. La science-histoire de l'univers . porteur d'un bouton hémisphérique, K, a mis à sa face arrière un autre disque en platine, H. Thissecond disque en platine repose sur la face avant du disque noir lampel LW AT , G, Et le bouton, K, appuie fermement contre le centre du diaphragme, D. les plaques, F et H, forment les bornes de l'émetteur, Et comme le dia-phragm, D, vibre, il provoque des variations dans la pression DU TÉLÉPHONE 271 et des changements correspondants dans la résistance du cir-cuit, produisant ainsi les ondulations de courant souhaitées. Le professeur David B. Hughes a fait un con-tr très précieux

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. The science-history of the universe . carrying a hemispherical button, K, has at-tached to its rear face another platinum disk, H. Thissecond platinum disk rests against the front face of thelampblack disk, G, and the button, K, presses firmlyagainst the center of the diaphragm, D. The plates, F andH, form the terminals of the transmitter, and as the dia-phragm, D, vibrates, it causes variations in the pressure THE TELEPHONE 271 and corresponding changes in the resistance of the cir-cuit, thus producing the desired undulations of current. Professor David B. Hughes made a most valuable con-tribution tending toward the perfection of the battery-transmitter. By a series of interesting experiments hedemonstrated conclusively that a loose contact betweenthe electrodes, no matter of what substance they are com-posed, is far preferable to a firm, strong current. Theapparatus used in one of his earlier experiments, made in1878, is shown in Fig. 39, and consists simply of threewire nails, of which A and B form the terminals of the. Fig. 38 —Phei.ps-Edison Transmitter. (From MillersAmerican Telephone Practice.) circuit containing a battery and a receiving instrument.The circuit was completed by a third nail, C, which waslaid loosely across the other two. Any vibrations in theair in the vicinity caused variations in the intimacy ofcontact between the nails, and corresponding variationsin the resistance of the circuit. This was a very ineffi-cient form of transmitter, but it demonstrated the princi-ple of loose contact very cleverly. It was found that carbon was, for various reasons, byfar the most desirable substance for electrodes in theloose-contact transmitter, and nothing has ever been foundto approach it in efficiency and desirability. 272 ELECTRICITY Another form of transmitter devised by Hughes, andcalled by him the microphone, is shown in Fig. 39. Thisconsists of a small pencil of gas carbon, A, pointed ateach end, and two blocks, B, B, of carbon fastened to adiaph

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