station, on the other hand, has
neither condenser, induction coil, nor key. When the apparatus is in
operation, the coherer switch is closed, and the instant a current
passes through the coherer and operates the telegraph sounder, the
galvanometer indicates the current.

Of course, when the coherer switch is closed, the battery operates the
decoherer.

HOW THE CIRCUITS ARE FORMED.--By referring again to Fig. 76, it will be
seen that when the key is depressed, a circuit is formed from the
battery through wire B to the primary coil, and back again to the
battery through wire D. The secondary coil is thereby energized, and,
when the full potential is reached, the current leaps across the gap
formed between the two knobs (F, H), thereby setting up a disturbance in
the ether which is transmitted through space in all directions.

It is this impulse, or disturbance, which is received by the coherer at
the receiving station, and which is indicated by the telegraph sounder.

CHAPTER XII

THE TELEPHONE

VIBRATIONS.--Every manifestation in nature is by way of vibration. The
beating of the heart, the action of the legs in walking, the winking of
the eyelid; the impulses from the sun, which we call light; sound, taste
and color appeal to our senses by vibratory means, and, as we have
hereinbefore stated, the manifestations of electricity and magnetism are
merely vibrations of different wave lengths.

THE ACOUSTIC TELEPHONE.--That sound is merely a product of vibrations
may be proven in many ways. One of the earliest forms of telephones was
simply a "sound" telephone, called the _Acoustic Telephone_. The
principle of this may be illustrated as follows:

Take two cups (A, B), as in Fig. 78, punch a small hole through the
bottom of each, and run a string or wire (C) from the hole of one cup to
that of the other, and secure it at both ends so it may be drawn taut.
Now, by talking into the cup (A) the bottom of it will vibrate to and
fro, as shown by the dotted lines and thereby cause the bottom of the
other cup (B) to vibrate in like manner, and in so vibrating it will
receive not only the same amplitude, but also the same character of
vibrations as the cup (A) gave forth.

[Illustration: _Fig. 78._ ACOUSTIC TELEPHONE]

[Illustration: _Fig. 79._ ILLUSTRATING VIBRATIONS]

SOUND WAVES.--Sound waves are long and short; the long waves giving
sounds which are low in the musical scale, and the short waves high
musical t