The telephonic successes recently achieved by Mr. Gray were in part anticipated by La Cour, of Copenhagen, whose method may be thus described: At the transmitting station a tuning-fork is set in vibration. At each vibration one of the prongs touches a fine strip of metal completing a circuit. At the receiving station the wire conveying the electric current is coiled round the prongs of another tuning-fork of the same tone, but without touching them. The intermittent current, corresponding as it does with the rate of vibration proper to the receiving fork, sets this fork in vibration; and in La Cour’s instrument the vibrations of the receiving fork were used to complete the circuit of a local battery. His object was not so much the production of tones as the use of the vibrations corresponding to different tones, to act on different receiving instruments. For only a fork corresponding to the sending fork could be set in vibration by the intermittent current resulting from the latter’s vibrations. So that, if there were several transmitting forks, each could send its own message at the same time, each receiving fork responding only to the vibrations of the corresponding transmitting fork. La Cour proposed, in fact, that his instrument should be used in combination with other methods of telegraphic communication. Thus, since the transmitting fork, whenever put in vibration, sets the local battery of the receiving station at work, it can be used to work a Morse instrument, or it could work an ordinary Wheatstone and Cook instrument, or it could be used for a pantelegraph. The same wire, when different forks are used, could work simultaneously several instruments at the receiving station. One special use indicated by La Cour was the adaptation of his system to the Caselli pantelegraph, whereby, instead of one style, a comb of styles might be carried over the transmitting and recording plates. It would be necessary, in all such applications of his method (though, strangely enough, La Cour’s description makes no mention of the point), that the vibrations of the transmitting fork should admit of being instantly stopped or “damped.”
Mr. Gray’s system is more directly telephonic, as aiming rather at the development of sound itself than at the transmission of messages by the vibrations corresponding to sound. A series of tuning-forks are used, which are set in separate vibration by fingering the notes of a key-board. The vibrations are transmitted to a receiving instrument consisting of a series of reeds, corresponding in note to the series of transmitting forks, each reed being enclosed in a sounding-box. These boxes vary in length from two feet to six inches, and are connected by two wooden bars, one of which carries an electro-magnet, round the coils of which pass the currents from the transmitting instrument. When a tuning-fork is set in vibration by the performer at the transmitting key-board, the electro-magnet is magnetized and demagnetized synchronously with the vibrations of the fork. Not only are vibrations thus imparted to the reed of corresponding note, but these are synchronously strengthened by thuds resulting from the lengthening of the iron when magnetized.