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receded, it was of course withdrawn from the mercury, and the current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
distance in a telegraphic circuit, a sound of a given pitch. It is
therefore a true telephone.