Page:Heroes of the telegraph (IA cu31924031222494).djvu/230

 electro-magnet, by causing the currents to alter the friction between the two rubbing surfaces, and so actuate a marker, which registered the message as in the Morse system.

This instrument was called the 'electromotograph,' and it occurred to Edison that in a similar way the undulatory currents from his carbon transmitter might, by varying the friction between a metal stylus and the prepared surface, put a tympanum in vibration, and reproduce the original sounds. Wonderful as it may appear, he succeeded in doing so by the aid of a piece of chalk, a brass pin, and a thin sheet or disc of mica. He attached the pin or stylus to the centre of the mica, and brought its point to bear on a cylindrical surface of prepared chalk. The undulatory current from the line was passed through the stylus and the chalk, while the latter was moved by turning a handle; and at every pulse of the electricity the friction between the pin and chalk was diminished, so that the stylus slipped upon its surface. The consequence was a vibration of the mica diaphragm to which the stylus was attached. Thus the undulatory current was able to establish vibrations of the disc, which communicated themselves to the air and reproduced the original sounds. The replica was loud enough to be heard by a large audience, and by reducing the strength of the current it could be lowered to a feeble murmur. The combined transmitter and receiver took the form of a small case with a mouthpiece to speak into, an car-piece on a hinged bracket for listening to it, press-keys for manipulating the call-bell and battery, and a small handle by which to revolve the little chalk cylinder. This last feature was a practical drawback to the system, which was patented in 1877.

The Edison telephone, when at its best, could transmit all kinds of noises, gentle or harsh; it could