so that the wires and the coils form a complete circuit. Words
are spoken into A. The air vibrations, passing through the central hole
in the cover, make the diaphragm vibrate towards and away from the
magnet. The distances through which the diaphragm moves have been
measured, and found not to exceed in some cases more than 1/10,000,000
of an inch! Its movements distort the shape of the "lines of force" (see
p. 118) emanating from the magnet, and these, cutting through the turns
of the coil, induce a current in the line circuit. As the diaphragm
approaches the magnet a circuit is sent in one direction; as it leaves
it, in the other. Consequently speech produces rapidly alternating
currents in the circuit, their duration and intensity depending on the
nature of the sound.

Now consider telephone B. The currents passing through its coil increase
or diminish the magnetism of the magnet, and cause it to attract its
diaphragm with varying force. The vibration of the diaphragm disturbs
the air in exact accordance with the vibrations of A's diaphragm, and
speech is reproduced.

THE EDISON TRANSMITTER.

The Bell telephone may be used both as a transmitter and a receiver, and
the permanent magnetism of the cores renders it independent of an
electric battery. But currents generated by it are so minute that they
cannot overcome the resistance of a long circuit; therefore a battery is
now always used, and with it a special device as transmitter.

If in a circuit containing a telephone and a battery there be a loose
contact, and this be shaken, the varying resistance of the contact will
cause electrical currents of varying force to pass through the circuit.
Edison introduced the first successful _microphone_ transmitter, in
which a small platinum disc connected to the diaphragm pressed with
varying force against a disc of carbon, each disc forming part of the
circuit. Vibrations of the diaphragm caused current to flow in a series
of rapid pulsations.

[Illustration: FIG. 63.--Section of a granular carbon transmitter.]

THE GRANULAR CARBON TRANSMITTER.

In Fig. 63 we have a section of a microphone transmitter now very widely
used. It was invented, in its original form, by an English clergyman
named Hunnings. Resting in a central cavity of an ebonite seating is a
carbon block, C, with a face moulded into a number of pyramidal
projections, P P. The space between C and a carbon diaphragm, D, is
packed with carbon granules, G