Page:Popular Science Monthly Volume 17.djvu/21

Rh to the accomplishment of that end were begun nearly twenty-years ago.

The first successful experiments were made by Philip Reis, of Fredericksdorf, Germany, in 1861. He argued that if it could be found practicable to convert sound pulsations into electric pulsations, and then convert these pulsations back again into sound pulsations, the same effect would be produced as if the vibrations had been actually transmitted through the air. In his instruments a membrane rigidly secured on the sides was caused to vibrate in the center by the motion of the air produced by any sound. In the center of this membrane was a delicate circuit-breaker so arranged as to break the circuit of an electric telegraph line at every vibration, thus successively magnetizing and demagnetizing an electro-magnet at the receiving station, and causing its armature to vibrate in accordance with the vibrations of the membrane at the transmitting station. The vibrations of this armature, properly mounted on a sounding-board, set into vibration the surrounding air, which carried the sound to the ear. His first instrument is represented in Fig. 1. A is the transmitting and B the receiving instrument, supposed to be placed at different stations and connected with each other by a metallic conductor. A conical tube, a b, six inches long, four inches in diameter at the larger, and one and a half inch in diameter at the smaller end, is closed at b by a collodion membrane o, against the center of which rests one end, c, of the lever c d. This lever has electric connection with the wire of the line joining the two stations at its point of support, e. The end d of the lever rests against the flat springy, which can be properly adjusted by means of the screw h, and which, through the metal standard f, is connected with the battery C. At station B the conducting wire passed around the electro-magnet m, which is mounted on a sounding-box W; thence to the ground. Attached to the armature at the pole of the magnet is a thin plate i, which is hung on an horizontal axis projecting from the upright k; the motion of the plate can be regulated by the screw l and the spring s. The best dimensions and most suitable adjustments of the instrument were determined by experiment. Its operation is as follows: When at rest the small spring n keeps the lever c d in contact at g, the circuit is closed, and the magnet in attracts the armature i. But, when by speaking into the tube a b, the air in the tube and therefore the membrane o is set into vibration, the contact at g is alternately broken and closed, and consequently the magnet at B is demagnetized and magnetized, alternately releasing and attracting the armature i. It is evident that the vibrations of i correspond in number and interval to the vibrations of the membrane o; so that the sound which enters the tube a b is reproduced at B so far as its pitch is concerned. But as the strength of the current is constant, neither the intensity nor the quality of the sound is reproduced.

In 1874 Elisha Gray, of Chicago, accomplished the reproduction of