the
current made the circuit in the coil of the electro-magnet. The magnet
attracting the armature away from the button broke the circuit, which of
course destroyed the magnetism of the magnet, and allowed the spring to
fly back against the button, to complete the circuit and reproduce the
same series of changes. The rapidity with which the current may be
broken in this way is only limited by the strength of both spring and
current. The greater the tension of the spring with a given current,
the greater number of vibrations will it make.

[Illustration: FIG. 11.]

Suppose such an intermittent current to pass through the coil
surrounding the soft iron rod, 256 times per second; then the rod would
evidently give 256 clicks per second, which would have the pitch of C.
When these clicks are produced in the rod hold in the hand, the sound is
hardly perceptible, being like that of a sounding tuning-fork when held
thus. In order to strengthen it, it is necessary to place it on some
resonant surface. It is customary to mount it upon an oblong box with
one or two holes in its upper surface, inasmuch as such a form is found
to give a louder response than any other, and is the shape usually given
to AEolian harps. The accompanying cut shows the combination of battery
B, the circuit-breaker, and the rod mounted upon the box. The wire W may
evidently be of any length, the magnetized rod and box responding to the
number of vibrations of the spring S, how long soever the circuit may
be.

HELMHOLTZ' ELECTRIC INTERRUPTOR.

In some of Helmholtz' experiments, it was essential to maintain the
vibrations of a tuning-fork for a considerable time. He effected this by
placing a short electro-magnet between the prongs of the fork, and
affixing a platinum point at the end of one prong in such a manner,
that, as the prong descended in its vibration, the platinum point dipped
into a small cup of mercury that completed the circuit. When the prong
receded, it was of course withdrawn from the mercury, and the current
was broken. As it is not possible for a tuning-fork to vibrate in more
than one period, such an arrangement would evidently make and break the
current as many times per second as the fork vibrated. When, therefore,
such an interruptor is inserted in the circuit with the click-rod on its
resonant box, the latter must give out just such a sound as the fork is
giving. With such a device, it is possible to reproduce at almost any
dis