Suppose we introduce a telephone a into the primary circuit of an ordinary induction coil, such as used in physiological laboratories, and another telephone b into the circuit of the secondary coil, and suppose the wires connecting telephone a with the primary and connecting telephone b to be many yards in length. If one observer speaks into a, another observer will hear every word if b is applied to his ear. Every one is acquainted with this fact, that the currents awakened in a induce corresponding currents in the secondary coil, which are transmitted to b. If we substitute for a a microphone-transmitter c, and speak to it, the tones are reproduced by telephone b.

Further, as I showed to the Society in February last, if we suspend a microphone-transmitter over the phonograph, when the latter is in motion and giving out tones, the variations in resistance in the microphone-transmitter produce such alterations in the current flowing through it to the coils of an electro-magnet as to make it possible, mechanically, to record these variations.

These considerations led me some months ago to try the following experiment. The microphone-transmitter was suspended over the phonograph disk, and it was introduced into the circuit of the primary coil of the induction machine along with four Obach's cells (Q type). The terminals of the secondary coil were carried to two strips of platinum foil immersed in two insulated vessels (glass beakers or shallow vulcanite troughs) containing a ·75 per cent. solution of common salt or sulphuric acid (1:10 of water).