20 THE POPULAR SCIENCE MONTHLY, 



gram, 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-but- 

 ton 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 an- 

 swers by depressing his bell-button and ringing A's bell. When A 

 speaks he depresses his push-button p connecting his primary and ter- 

 tiary, 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 prop- 

 erty of the carbon button as varying pressure is put upon it by the 

 vibrating diaphragm actuated by the voice. This electric wave-im- 

 pulse, 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 trans- 

 mits 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 varia- 

 tions of pressure on the carbon button as to render the instrument in- 

 articulate 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 car- 

 bon button, led to the invention of the micro-tasimeter. If the car- 

 bon button would respond to changes of pressure as small as those 

 caused by molecular action in the handle of the telephone, it would 



