of the wire through it and clip
it. The block will then resemble Fig. 7 C. There should be three or four
inches of wire at each end for convenience in connection, and when the
eight blocks are wound they are to be mounted on end at the rear side of
a base-board measuring ten inches long, three inches wide at the ends,
and nine at the middle (or across the face of the switchboard to the
rear edge behind the blocks). Use slim steel-wire nails and glue to
attach the blocks to the base; or slender screws may be employed. Across
the top lay a piece of wood a quarter of an inch in thickness, and drive
small nails or screws down through it and into the blocks.
[Illustration: _=Fig. 8=_
_=Fig. 10=_
_=Fig. 9=_
COMPACT FORMS OF RHEOSTATS]
Connect the ends of the coils together in series, as already described,
and carry the wires under the base-plate in grooves cut with a
[V]-shaped chisel. If the sunken wires are bothersome, the work may be
avoided by running the wires direct to the foot of the contact-points
and elevating the rheostat on four small blocks that may be screwed, or
nailed and glued, under the corners, as shown in Fig. 8. These will
raise the base half an inch or more above the table on which the
rheostat will rest so as to allow room for the under wires.
A rheostat of round blocks standing on end is shown at Fig. 9 A. These
are pieces of curtain-pole, four inches long and wound with loops of No.
16 or 18 wire, as shown at Fig. 9 B. The loop and loose ends are caught
with staples, and when arranged on a base-board they are to be connected
in series as before described. One long, slim screw passed up through
the base-board and into the lower end of the block will hold each block
securely in place. To keep it from twisting, a little glue may be placed
under the blocks so that when the screw draws the block down to the base
it will stay there permanently upon the hardening of the glue. The
leading wires should be slipped under the washers forming the
contact-points of the switch; or they may be carried under the board to
the nuts that hold the lower ends of the bolts.
Another form of rheostat (Fig. 10 A) is made by sawing a one-inch
curtain-pole into four-inch lengths and cross-cutting each piece with
eight or ten notches, as shown at Fig. 10 B. These pieces are screwed
and glued fast along each side of a base-board eight inches wide and
fourteen inches long, so that the notches face the outer edges of the
board. The strand of wire passes round these upright blocks and fits
into the notches so as to prevent them from falling down.
The top end of wire at each pair of blocks is made fast by a turn or two
of another piece of wire and a twist to hold it securely; then the
loose end is carried down through a hole and along under the board to
the foot of a contact-point.
Any number of these upright coils may be made, and on a long board the
switch may be arranged at one side instead of at the end, as shown in
Fig. 10 A. When making ten or more coils it is best to use three or four
sizes of wire, beginning with fine and ending with coarse. For instance,
in a twelve-coil rheostat make three coils of No. 26, three of No. 22,
and three of No. 18; or if coarser wire is required use Nos. 20, 16, and
12.
German-silver comes bare and insulated. It is preferable to have the
fine wire insulated, but the heavier sizes may be bare, as it is
cheaper; moreover, if heated too much the insulation will burn or char
off. When cutting out the coils always begin at the end where the finer
wire is wound; then as the current is admitted more freely the heavier
wires will conduct it without becoming overheated.
For running a sewing-machine, fan, or other small direct-current motor
wound for low voltage, the house current (if electric lights are used in
the house) may be brought down to the required voltage with
German-silver rheostats similar to these already described. Another and
very simple method is to arrange sixteen-candle-power lamps in series,
as shown in Fig. 11. Six porcelain lamp-sockets are screwed fast to a
wood base and the leading in and out wires brought to binding-posts or
the contact-points of a switch. The leading-in wire to the series is
made fast at binding-post A, which in turn is connected with screw B,
under the head of which the switch-arm is held. When the switch is
thrown over to contact-point C the current passes through lamp No. 1
back to point D; through lamp No. 2 back to E; then through lamps Nos.
3, 4, 5, and 6, and out through point I to post J. A turn of the switch
to D cuts out lamp No. 1, to E cuts out No. 2, and so on. The filaments
of incandescent lamps in their vacuum are among the very best mediums of
resistance, and with a short series of lamps a current of 220 volts can
quickly be cut down to a few volts for light experimental work or to run
some small piece of apparatus.
[Illustration: _=Fig. 11=_]
[Illustration: _=Fig. 12=_]
[Illustration: _=Fig. 14=_]
Lamps in series are often used to cut down the current for operating
electric toys and trains. The adjustment of the current should never be
left to children, however, but should be attended to by some one
qualified to look after the apparatus. Otherwise an unpleasant or even
dangerous shock may be received. Another simple form of resistance
apparatus is made from the carbon pencils used for arc lights. Short
pieces will answer very well, but if the long bare ones can be had they
will be found preferable. Do not use the copper-plated ones as they
would conduct the current too freely; they should be bare and black. Now
around the ends of each piece take several turns of copper wire for the
terminals and cut-out wires. Fasten those pencils down on a board (as
shown at Fig. 12) by boring small holes through the board, passing a
loop of copper wire down through the holes, and giving the ends a twist
underneath. The leading wires to and from the contact-points should be
insulated and may be above or below the board. From the descriptions
already given, the connections of this rheostat can readily be
understood.
The rheostat shown in Fig. 13 is perhaps the most complete and practical
apparatus that a boy could make or would need. It is composed of a
frame, six porcelain tubes, a switchboard, and the necessary
German-silver and copper wire.
From an electrical supply-house obtain six porcelain tubes fourteen by
three-quarter inch. Porcelain tubes and rods warp in the firing and are
seldom straight; in purchasing these select them as nearly perfect as
possible in shape, size, and length.
[Illustration: _=Fig. 13=_
A PANEL RHEOSTAT]
Buy, also, twelve small porcelain knobs that are the right size to fit
inside the large tubes. These should have holes bored through them to
admit screws. Construct a frame of hard-wood to accommodate the tubes,
as shown in the drawing, and leave one end loose. With slim screws make
the porcelain knobs fast to the top and bottom strips of the frame, as
shown in Fig. 14. The porcelain rods will fit over these and will thus
be held securely in the frame, one small knob entering the tube at each
end, as indicated by the dotted lines in Fig. 14.
The first porcelain tube to the left is wound with No. 22 German-silver
wire, the next with No. 20, the third with No. 18, then Nos. 16, 14, and
12; so that in this field a broad range can be had for a current of 110
volts.
The coils are connected in series, as explained for the other rheostats,
and the leading wires brought down to the back of a switchboard of which
Fig. 13 A is the front and Fig. 13 B the rear view. The switchboard is
made of thin slate or soapstone; or a fibre-board may be employed.
Fibre-board is especially made for electrical work, and can be had from
a large supply-house in pieces of various thickness, three-eighths of an
inch being about right for this board. Brass bolts and nuts and copper
washers are used for the contact-poles, and when the ends of the leading
wires are looped around the bolts the nuts are to be screwed down
tightly so as to make good contacts. This rheostat may be used when
lying on a table, or it can be hung up by means of two screw-eyes driven
in the top of the frame, as shown in Fig. 13 A.
A convenient form of rheostat for fine wire and high resistance is shown
in Fig. 15. This is on the plan of the well-known Wheatstone rheostat
and does not require a switchboard nor a series of coils. Two rollers,
one of wood the other of metal or brass-covered wood, are set in a
frame, and by means of a handle and projecting ends with square
shoulders, one or the other of the rollers may be turned so that the
wire on one winds up while on the other it unwinds.
The wooden roller may be made from a piece of curtain-rod one inch in
diameter, and it should have a thread cut on it. This will have to be
done on a screw-cutting lathe, and any machinist will do it for a few
cents. There should be from twelve to sixteen threads to the inch--no
more--although there may be as few as eight. Twelve will be found a good
number, as that does not crowd the coils and the risk of their touching
is minimized. The ends of the roller should have bearings that will fit
in holes made in the end-pieces of the frame, and at one end of each
roller a square shoulder is to be cut, as shown at A in Fig. 16. A short
handle may be made from two small pieces of wood, as shown at B in Fig.
16. It must be provided with a square hole so that it will fit on the
roller ends. The metal roller may be made from a piece of light brass
tubing one inch in diameter through which a wooden core is slipped; or
it can be a piece of brass-covered curtain-pole with the ends shaped the
same as the wooden one. The wood roller should have a collar of thin
brass or copper (or other soft metal except lead) attached to the front
end; or several turns of wire may be made about the roller so as to form
a contact-point. A piece of spring brass, copper, or tin rests on this
collar and is held fast under a binding-post, which in turn is screwed
to the wooden frame. A similar strip of spring metal is held under
another post on the opposite side of the frame and bears on the metal
roller.
[Illustration: _=Fig. 15=_]
[Illustration: _=Fig. 16=_]
German-silver wire is wound on the wooden roller, one end having been
made fast to the metal collar; and when all the thread grooves on the
wood roller are filled the opposite end of the wire is attached to the
rear end of the metal roller. The current entering at binding-post No. 1
crosses on the strip of spring metal to the collar, travels along the
coil of wire, and crosses to the metal roller and is conducted out at
binding-post No. 2 (see Fig. 15). If the resistance is too great slip
the handle over the end of the metal roller and give it several turns.
The current will then pass with greater freedom as the wire on the
wooden roller becomes shorter. This may be readily seen by connecting a
small lamp in series with a battery and this rheostat. As the metal
cylinder is turned the current flows more freely and the filament
becomes red, then white, and finally burns to its full capacity. Take
care, however, not to admit too much current as it will burn out the
lamp. Some sort of adjustment should be made to prevent the rollers
turning of themselves and thus allowing the wire coils to slacken. This
may be done by boring the two holes for the rollers to fit in and then,
with a key-hole saw, cutting the stick as shown at C in Fig. 16, taking
care not to split it at the ends. The result will be a long slot which,
however, has nothing to do with the bearings. Down through the middle of
the stick make a hole with an awl, so that the screw-eye will move
easily in the upper half but will hold in the lower half. Under the head
of the eye place a small copper washer; then with the thumb and finger
drive the screw-eye down until the head rests on the washer.
A slight turn of the eye when it is in the right place will draw the
upper and lower parts of the stick together and bind the wood about the
bearing ends of the rollers. The rollers should not be held too tightly
as that would strain the wire when winding it from one to the other. It
should be just tight enough to keep the wire taut.
Two or more of these roller resistance-frames may be made and connected
in series so that a close adjustment can be had when using battery
currents for experimenting.
Liquid Resistance
Apart from metallic, mercurial, or carbon resistance a form of liquid
apparatus is frequently used in laboratory and light experimental work.
[Illustration: _=Fig. 17=_]
[Illustration: _=Fig. 19=_]
This style of resistance equipment is the least expensive to make, and
will give excellent satisfaction to the boy who is using light currents
for induction-coils, lamps, galvanometers, and testing in general. The
simplest form of liquid resistance is made by using a glass bottle with
the upper part cut away. The cutting may be done with a steel-wheel
glass-cutter. The bottle should then be tapped on the cut line until the
top part falls away. Go over the sharp edges with an old file to chafe
the edge and round it; then solder a tin, copper, or brass disk to a
piece of well-insulated wire and drop it down in the bottom of the
receptacle, as shown at Fig. 17. Cut a smaller disk of metal, or use a
brass button, and suspend it on a copper wire which passes through a
small hole in a piece of wood at the top of the jar. Notches should be
cut at the under side of this wood cross-piece so that it will fit on
top of the jar and stay in place. The jar is to be nearly filled with
water, having a teaspoonful of sulphate of copper dissolved in it. This
will turn the water a bluish color and make it a slightly better
conductor, particularly when the button is lowered close to the round
disk. If a high resistance is desired the copper may be omitted leaving
the water in its pure state. The wires leading in and out of the jar
should be connected between the apparatus and the battery so that the
proper amperage can be had by raising or lowering the button. A series
of these liquid resistance-jars may be made of glass tubes an inch in
diameter and twelve inches long. One end of them may be stopped with a
cement made of plaster of Paris six parts, ground silex or fine white
sand two parts, and dextrine two parts. Mix the ingredients together
when dry, taking care to break all small lumps in the dextrine; then add
water until it is of a thick consistency like soft putty. Solder the
ends of some copper wires to disks of copper or brass and set them on
the middle of bone-buttons; these in turn are to be imbedded in the
mixture after the wire has been passed through a hole in the bottom.
Their location can be seen in the bottom of the tubes Fig. 18, and Fig.
19 A is an enlarged figure drawing of the plate, button, and wire. The
wires are brought out under the lower edge of the tubes, and enough of
the composition is floated about the bottom and outer edge of the tube
to form a base, as shown in the drawing. A base-board is made six inches
wide and long enough to accommodate the desired number of tubes. Two
pieces of wood one inch wide and three-quarters of an inch thick have
hollow notches cut from them at one side, as shown at Fig. 19 B. In
these notches the tubes are gripped. Screws are passed through one stick
and into the other so as to clamp the wood and tubes securely together.
The rear stick is supported on two uprights which are made fast to the
rear edge of the base-plate with screws and glue.
Along the front of the base-board small metal contact plates, or
binding-posts, are arranged (see Binding-posts, chapter iii.) and the
wires led to them from the tubes, as shown in the drawing. The top or
drop wires in the tubes are provided with metal buttons at the ends; or
the end of the wire may be rolled up so as to form a little knob. The
manner of connecting the wires was freely explained in the
resistance-coil descriptions and may be studied out by examining the
drawing closely. In this resistance-apparatus there are two ways of
cutting out a medium--first, by lowering the wire in the tube so that
both contact-points meet; and second, by cutting out the first tube
altogether by connecting the incoming wire with the second binding-post.
Then again the resistance may be regulated quite accurately by raising
or lowering the wires in the liquid.
For example, there is too much resistance if the current has to travel
through all the tubes. If it is too strong when one tube is cut out, the
wire in tube No. 1 is lowered so that the contacts are an inch apart.
Then the more accurate adjustment is made by dropping the wire in the
second tube, as shown in Fig. 18. The wires leading out at the top of
the tubes are pinched over the edge to hold them in place. They should
be cotton insulated and the part that is in the liquid should be coated
with hot paraffine.
The water may be made a slightly better conductor if a small portion of
sulphate of zinc, or sulphate of copper, is added to each tubeful.
[Illustration: _=Fig. 18=_]
Hittorf’s resistance-tube is one of the oldest on these lines, and two
or more of them are coupled in series, as described for this water-tube
resistance; glass tubes are employed that have one end sealed with a
permanent composition, as described for Fig. 18. A metallic cadmium
electrode is placed at the bottom of the tube, and the tube is then
filled with a solution of cadmium iodide one part and amylic alcohol
nine parts, and then corked. A wire passing down through or at the side
of the cork is attached to another small piece of metallic cadmium,
which touches the top of or is suspended a short distance in the liquid.
As the alcohol is volatile the cork cannot be left out of the tube, and
the wire must be drawn through the cork with a needle so that no opening
is left for evaporation. A number of these tubes may be made and coupled
in series and the wires led down to the contact-points of a switch.