THE TELEPHONE
For direct communication over short or moderately long distances,
nothing has been invented as yet that will take the place of the
telephone. A few years ago, when this instrument was first brought out,
it was the wonder of the times, just as wireless telegraphy is to-day.
Starting with the simple form of the two cups with membranes across the
ends, and a string or a wire connecting them, we have to-day the complex
and wonderful electric telephone, giving perfect service up to a
distance of two thousand miles. Some day inventors in the science of
telephony will make it possible to communicate across or under the
oceans, and when the boys of to-day grow to manhood they should be able
to transact business by ’phone from San Francisco to the Far East, or
from the cities near the Atlantic coast to London, Paris, or Berlin.
It is hardly necessary to enter into the history of telephones, as this
information may be readily found in any modern encyclopædia or reference
work. But the boy who is interested in electricity wants to know how to
make a telephone, and how to do it in the up-to-date way, with the wire
and ground lines, switches, cut-outs, bell connections, and other vital
parts, properly constructed and assembled. In this laudable ambition we
will endeavor to help him.
The general principle of the telephone may be explained in the statement
that it is an apparatus for the conveyance of the human voice, or indeed
any sounds which are the direct result of vibration.
Sound is due to the vibrations of matter. A piano string produces sound
because of its vibration when struck, or pulled to one side and then
released. This vibration sets the air in rapid motion, and the result is
the recording of the sound on our ear-drums, the latter corresponding to
the film of sheepskin or bladder drawn over the hollow cup or cylinder
of a toy telephone. When the head of a drum is struck with a small stick
it vibrates. In this case the vibrations are set in motion by the blow,
while in the telephone a similar phenomenon is the result of vibratory
waves falling from the voice on the thin membrane, or disk of metal, in
the transmitter. When these vibrations reach the ear-drum the nervous
system, corresponding to electricity in the mechanical telephone,
carries this sound to our brains, where it is recorded and understood.
In the telephone the wire, charged with electricity, carries the sound
from one place to another, through the agencies of magnetism and
vibration.
Over short distances, however, magnetism and electricity need not be
employed for the transmission of sound. A short-line telephone may be
built on purely vibratory principles. Almost every boy has made a
“phone” with two tomato-cans over which a membrane is drawn at one end
and tied. The middle of the membrane is punctured, and a button, or
other small, flat object, is arranged in connection with the wires that
lead from can to can.
A Bladder Telephone
A really practical talking apparatus of this simple nature may be made
from two fresh beef bladders obtained from a slaughter-house or from the
butcher. You will also need two boards with holes cut in them, two
buttons, some tacks, and a length of fine, hard, brass, copper, or
tinned iron wire. The size should be No. 22 or No. 24. The boards should
be ten by fourteen inches and half an inch in thickness. Cut holes in
them eight inches in diameter, having first struck a circle with a
compass. This may be done with a keyhole saw and the edges sand-papered
to remove rough places. Prepare the bladders by blowing them up and
tieing them. Leave them inflated for a day or two until they have
stretched, but do not let them get hard or dry.
When the bladders are ready, cut off the necks, and also remove about
one-third of the material, measuring from end to end. Soak the bladders
in warm water until they become soft and white. Stretch them, loosely
but evenly, over the opening in the boards, letting the inside of the
bladder be on top, and tack them temporarily all around, one inch from
the edge of the opening. Test for evenness by pushing down the bladder
at the middle. If it stretches smoothly and without wrinkles it will do;
otherwise the position and tacks must be changed until it sets perfectly
smooth.
[Illustration: _=Fig. 1=_]
[Illustration: _=Fig. 2=_]
[Illustration: _=Fig. 3=_]
[Illustration: _=Fig. 4=_]
The bladder must now be permanently fastened to the board by means of a
leather band half an inch wide and tacks driven closely, as shown in
Fig. 1. With a sharp knife trim away the rough edges of the bladder that
extend beyond the circle of leather. Attach a piece of the fine wire to
a button, as shown in Fig. 2, and pass the free end through the centre
of the bladder until the button rests on its surface. Then fasten an
eight-pound weight to the end of the wire and set in the sun for a few
hours, until thoroughly dry, as shown at Fig. 3.
When both drums are complete, place one at each end of a line, and
connect the short wires with the long wire, drawing the latter quite
taut. The course of the main wire should be as straight as possible, and
should it be too long it may be supported by string loops fastened to
the limbs of trees, or suspended from the cross-piece of supports made
in the form of a gallows-tree or letter F. To communicate it will be
necessary to tap on the button with a lead-pencil or small hard-wood
stick. The vibration will be heard at the other end of the line and will
attract attention.
By speaking close to the bladder in a clear, distinct tone, the sound
will carry for at least a quarter of a mile, and the return vibrations
of the voice at the other end of the line can be clearly recognized.
A Single (Receiver) Line
The principal parts of the modern telephone apparatus are the
transmitter, receiver, induction-coil, signal-bell, push-button,
batteries, and switch. The boxes, wall-plates, etc., etc., are but
accessories to which the active parts are attached.
The first telephone that came into general use was the invention of
Graham Bell, and the principle of his receiver has not been materially
changed from that day to this, except that now a double-pole magnet and
two fine wire coils are employed in place of the single magnet and one
coil. A practical form of single magnet receiver that any boy can
easily construct is shown in Fig. 4, and Fig. 5 is a sectional drawing
of the receiver drawn as though it had been sliced or sawed in two, from
front to rear.
It is made from a piece of curtain-pole one inch and an eighth in
diameter and three inches and a half long. A hole three-eighths of an
inch in diameter is bored its entire length at the middle, and through
this the magnet passes. At one end of this tube a wooden pill-box (E) is
made fast with glue, or a wooden cup may be turned out on a lathe and
attached to the magnet tube. If the pill-box is employed it should be
two inches and a half in diameter, and at four equidistant places inside
the box small lugs of wood are to be glued fast. Into these lugs the
screws employed to hold the cap are driven. The walls of pill-boxes are
so thin that without these lugs the cap could not be fastened over the
thin disk of metal (D) unless it were tied or wired on, and that would
not look well. If the cup is turned the walls should be left thick
enough to pass the screws into, and the inside diameter should then be
one inch and three-quarters.
[Illustration: _=Fig. 5=_]
[Illustration: _=Fig. 6=_]
The cap (B) is made from thin wood, fibre, or hard rubber. It is
provided with a thin rim or collar to separate its inner side from the
face of the disk (D). Four small holes are bored near the edge of this
cap, so that the screws which hold it fast to the cup (E) may pass
through them. The magnet (M) is a piece of hard steel three-eighths of
an inch in diameter and four inches and a quarter long. This may be
purchased at a supply-house, and if it is not hard enough a blacksmith
can make it so by heating and plunging it in cold water several times.
It may be magnetized by rubbing it over the surface of a large horseshoe
magnet, or if you live near a power station you can get one of the
workmen to magnetize it for you at a trifling cost. Should you happen to
possess a bar magnet of soft iron with a number of coils of wire, and
also a storage-battery, the steel bar may be substituted for the soft
iron core and the current turned on. After five minutes the steel can
be withdrawn. It is now a magnet, and will hold its magnetism
indefinitely.
Now have a thin, flat spool turned from maple or boxwood to fit over one
end of the rod, and wind it with a number of layers of No. 36 copper
wire insulated with silk. This is known in the electrical supply-houses
as “phone”-receiver insulated wire, and will cost about fifty cents an
ounce. One ounce will be enough for two receivers. It should be wound
evenly and smoothly, like the strands of thread on a spool, and this may
be done with the aid of the winder described on page 58.
When the wire is in place a drop of hot paraffine will hold the end so
that the wire will not unwind. The ends of this spool-winding should be
made fast to heavier wires, which are run through small holes in the
tube (A) and project out at the end, as shown at F F. The magnet, with
its wire-wound spool on the end, is then pushed through the hole in A
until the top end of the rod is slightly below the edges of the cup (E),
so that when the metal disk (D) is laid over the cup (E) the space
between the magnet and disk, or diaphragm (D), is one-sixteenth of an
inch (see Fig. 5). Put some shellac on the magnet, so that when it is in
the right place the shellac will dry and hold it fast.
The cap (B) holds the disk (D) in place, and protects the spool and its
fine wire from being damaged and from collecting dust. After giving the
exterior a coat of black paint and a finishing coat or two of shellac,
the receiver will be ready for use.
The original telephone apparatus was made up of these receivers
only--one at each of a line in connection with a battery, bell,
push-button, and switch. On a window-casing, or the wall through which
the wires passed, a lightning-arrester was arranged and made fast. Using
receivers only, it was necessary to speak through the same instrument
that one heard through, and for a few years this unhandy method of
communication was the only one possible. Then the transmitter was
invented.
Plan of Installation
Many of these single-receiver lines are still in use, and as they
require but a small amount of constructive skill a diagram of the wiring
and the plan of arrangement is shown in Fig. 6.
At the left side, R is the receiver at one end of the line and R 2 that
at the other, line No. 1 being a continuous wire between the two
receivers. When the boy at R wishes to call his friend at R 2 he uses
his push-button (P B), and the battery (B B) operates the electric bell
(E B 2) at the other end. In order to have the bell connections
operative, the switch (S 2) must be thrown over to the left when the
line is “quiet,” while the switch (S) should be thrown to the right.
With the switches in this position the boy at either end may call his
friend at the opposite end.
With the switch (S 2) thrown to the left (the position it should be in,
except when talking over the line), the boy at the other end pushes his
button (P B), first throwing switch S to the left. This makes connection
for the battery (B B), and the circuit is closed through wires that join
line No. 1 and line No. 2 at 1 and 2. The branch lines to the bell (E B
2) join the main lines at 3 and 4, through switch S 2, when the bar is
thrown to the left. The circuit being complete, the batteries (B B) at
one end of the line ring the bell (E B 2) at the other end of the line.
In the reverse manner, when the switch (S) is thrown to the right, the
boy at the opposite end rings the bell (E B) by pressing on the button
(P B 2), first throwing switch S 2 over to the right. If the boy at the
left is calling up the boy at the right, the switch (S) should be thrown
to the left, and he keeps ringing until the other operator throws switch
S 2 over to the right. If now he has the receiver (R) up to his ear he
can hear the vibration of the bell (E B 2) ringing through the receiver
(R) at his end of the line. But when the boy summoned to R 2 takes up
the receiver and places it to his ear, he throws switch S 2 over to the
right side, and the boy at R leaves switch S over on the left side. This
brings the lines into direct connection with the receivers in series. Be
careful, when setting up this line, to have the batteries (B B) in
series with B 2 B 2; otherwise there would be counter-action. The carbon
of one cell should be connected with the zinc of the next cell, and so
on.
Another receiver is shown at Fig. 7. The tube (A) and the cup are turned
from one piece of wood, and the cap (B) from another piece. The length
of the receiver is five inches, and the cap is two inches and a half
across. The shank, or handle, through which the magnet is passed
measures one inch and a quarter in diameter.
These wood parts will have to be made by a wood-turner; and before the
long piece is put in a lathe the hole, three-eighths of an inch in
diameter, should be bored. It must be done carefully, so that the wood
shell will be of even thickness all around the hole. Also two small
holes should be made the entire length of the handle, through which the
wires leading from the coil to the binding-posts may pass.
[Illustration: _=Fig. 7=_]
The spool for the fine insulated wire coil is turned from box-wood or
maple, and wound as described in chapter iv., on Magnets and
Induction-coils. Small binding-posts (F F) with screw ends should be
driven down into the holes at the end of the handle and over the bare
ends of the wires that project out of the holes. The magnet (M) is
three-eighths of an inch in diameter, and is provided with the spool and
coil (C) at the large end of the receiver.
The disk (D) is of very thin iron, and is held in place by the cap (B)
and four small brass screws driven through the edge of B and into the
cup end of A. A screw-eye should be driven into the small end of the
receiver from which it may hang from a hook. If a double hook and bar is
employed the receiver will hang in the fork, being held there by the
rim of wood turned at the small end of A.
A Double-pole Receiver
[Illustration: _=Fig. 8=_]
Another form of receiver is shown at Fig. 8. This is a double-pole
receiver, with the coils of fine wire arranged on the ends of a bent
band of steel and located in the cup (A), so that the ends of the magnet
are close to the diaphragm (D). Fig. 8 is a sectional view of an
assembled receiver, but a good idea can be had from the drawings of the
separate parts. The magnet (M) is of steel one-eighth of an inch thick
and five-eighths of an inch wide. A blacksmith will make this at a small
cost. It should measure two and one-half inches wide, two and one-half
inches long, the ends being five-eighths of an inch apart.
Thin wooden spools are made from wood or fibre to fit over the steel
ends, and are wound with No. 36 silk-insulated wire. A wooden cup, or
shell (A), is turned from cherry, maple, or other close-grained wood,
and at the back a hole is cut just large enough for the magnet ends to
slip through exclusive of the coils wound on them. A plug of wood (A A)
is driven between the ends of the magnet to hold them in place. Some
shellac on the edges of the hole and the plug will harden and keep the
parts in place.
The coils (C C) are placed on the magnet ends, and the fine wires are
made fast to the binding-posts (E E), the latter being screwed fast to
the shell (A). The diaphragm (D) is then arranged in place and held with
the cap (B) and the small screws which pass through it and into the
shell (A).
The Transmitter
With any one of these receivers a more complete and convenient telephone
can be made by the addition of a transmitter and an induction-coil.
Following the invention of the receiver, several transmitters were
designed and patented, among them being the Edison, Blake, Clamond,
Western Union, and Hunning. The Edison and Hunning are the ones in
general use, and as either of them can easily be made by a boy a
simplified type of both is shown in Figs. 9 and 11.
[Illustration: FIG. 9
FIG. 10
FIG. 11
FIG. 12
SIMPLIFIED TYPE OF TRANSMITTER]
Some small blocks of wood, tin funnels, small screws, granulated or
powdered carbon, some thin pieces of flat carbon, and a piece of very
thin ferrotype plate will be the principal things needed in making a
transmitter similar to the one shown in Fig. 9. All that is visible from
the outside is a plate of wood screwed to a block of wood, and a
mouth-piece made fast to the thin board.
In Fig. 10 an interior section is shown, which when once understood will
be found extremely simple. The block (A) is of pine, white-wood, birch,
or cherry, and is two inches and three-quarters square and five-eighths
or three-quarters of an inch thick. A hole seven-eighths of an inch in
diameter is bored in the centre of this block, half an inch deep, and a
path is cut at the face of the block one inch and a half in diameter and
one-eighth of an inch deep. Be careful to cut these holes accurately and
smoothly, and if it is not possible to do so, it would be well to have
them put in a lathe and turned out.
The face-plate (B) is two inches square, with a three-quarter-inch hole
in it, and the under-side is cut away for one-eighth of an inch in depth
and one inch and a half in diameter. The object of these depressions in
block A and face-plate B is to give space for the diaphragm (D) to
vibrate when the voice falls on it through the mouth-piece (C).
From carbon one-eighth of an inch in thickness two round buttons are cut
measuring three-quarters of an inch across. A small hole is bored in the
centre of each button, and one of them is provided with a very small
brass screw and nut, as shown at F F. One side of the button-hole is
countersunk, so that the head of the screw will fit down into it and be
flush with the face of the carbon. With a small three-cornered or square
file cut the surface of the buttons with criss-cross lines, as shown at
F F. When the buttons are mounted in the receiver these surfaces will
face each other. Cut a small washer from felt or flannel, and place it
in the bottom of the hole in block A. Line the side of the hole with a
narrow strip of the same goods; then place the button (F F) in the hole,
pass the screw through the hole and through the block (A), and make it
fast with the nut, as shown at F. Place a thin, flat washer under the
nut, and twist a fine piece of insulated copper wire between washer and
nut for terminal connections, taking care that the end of the wire under
the nut is bare and bright, so that perfect contact is assured. Since
the practice of telephony involves such delicate and sensitive vibratory
and electrical phenomena, it is best to solder all joints and unions
wherever practicable, and so avoid the possibility of loose connections
or corrosion of united wires.
From very thin ferrotype plate cut a piece two inches square, and at the
middle of it attach the other carbon button by means of a small rivet
which you can make from a piece of copper wire. Or a very small brass
machine screw may be passed through the button and plate; then gently
tapped at the face of the plate to rivet it fast, as shown at E. Lay the
block down flat and partly fill the cavity with carbon granules until
the button is covered. Do not fill up to the top of the hole. Over this
lay the disk (D), so that the carbon button at the under side of it will
fit in the top part of the hole between the sides of felt or flannel.
Make the disk fast to the block (A) with small pins made by clipping
ordinary pins in half and filing the ends.
A slim bolt (G) is passed through the block (A), and a wire terminal is
caught under a nut and between a washer at the back of the block, as
described for F. The japan or lacquer must be scraped away from the disk
(D) where the bolt-head touches it, so that perfect electrical contact
will be the result.
A small tin funnel is cut and made fast to the face-plate (B), or if an
electrical supply-house is at hand a mouth-piece of hard rubber or
composition may be had for a few cents. The block (B) is then screwed
fast to A, forming the transmitter shown at Fig. 9. When this
transmitter stands in a vertical position the granules, or small
particles of carbon, drop down between the buttons of carbon, packing
closely at the bottom of the cavity. At the middle they are loosely
placed, and at the top there are none. As the high or low vibrations of
the voice fall on the disk (D) they act accordingly on the carbon
granules, which in turn conduct the vibrations to the rear carbon
button, and, by the aid of electricity reproduce the same sound, in high
or low tone, through the receiver at the other end of a line.
This improved transmitter makes it possible to talk in a moderate tone
of voice over distances up to one thousand miles, while with the old
form of the instrument it was necessary to talk very loud in order to be
heard only a few miles away. Where a portable apparatus is desired, this
block may be attached to a box or an upright staff.
This transmitter will not work when on its back or so that the funnel is
on top, because the particles of carbon would settle on the rear button
and not touch the front one. It is essential that the carbon grains
should touch both buttons at the same time, and at the lower part of the
cavity they should lie quite solid. It is not necessary, however, to
pack it in, for the vibratory action of the voice, or other sounds, will
cause the particles to adjust themselves and settle in a compact mass.
Another Form of Transmitter
In Fig. 11 another style of transmitter is shown. It is assembled on the
front of a box. This front or cover swings on hinges, and can be opened
so that the mechanism in the interior of the box may be gotten at
easily.
A sectional view of this transmitter is shown in Fig. 12. A hole one
inch and a half in diameter is cut in the cover (A). A round or square
block (B) two inches and a quarter across and half an inch thick is made
fast to the rear of the cover, and in this a hole is bored seven-eighths
of an inch in diameter and one-quarter of an inch deep.
The sides and bottom of this hole are lined with flannel or felt, and a
carbon button with roughened surface, as shown at F F, is made fast in
it by a small machine screw and nut (F). A diaphragm (D) is cut from
thin ferrotype plate, and a carbon button is made fast to the middle of
it by a small machine screw or a rivet made from soft copper or brass.
When the block (B) has been screwed fast to A, place some granules of
carbon in the space (H); then lay the diaphragm over the opening, and
make it fast with small screws or pins driven around the edge.
From a small tin funnel and a tin-can cap make a mouth-piece (C) by
cutting a hole in the cap and slipping the funnel through it, then
cutting the end of the funnel that projects through the hole and bending
back the ears so that they lap on the inner side of the cap. These small
ears may be soldered to the cap so as to hold the mouth-piece securely
in place. From felt or flannel cut a washer the size of the can top and
about three-eighths of an inch in width. Lay this over the diaphragm;
then place the mouth-piece on it and fasten it to the door (A) with
small screws. The use of this washer is to prevent any false vibrations
in the mouth-piece affecting the sensitive diaphragm. Make a small hole
through A and B and pass a bolt (E) through this hole, taking care to
lap a thin piece of sheet-brass on the diaphragm (D), bending it over so
that it will lie under the head of the bolt (E). The diaphragm must be
scraped where the metal touches it, so as to make perfect electrical
connection between D and E. At the rear end of E arrange a washer and
nut (G), so that the current passing in at G travels through E and D,
then through the carbon buttons and granules, and out at F.
From pine or white-wood one-quarter or three-eighths of an inch thick
make a box four inches wide, six inches high, and two inches and a half
deep. To the front of this attach a cover, which should measure a
quarter of an inch larger all around than the width and height of the
box. Use brass hinges for this work so that the cover may be opened.
Fasten a transmitter to the front of the cover, or make one on the
cover, as shown in Fig. 11, and attach the box to a back-board or
wall-plate five inches wide and seven inches high made of pine or
white-wood half an inch in thickness (see Fig. 13).
[Illustration: FIG. 13]
[Illustration: FIG. 14]
[Illustration: FIG. 15]
[Illustration: FIG. 16]
At the left side of the box cut a slot through the wood, so that a lever
and hook may project and work up and down. The end of this lever is
provided with a hook on which a receiver may be hung, as shown in Fig.
13, and the inside mechanism is arranged as shown at Fig. 14. A is an
angle-piece of brass or copper, which acts as a bracket and which is
screwed fast to the inside of the box. B is the lever and hook, which is
cut from a strip of brass. The attached end is made wider, and an ear
(C), to which a wire is soldered, projects down beyond the screw.
A view looking down on this lever and bracket is shown at Fig. 15. A is
the bracket, B the lever, and E the screw or bolt holding the two parts
together, with a thin copper washer between them to prevent friction.
When the lever and bracket are made fast to the box, a spring (D) should
be arranged, so that when the receiver is removed from the hook the
lever will be drawn up to the top of the slot. A small contact-plate (F)
is made of brass, and fastened at the lower end of the slot. On this the
lever should rest when the receiver is on the hook. A contact-wire is
soldered to this plate, which in turn is screwed fast to the inside of
the box. This mechanism is part of a make-and-break switch to cut out
and cut in the bells or telephone, and will be more clearly understood
by referring to the diagram in Fig. 17. At the right side of the box a
small push-button is made fast, and this, with two binding-posts at the
top and four at the underside of the box, will complete the exterior
equipment of one end of a line.
The construction of the push-button is shown in Fig. 16, A being the box
and B the button which passes through a small hole made in the side of
the box. C is a strip of spring-brass screwed fast to the box. It must
be strong enough to press the small bone or hard rubber button towards
the outside of the box. A wire is caught under one screw-head, and
another one is passed under the screw-head which holds the other spring
(D) to the box. When the button (B) is pushed in, it brings spring C
into contact with D, and the circuit is closed. Directly the finger is
removed from B the spring (C) pushes it out and breaks the circuit. This
button is used only in connection with the call-bells, and has nothing
to do with the telephone. The wires leading from the interior of the box
pass through the wall-plate and along in grooves to the foot of the
binding-posts, which are arranged below the box on the back-board, as
shown in Fig. 13.
A buzzer or bell is made fast to the inside of the box, unless it is too
large to fit conveniently, in which case it may be attached to the wall
above or below the box.
The Wiring System
Fig. 17 shows the wiring system for this outfit, which, when properly
set up and connected, should operate on a circuit or line several miles
in length, provided that the batteries are strong enough.
This system may be installed in the box shown in Fig. 13, the flexible
cord containing two wires being attached to the binding-posts at the top
of the box and to the posts at the end of the receiver. This system
differs from the one shown in Fig. 6 only in the addition of receivers T
and T 2, and in the substitution of the automatic lever-switches (L S
and L S 2) for the plain switches (S and S 2) in Fig. 6. When the line
is “quiet” the receiver (R) should be hanging on the lever-switch (L S),
which rests on the contact-plate (A). At the opposite side of the line
the receiver (R 2) hangs on the lever-switch (L S 2), which in turn
rests on the contact-plate (A A). This puts the bell circuit in
service.
[Illustration: FIG. 17
PLAN OF TELEPHONE CIRCUIT, COMPRISING RECEIVERS, TRANSMITTER, ELECTRIC
BUZZERS OR BELLS, LEVER-SWITCHES, PUSH-BUTTONS AND BATTERIES FOR
STATIONS NOT OVER FIVE MILES APART.]
If the boy at the left wishes to call up the boy at the right he removes
the receiver (R) from the hook (L S) and presses on the button (P B).
This closes the circuit through the battery (C C C), and operates the
electric buzzer or bell (E B 2) at the other end of the system, through
line No. 1 and line No. 2. The operation may be clearly understood by
following the lines in the drawing with a pointer. The boy at the left
may keep on calling the boy at the right so long as the receiver (R 2)
hangs on the lever (L S 2) and holds it down against the plate (A A).
But directly the receiver (R 2) is removed, the lever (L S 2) flies
up--being drawn upward by the spring (D) shown in Fig. 14--and closes
the telephone circuit through the spring-contact (B B), at the same time
cutting out the bell circuit. The boy at the left having already removed
his receiver, the telephone circuit is then complete through lines Nos.
1 and 2 and batteries C C C and C 2 C 2 C 2, the boys at both ends
speaking into the transmitters and hearing through the receivers. The
contacts B and B B are made from spring-brass or copper, and are
attached inside the boxes at the back, so that when the levers are up
contact is made, but when down the circuit is broken or opened. In Fig.
18 an interior view of a box is shown, the door being thrown open and
the receiver left hanging on the hook.
[Illustration: FIG. 18
FIG. 19
FIG. 20
TELEPHONE INSTALLATION. INTERIOR VIEW OF BOX]
The arrangement of the several parts will be found convenient and easy
of access. E B is the electric buzzer, L S the lever-switch, P B the
push-button, T the transmitter, and R the receiver. Nos. 1, 2, 3, 4, 5,
6, 7, 8 are binding-posts or terminals, and B is the spring-contact
against which the lever-switch (L S) strikes when drawn up by the spring
(D).
The wires that pass from 6 to 7 and from 4 to 8 should be soldered fast
to one side of the hinge, and those running from the terminals or nuts
at the back of the transmitter (T) to 7 and 8 should be similarly
secured. Small brass hinges are not liable to become corroded at the
joints, but to insure against any such possibility the ends of several
fine wires may be soldered to each leaf of the hinge, so that when the
door is closed the wires will be compressed between the hinge-plates.
For long-distance communication it will be necessary to install an
induction-coil, so that the direct current furnished by the batteries,
in series with the transmitter, can by induction be transformed into
alternating current over the lines connecting the two sets of apparatus.
This system is somewhat more complicated and requires more care in
making the connections, but once in operation it will be found far
superior to either of the systems hitherto described.
A Telephone Induction-coil
It will be necessary to make two induction-coils, as described in
chapter iv., page 62, Fig. 8. A telephone coil for moderately
long-distance circuits is made on a wooden spool turned from a piece of
wood three inches and a half long and one inch square, as shown at Fig.
19. The core-sheath is turned down so that it is about one-sixteenth of
an inch thick. This spool is given a coat or two of shellac, and two
holes are made at each end, as shown in the drawing. The first winding
or primary coil is made up of two layers of No. 20 double-insulated
copper wire, one end projecting from a hole at one end of the spool, the
other from a hole at the other end. This coil is given two or three thin
coats of shellac to bind the strands of wire and thoroughly insulate
them, and over the layer a piece of paper is to be wrapped and
shellacked. The secondary coil is made up of twelve layers of No. 34
silk-insulated copper wire, and between each layer a sheet of paper
should be wound so that it will make two complete wraps. Each paper
separator should be given a coat of shellac or hot paraffine; then the
turns of wire should be continued just as thread is wound upon a spool,
smoothly, closely, and evenly, until the last wrap is on. Three or four
wraps of paper should be fastened on the coil to protect it, and it may
then be screwed fast inside a box. The core-hole within the coil should
be packed with lengths of No. 24 soft Swedes iron wire three inches and
a half long. In Fig. 19 the wires are shown projecting from the end of a
spool, and Fig. 20 depicts a completed telephone induction-coil. The
installation of the induction-coils is shown in Fig. 21.
[Illustration: FIG. 21
PLAN OF TELEPHONE CIRCUIT, COMPRISING RECEIVERS, TRANSMITTERS, ELECTRIC
BUZZERS OR BELLS, LEVER-SWITCHES, INDUCTION-COILS, PUSH-BUTTONS, AND
BATTERIES FOR STATIONS UP TO FIVE HUNDRED MILES APART.]
The wiring is comparatively simple, and may be easily followed if the
description and plan are constantly consulted when setting up the line.
R and R 2 are the receivers, T and T 2 the transmitters, C 1 and C 2 the
batteries, E B and E B 2 the buzzers or bells, P B and P B 2 the
push-buttons, and L S and L S 2 the lever-switches. For convenience of
illustration the induction-coils are separated. The primary coil (P C)
is indicated by the heavy spring line and the secondary coil (S C) by
the fine spring line. When the line is “dead” both receivers are hanging
from the hooks of the lever-switches. If the boy at the left wishes to
call the boy at the right he lifts the receiver (R) from the hook (L S)
and presses the button (P B). This throws the battery (C 1 C 1 C 1) in
circuit with lines Nos. 1 and 2, and operates the buzzer (E B 2). When
the boy at the right lifts his receiver (R 2) from the hook (L S 2), the
bell circuit is cut out and the ’phone circuit is cut in. When the
lever-switches are drawn up against the contact-springs (A, B, and C and
A A, B B, and C C), both batteries are thrown into circuit with the
transmitters at their respective ends through the primary coils (P C and
P C 2). By inductance through the secondary coils (S C and S C 2), lines
Nos. 1 and 2 are electrified, and when the voice strikes the disks in
the transmitters the same tone and vibration is heard through the
receivers at the other end of the line. While conversation is going on
the batteries at either end are being drawn upon or depleted; but as
soon as the receivers are hung on the hooks and the lever-switches are
drawn away from the contact-springs, the flow of current is stopped. The
buzzers or bells consume but a small amount of current when operated,
and in dry cells the active parts recuperate quickly and depolarize. The
greatest drain on a battery, therefore, is when the line is closed for
conversation.
An Installation Plan
A simple manner in which to install this apparatus in boxes is shown in
Fig. 22. The box is depicted with the front opened and with the receiver
hanging on the hook. When the lever-switch (L S) is down it rests on
the contact-spring (A), thus throwing in the bell circuit. When the boy
at the other end of the line pushes the button on his box it operates
the buzzer (E B). This can be understood by following with a pointer the
wires from the buzzer to the outlet-posts (Nos. 1 and 3) at the bottom
of the wall-plate.
[Illustration: _=Fig. 22=_]
When the receiver (R) is lifted from the hook (L S), it cuts out the
bell circuit and cuts in the telephone circuit, through the
spring-contacts (B and C). This circuit may easily be followed through
the wires connecting transmitter, receiver, induction-coil, and
batteries. The heavy lines leading out from the induction-coil are the
primary coil wires, and the fine hair lines are those forming the
secondary coil. The medium lines are those that connect the
binding-posts, batteries, and lines.
When the bell circuit is connected the impulse coming from the other end
of the line enters through wire No. 10 to post No. 3, thence to strip E
and plate G, and so on to E B, which it operates. The current then
passes from E B to contact A, through L S to post No. 1, and out on wire
No. 11.
To operate the buzzer at other end of the line the button (P B) is
pushed in. This moves the spring (E) away from the plate (G), and brings
it into contact with F. This connects the circuit through the battery
wire (No. 8) to post No. 1 to line No. 11 without going into the box,
and from wire No. 9 to post No. 2; thence to hinge No. 7 to plate F,
through E, down to post No. 3, and out through wire No. 10. In this
manner the current is taken from the batteries at the foot of wires Nos.
8 and 9, and used to ring the buzzer at the other end of the line.
When the hook (L S) is up the circuit is closed through T, I C, and
battery. The current runs from the battery through wire No. 8 to post
No. 1, to L S, through C and primary coil out to hinge No. 6, through
transmitter to hinge No. 7, to post No. 2, and back to battery through
wire No. 9.
By inductance the sound is carried over the line, in at wire No. 10, to
post No. 3, through secondary coil to post No. 4, through receiver R to
post No. 5, through B and L S to post No. 1, and out through wire No.
11. At the other end of the line it goes through the same parts of the
apparatus.
A Portable Apparatus
For convenience it is often desirable to have a portable transmitter,
and so avoid the inconvenience of having to stand while speaking. A neat
portable apparatus that will stand on a ledge or table, and which may be
moved about within the radius of the connecting lines, is shown in Fig.
23.
The wooden base is four inches square and the upright one inch and a
half square. The stand is twelve inches high over all, and on the bottom
a plate of iron or lead must be screwed fast to make it bottom-heavy, so
that it will not topple over.
The lever-switch may be arranged at the back of the upright and the
push-button at the front near the base, as shown at A. The wall-box
contains the buzzer and induction-coil, and within it the wiring is
arranged from the portable stand to the batteries and line as shown at
C. This illustration is too small, however, to show the complete wiring,
and the young electrician is therefore referred to Fig. 22. The battery
(B) is composed of as many dry or wet cells as may be required to
operate the line. These must be connected in series at both ends. At D a
rear view of the upright and transmitter is shown to illustrate the
manner in which the wiring can be done. If a hollow upright is made of
four thin pieces of wood a much neater appearance may be secured by
enclosing the wires.
[Illustration: FIG. 23
A PORTABLE APPARATUS]
In all of these telephone systems one wire must lead to the ground, or
be connected with a water-pipe, taking care, however, to solder the wire
to a galvanized pipe so that perfect contact will be the result. If the
wire is carried directly to the ground it must be attached to a plate,
which in turn is buried deep enough to reach moist earth, as described
in the chapter on Line and Wireless Telegraphs, page 215.
Care and accuracy will lead to success in telephony, but one slip or
error will throw the best system out of order and render it useless.
This, indeed, applies to all electrical apparatus; there can be no
half-way; it will either work or it won’t.