When we speak into the mouthpiece of the transmitter, the vibrations of
the air cause the diaphragm to vibrate very rapidly, and, of course,
every movement of the diaphragm presses _more or less_ against the
carbon button, in consequence of which the currents passing through the
primary of the induction-coil are constantly increased or diminished
and thus produce similar effects, but magnified, in the secondary.
The effect of this is that the magnet in the receiver of the other
telephone is receiving a rapidly changing current, which, producing
corresponding magnetic changes, makes the magnet alternately weaker
or stronger. This influences, by magnetism, the iron diaphragm
accordingly, and makes it reproduce the same vibrations that were
caused by the speech at the transmitter of the sending telephone. Thus,
the same vibrations being _reproduced_, the original sounds are given
out, and we can hear what the person at the sending telephone is saying.
The action of the telephone illustrates well the wonderfully quick
action of the electric current by the reproduction of these sound
waves, or air vibrations, for they number many thousands in one
minute's speech.