Not many years ago, the telephone was the wonder of the times just as
the aeroplane and the wireless are to-day. The original telephone
apparatus was an exceedingly crude and simple arrangement which has
gradually developed into a wonderful and complex system which makes it
possible to carry on a conversation with almost any other point in the
United States as easily as it is face to face.
[Illustration: FIG. 93.—The Wooden Back for the Telephone.]
Many of my young readers have undoubtedly already constructed a set of
telegraph instruments and rigged up a line with some nearby chum. Like
the telegraph, the principle of the telephone is that of a current of
electricity flowing over a line wire into a pair of electro-magnets, and
it is not any more difficult to install a short telephone line than a
telegraph. Of course there are many important differences between a
telephone and a telegraph but they are not intricate or complex.
The telephone is exceedingly sensitive when compared to the telegraph.
An ordinary telegraph relay requires about one hundredth of an ampere to
operate it, whereas a telephone receiver will produce an audible sound
on a current of less than one millionth of an ampere.
[Illustration: FIG. 94.—The Complete Telephone.]
The telephone may be defined as an apparatus for transmitting speech to
a distance by means of electricity. The part of the apparatus which
takes up the sounds and changes them into electric currents is the
transmitter. Where words are spoken into the mouth-piece of the
transmitter, they strike a thin metal diaphragm, to the back of which is
fastened a small cup-shaped piece of carbon. A second cup is mounted in
a rigid position directly back of the first and the space between is
filled with small polished granules of carbon. When the granules are
undisturbed and are lying loosely together their resistance to an
electric current is so great that they allow almost none to flow.
When the vibrations of the voice strike the diaphragm they move it back
and forth and cause the carbon cup mounted on its back to move and exert
a varying pressure on the granules. When the granules are compressed
their resistance is greatly lowered and they permit current to pass.
That part of the apparatus which transforms the pulsating current back
into sound waves is called the receiver. It consists of a thin iron
disk, placed very near but not quite touching the end of a small
permanent magnet about which is wound a coil of fine insulated wire.
[Illustration: FIG. 95.—Details of the Receiver Hook.]
If the transmitter and receiver are connected together in series with a
battery and words are spoken into the transmitter, the little carbon
granules are immediately thrown into motion, and being alternately
compressed and released cause corresponding changes in the current
flowing through the receiver from the battery.
The strength of the magnetic pull exerted by the combined permanent
magnet and coil of wire varies with each change in the electric current
and thus by alternately attracting and repelling the diaphragm causes it
to vibrate and emit sounds.
This is the general principle of the telephone. The telephones in actual
service to-day are complicated with bells, induction coils, condensers
and relays, which render them more efficient, but the general principle
of the transmission of the speech is the same.
I would not advise the young experimenter to attempt to build his own
transmitter and receiver except for the knowledge and experience gained
thereby. Homemade instruments of this sort are not likely to prove
sensitive enough for practical purposes over a long line.
A first class telephone receiver can be purchased for 90 cents and an
equally good transmitter for $2.25 These, together with an ordinary
electric bell can be built into a first class telephone instrument which
will give excellent service over a long line.
[Illustration: FIG. 96.—Showing how the Push Button is arranged.]
Any boys can easily build and install such an arrangement. When
completed it will prove invaluable as a source of communication from one
part of the house to another, from the house to the barn or between two
chums living in the same neighborhood. Figure 93 shows the wooden back
of the instrument. It is made from a piece of board 12 inches long and 7
inches wide, by cutting into the shape shown. This back serves to mount
the transmitter, receiver, bell, etc.
*The Bell* is for the purpose of giving an audible signal so that the
person hearing it will know that some one at the other end of the line
wishes, to talk.
Figure 94 is a front view of the completed instrument. The bell is
mounted on the upper part of the back as shown in the illustration.
*The Receiver Hook* is shown in detail in Figure 95. It should be cut
out of heavy sheet brass and finished in the shape shown.
The receiver hook is mounted directly under the bell as shown in Figure
94. The hook should project out to the left beyond the edge of the back.
The hook is mounted by a round headed brass screw passing through the
hole in the right hand end.
[Illustration: FIG. 97.—Circuit showing how to connect two Telephone
Stations to the Line.]
The hook should move freely up and down between two contacts (marked 1
and 2 in Figure 94). These contacts are simply two small brass strips
bent at right angles and fastened about an inch apart along the left
hand edge of the wooden back by means of two small round headed brass
wood screws. The position of the contacts should be such that the hook
is in a horizontal position when it is half way between the two. A small
spring is arranged so as to pull the hook up against the upper contact
(No. 1) when the receiver is lifted off. When the receiver is hung on
the hook, it should overcome the lift of the spring and pull the hook
down against the lower contact (No. 2).
*The Receiver* may be purchased at almost any reliable electrical supply
house. It should be provided with a ring so that it can be hung onto the
hook.
*The Transmitter* is preferably of the "solid back" long distance type.
It also can be purchased ready made from almost any electrical supply
house.
The transmitter is mounted on the wooden back directly under the hook.
It is marked T in Figure 94.
*A Push Button*, made out of two round headed brass wood screws and a
strip of spring brass as shown in Figure 96, is mounted to the right of
the transmitter.
One end of the spring is fastened down firmly with a screw, the other
end is bent so that it is directly over but does not touch the second
screw mounted about seven-eighths of an inch away.
Four binding posts should be arranged along the bottom of the wooden
back and two more just to the left of the transmitter.
The set is now ready to "wire" or connect. The connections can be
understood best by following the diagrams in Figures 94 and 97.
Starting with the four binding posts along the bottom of the back, mark
the left hand one B1, mark the next one B2, the third E and the fourth
L.
Connect a wire from binding post B1 to the post P1 directly above it.
Run the wire from P1 to the brass strip on the push button. Connect
binding posts B2 and E together and from E run a wire to the right hand
binding post on the bell. Connect a wire from the left hand post on the
bell to the lower contact 2. The hook should be connected to binding
post L. The upper contact 1 is connected to one terminal of the
transmitter. The other terminal of the transmitter goes to binding post
P2. The telephone receiver should be connected to the posts P1 and P2 by
means, of flexible wire. The contact on the push button should be
connected to the upper hook contact 1.
The set is now complete, ready to connect to a similar outfit.
The battery is connected to the binding posts marked B1 and B2. E and L
are the line posts to which the wires connecting the two sets are led.
The wire connecting the two outfits should not be smaller than No. 18 B.
& S. gauge. No. 14 will prove better if the line is a long one.
The wire should be very carefully insulated. Ordinary porcelain knobs
will serve as suitable insulators.
If desirable, only one wire need be used, the circuit being completed by
connecting the post E on each instrument to the ground or earth. The gas
or water pipes will form the most suitable ground connection.
The number of batteries required will depend upon the length of the
line. The actual number necessary to ring the bell properly can be best
ascertained by experiment. It should be noticed that the battery
connections at each station are reversed so that the batteries will be
series. The negative of the battery at one station is connected to E and
the positive pole of the other battery at the other station to the
similar post.
When the stations are not in use the receivers should be left hanging on
the hooks. In order to call the other station, lift the receiver off the
hook. This throws the transmitter, receiver and battery into the
circuit. The resistance of the receiver and transmitter, however, are so
great that they will prevent enough current from flowing to ring the
bell at the other station. Pressing the push button will short circuit
the transmitter and receiver and allow enough current to flow to ring
the bell at the other station. The other person has merely to lift up
his receiver in order that the two parties may talk back and forth.
When the receivers are on the hooks both bells are in circuit ready for
either party to call up.