THE TELEPHONE, AND HOW IT TRANSMITS SPEECH.
=119. The Telephone= is an instrument for reproducing sounds at a
distance, and electricity is the agent by which this is generally
accomplished. The part spoken to is called the _transmitter_, and
the part which gives sound out again is called the _receiver_. Sound
itself does not pass over the line. While the same apparatus can be
used for both transmitter and receiver, they are generally different in
construction to get the best results.
[Illustration: Fig. 125.]
[Illustration: Fig. 126.]
[Illustration: Fig. 127.]
=120. The Bell or Magneto-transmitter= generates its own current, and
is, strictly speaking, a dynamo that is run by the voice. It depends
upon induction for its action.
[Illustration: Fig. 128.]
Fig. 125 shows a coil of wire, H, with soft iron core, the ends of the
wires being connected to a delicate galvanoscope. If one pole of the
magnet H M be suddenly moved up and down near the core, an alternating
current will be generated in the coil, the circuit being completed
through the galvanoscope. As H M approaches the core the current will
flow in one direction, and as H M is withdrawn it will pass in the
opposite direction. The combination makes a miniature alternating
dynamo.
[Illustration: Fig. 129.]
If we imagine the soft iron core of H, Fig. 125, taken out, and one
pole of H M, or preferably that of a bar magnet stuck through the coil,
a feeble current will also be produced by moving the soft iron back and
forth near the magnet's pole. This is really what is done in the Bell
transmitter, soft iron in the shape of a thin disc (D, Fig. 126) being
made to vibrate by the voice immediately in front of a coil having
a permanent magnet for a core. The disc, or _diaphragm_, as it is
called, is fixed near, but it does not touch, the magnet. It is under
a constant strain, being attracted by the magnet, so its slightest
movement changes the strength of the magnetic field, causing more or
less lines of force to shoot through the turns of the coil and induce a
current. The coil consists of many turns of fine, insulated wire. The
current generated is an alternating one, and although exceedingly small
can force its way through a long length of wire.
[Illustration: Fig. 130.]
Fig. 127 shows a section of a regular transmitter, and Fig. 128 a form
of compound magnet frequently used in the transmitter. Fig. 129 shows a
transmitter with cords which contain flexible wires.
[Illustration: Fig. 131.]
=121. The Receiver=, for short lines, may have the same construction as
the Bell transmitter. Fig. 130 shows a diagram of two Bell receivers,
either being used as the transmitter and the other as the receiver.
As the alternating current goes to the distant receiver, it flies
through the coil first in one direction and then in the other. This
alternately strengthens and weakens the magnetic field near the
diaphragm, causing it to vibrate back and forth as the magnet pulls
more or less. The receiver diaphragm repeats the vibrations in the
transmitter. Nothing but the induced electric current passes over the
wires.
[Illustration: Fig. 132.]
=122. The Microphone.= If a current of electricity be allowed to
pass through a circuit like that shown in Fig. 131, which includes a
battery, a Bell receiver, and a microphone, any slight sound near the
microphone will be greatly magnified in the receiver. The microphone
consists of pieces of carbon so fixed that they form loose contacts.
Any slight movement of the carbon causes the resistance to the current
to be greatly changed. The rapidly varying resistance allows more or
less current to pass, the result being that this pulsating current
causes the diaphragm to vibrate. The diaphragm has a constantly varying
pull upon it when the carbons are in any way disturbed by the voice, or
by the ticking of a watch, etc. This principle has been made use of in
carbon transmitters, which are made in a large variety of forms.
[Illustration: Fig. 133.]
=123. The Carbon Transmitter= does not, in itself, generate a
current like the magneto-transmitter; it merely produces changes in
the strength of a current that flows through it and that comes from
some outside source. In Fig. 132, X and Y are two carbon buttons, X
being attached to the diaphragm D. Button Y presses gently against X,
allowing a little current to pass through the circuit which includes
a battery, D C, and a receiver, R. When D is caused to vibrate by the
voice, X is made to press more or less against Y, and this allows more
or less current to pass through the circuit. This direct undulating
current changes the pull upon the diaphragm of R, causing it to vibrate
and reproduce the original sounds spoken into the transmitter. In
regular lines, of course, a receiver and transmitter are connected at
each end, together with bells, etc., for signaling.
[Illustration: Fig. 134.]
=124. Induction Coils in Telephone Work.= As the resistance of long
telephone lines is great, a high electrical pressure, or E.M.F. is
desired. While the current from one or two cells is sufficient to work
the transmitter properly, and cause undulating currents in the short
line, it does not have power enough to force its way over a long line.
To get around this difficulty, an induction coil, Fig. 133, is used
to transform the battery current, that flows through the carbon
transmitter and primary coil, into a current with a high E. M. F. The
battery current in the primary coil is undulating, but always passes in
the same direction, making the magnetic field around the core weaker
and stronger. This causes an alternating current in the secondary coil
and main line. In Fig. 133 P and S represent the primary and secondary
coils. P is joined in series with a cell and carbon transmitter; S
is joined to the distant receiver. One end of S can be grounded, the
current completing the circuit through the earth and into the receiver
through another wire entering the earth.
[Illustration: Fig. 135.]
=125. Various forms= of telephones are shown in Figs. 134, 135, 136.
Fig. 134 shows a form of desk telephone; Fig. 135 shows a common form
of wall telephone; Fig. 136 shows head-telephones for switchboard
operators.
[Illustration: Fig. 136.]