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 When the prong receded, it was of course withdrawn from the mercury, and the current was broken.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. REISS' TELEPHONE. The ease with which membranes are thrown into vibrations corresponding in period to that of the sounding body has already been alluded to on p. 80; and several attempts have been made, at different times, to make membranes available in telephony. The first of these attempts was made by Philip Reiss of Friedrichsdorf, Germany, in 1861. His apparatus consisted of a hollow box, with two apertures: one in front, in which was inserted a short tube for producing the sound in, and indicated by the arrow in the cut, Fig. 12; the other on the top. This was covered with the membrane _m_,--a piece of bladder stretched tight over it. Upon the middle of the membrane, a thin piece of platinum was glued; and this piece of platinum was connected by a wire to a screw-cup from which another wire went to a battery. [Illustration: FIG. 12.]