e the whole circuit. This induced electricity will
consist of a single wave or pulse, and its force will depend upon the
velocity of the approach of N S to M. A like pulse of electricity will
be induced in the coils when N S is made to move away from M; but this
current will move through the circuit in the opposite direction, so that
whether the pulsation goes from M to R, or from R to M, depends simply
upon the direction of the motion of N S.

The electricity thus generated in the wire by such vibratory movements
varies in strength proportional to the movement of the armature;
therefore the line wire between two places will be filled with
electrical pulsation exactly like the aerial pulsations in structure.
Fig. 10, p. 98, may be used to illustrate the condition of the wire
through which the currents pass. The dark part may represent the
strongest part of the wave, while the lighter part would show the
weaker part of the wave. The chief difference would be, that electricity
travels so fast, that what is there represented as one wave in air with
a length of two feet would, in an electric wave, be more than fifty
miles long.

These induced electric currents are but very transient (see p. 31); and
their effect upon the receiver R is to either increase or decrease the
power of the magnet there, as they are in one direction or the other,
and consequently to vary the attractive power exercised upon the iron
plate armature.

Let a simple sound be now made in the tube, consisting of 256 vibrations
per second: the membrane carrying the iron will vibrate as many times,
and so many pulses of induced electricity will be _imposed_ upon the
constant current, which will each act upon the receiver, and cause so
many vibrations of the armature upon it; and an ear held at P will hear
the sound with the same pitch as that at the sending instrument. If two
or more sound-waves act simultaneously upon the membrane, its motions
must correspond with such combined motions; that is, its motions will
be the resultant of all the sound-waves, and the corresponding
pulsations in the current must reproduce at R the same effect. Now, when
a person speaks in the tube, the membrane is thrown into vibrations more
complex in structure than those just mentioned, differing only in number
and intensity. The magnet will cause responses from even the minutest
motion; and therefore an ear at R will hear what is said at the tube.
This was the instrument exhi