Page:Popular Science Monthly Volume 17.djvu/30

20, Fig. 5, which represents the termini of a telephone-line; C, the induction coil, consisting of a primary, secondary, and tertiary circuit; T, the carbon transmitter; R, the electro-motograph receiver; B, battery; r, relay; b, bell; p, push-button; and p’, bell-button. The local circuit is represented in dotted lines (- - - -); the primary thus -  - ; the secondary thus  - - -  - - - ; and the tertiary thus. Suppose A, station 1, wishes to communicate with B, station 2. He depresses the bell-button p’, when, it will be seen, a circuit is completed over the line through B's relay, closing his local circuit and ringing his bell; B then answers by depressing his bell-button and ringing A's bell. When A speaks he depresses his push-button p connecting his primary and tertiary, which completes his local primary circuit passing through the transmitter, where the electric impulse is transformed, as it were, into electric waves of varying number and amplitude by the peculiar property of the carbon button as varying pressure is put upon it by the vibrating diaphragm actuated by the voice. This electric wave-impulse, in passing through A's primary coil, induces a corresponding current in his secondary, which is transmitted, as may be traced over the line, into B's coil, when induction again takes place in B's tertiary, and B will then hear from his receiver what A has to say, and transmits his answer by the same modus operandi. The second connection that A makes when he depresses his push-button p is for the purpose of keeping his tertiary closed in order that B might interrupt him at any time during the communication. The reason for the alternate contact of the primary and tertiary at p is that each contact gives a slight but harmless knock upon the chalk cylinder of the motograph receiver, which, if occurring simultaneously, tends to disrupt its surface. For talking, one of the two Callaud cells is used; for the bell the two are required. Mr. Edison has lately adopted a small electric engine instead of a crank for the motograph purposes, which occasions the use of an extra cell."

While Mr. Edison was experimenting with his telephone in order to ascertain the proper arrangement of the diaphragm, he found that the expansion or contraction of the rubber handle caused such variations of pressure on the carbon button as to render the instrument inarticulate and sometimes even inoperative. He then tried iron handles. The same trouble was experienced, and, in addition, the receiving instrument was found to emit a kind of sound, which was attributed to the molecular action of the iron during the process of expansion. The immediate result of this discovery was that the handle of the instrument was dispensed with; but it also furnished a suggestion which, calling prominent attention to the extreme delicacy of the carbon button, led to the invention of the micro-tasimeter. If the carbon button would respond to changes of pressure as small as those caused by molecular action in the handle of the telephone, it would