are of one kind or of the other.

An intermittent current is characterized by the alternate presence and
absence of electricity upon the circuit.

A pulsatory current results from sudden or instantaneous changes in the
intensity of a continuous current; and

An undulatory current is a current of electricity, the intensity of
which varies in a manner proportional to the velocity of the motion of a
particle of air during the production of a sound: thus the curve
representing graphically the undulatory current for a simple musical
note is the curve expressive of a simple pendulous vibration--that is, a
sinusoidal curve.

And here I may remark, that, although the conception of the undulatory
current of electricity is entirely original with myself, methods of
producing sound by means of intermittent and pulsatory currents have
long been known. For instance, it was long since discovered that an
electro-magnet gives forth a decided sound when it is suddenly
magnetized or demagnetized. When the circuit upon which it is placed is
rapidly made and broken, a succession of explosive noises proceeds from
the magnet. These sounds produce upon the ear the effect of a musical
note when the current is interrupted a sufficient number of times per
second....

[Illustration: Fig. 2]

For several years my attention was almost exclusively directed to the
production of an instrument for making and breaking a voltaic circuit
with extreme rapidity, to take the place of the transmitting tuning fork
used in Helmholtz's researches. Without going into details, I shall
merely say that the great defects of this plan of multiple telegraphy
were found to consist, first, in the fact that the receiving operators
were required to possess a good musical ear in order to discriminate the
signals; and secondly, that the signals could only pass in one direction
along the line (so that two wires would be necessary in order to
complete communication in both directions). The first objection was got
over by employing the device which I term a "vibratory circuit breaker,"
whereby musical signals can be automatically recorded....

I have formerly stated that Helmholtz was enabled to produce vowel
sounds artificially by combining musical tones of different pitches and
intensities. His apparatus is shown in Fig. 2. Tuning forks of different
pitch are placed between the poles of electro-magnets (_a1_, _a2_, &c.),
and are kept in continuous vibration by the acti