hat the solution of the
individual harmonic motion is difficult and laborious.

These photographs point out several facts very clearly. One is that
the alternations of currents in the telephone line, like the motions
of the molecules of air of the original sound, are highly complex and
are not, as musical tones are, regular recurrences of equal
vibrations. They show also that any vowel sound may be considered to
be a regular recurrence of certain groups of vibrations of different
amplitudes and of different frequencies.

CHAPTER III

ELECTRICAL SIGNALS

Electric calls or signals are of two kinds: audible and visible.

[Illustration: Fig. 15. Telegraph Sounder and Key]

[Illustration: Fig. 16. Vibrating Bell]

Audible Signals. _Telegraph Sounder._ The earliest electric signal
was an audible one, being the telegraph sounder, or the Morse register
considered apart from its registering function. Each telegraph sounder
serves as an audible electric signal and is capable of signifying more
than that the call is being made. Such a signal is operated by the
making and breaking of current from a battery. An arrangement of this
kind is shown in Fig. 15, in which pressure upon the key causes the
current from the battery to energize the sounder and give one sharp
audible rap of the lever upon the striking post.

_Vibrating Bell_. The vibrating bell, so widely used as a door bell,
is a device consequent to the telegraph. Its action is to give a
series of blows on its gong when its key or push button closes the
battery circuit. At the risk of describing a trite though not trivial
thing, it may be said that when the contact _1_ of Fig. 16 is closed,
current from the battery energizes the armature _2_, causing the
latter to strike a blow on the gong and to break the line circuit as
well, by opening the contact back of the armature. So de-energized,
the armature falls back and the cycle is repeated until the button
contact is released. A comparison of this action with that of the
polarized ringer (to be described later) will be found of interest.

[Illustration: Fig. 17. Elemental Magneto-Generator]

_Magneto-Bell._ The magneto-bell came into wide use with the spread of
telephone service. Its two fundamental parts are an
alternating-current generator and a polarized bell-ringing device.
Each had its counterpart long before the invention of the telephone,
though made familiar by the latter. The alternating-current gen