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