Page:Popular Science Monthly Volume 14.djvu/141

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SOME of the discoveries and inventions of Mr, Edison, that were made during His researches which culminated in the invention of the carbon-telephone, have already been published. We now propose to present a more complete description of the important forms of telephone upon which he then experimented, and to describe his more recent acoustic inventions.

The carbon-telephone is only one of many contrivances for reproducing articulate speech at a distance through the aid of electricity, but, owing to its clear and truthful articulation, its simplicity of construction, and the far greater volume of sound which it creates, is probably destined to be the most extensively used. Other instruments of Mr. Edison's invention, however, are not far behind it, and may by improvement be made equally effective.

As a rule, Mr. Edison has succeeded better with those telephones which produce a variation in the resistance of the circuit than with those which depend for their action upon a variation of the electro-motive force or static charge. An instrument very similar to the carbon-transmitting telephone is shown in Fig. 1, the essential difference being that the carbon is replaced by bibulous paper moistened with water. This semi-conductor, like the carbon, changes its resistance under the influence of varying pressure. The paper is kept moist by capillary action, a strip being used, one end of which dips into a reservoir of water.

In Fig. 2 is shown a form of the carbon-transmitting telephone, requiring no adjustment whatever. It operates well, notwithstanding the simplicity of its construction. A plate of metal rests on the bottom of a hollow vessel. On this is placed a block of prepared carbon, upon which a second and light plate is laid.