. American telephone practice . the other two. Any vibrations inthe air in the vicinity caused variations in the intimacy of contact 18 AMERICAN TELEPHONE PRACTICE. between the nails, and corresponding variations in the resistanceof the circuit. This was a very inefficient form of transmitter, butit demonstrated the principle of loose contact very cleverly. It was found that carbon was, for various reasons, by far themost desirable substance for electrodes in the loose contact trans-mitter, and nothing has ever been found to even approach it inefficiency and desirability. Another form of trans
. American telephone practice . the other two. Any vibrations inthe air in the vicinity caused variations in the intimacy of contact 18 AMERICAN TELEPHONE PRACTICE. between the nails, and corresponding variations in the resistanceof the circuit. This was a very inefficient form of transmitter, butit demonstrated the principle of loose contact very cleverly. It was found that carbon was, for various reasons, by far themost desirable substance for electrodes in the loose contact trans-mitter, and nothing has ever been found to even approach it inefficiency and desirability. Another form of transmitter devised by Hughes, and called byhim the microphone, is shown in Fig. 19. This consists of a smallpencil of gas carbon, A, pointed at each end, and two blocks, B B,of carbon fastened to a diaphragm or sounding board, C. Theseblocks are hollowed out in such a manner as to loosely hold betweenthem the pencil, A. The blocks, B B, form the terminals of the cir-cuit. This instrument, though crude in form, is of marvelous deli-. FIG. 18.—HUGHES NAIL MICROPHONE. cacy and is well termed microphone. The slightest noises in itsvicinity, and even those incapable of being heard by the ear alone,produce surprising effects in the receiving instrument. This partic-ular form of instrument is, in fact, too delicate for ordinary use, asany jar or loud noise will cause the electrodes to break contact andproduce deafening noises in the receiver. Nearly all carbon trans-mitters of to-day are of the loose-contact type, this having entirelysuperseded the first form devised by Edison, which was then sup-posed to depend on the actual resistance of a carbon block beingchanged under varying pressure. In speaking of Professor Hughes work on loose contacts and the THE BATTERY TRANSMITTER. 19 microphone, the Telegraph Journal and Electrical Review, an Eng-lish electrical paper, says in its issue of July I, 1878: The micro-phone is a striking illustration of the truth that in science any phe-nomenon
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