﻿audible in the Telephone. 287 



same in the two cases, we should expect, according to the 

 above law, that about 10 times as much current would be 

 required in T 2 as in T lt Both instruments are of the Bell 

 (unipolar) type, and comparison with other specimens shows 

 that there is nothing exceptional in their sensibility. 



In view of the immense discrepancies above recorded, it is 

 evident that what is required is not so much accuracy of 

 measurement as assured soundness in method. It appeared 

 to me that electromotive forces of the necessary harmonic 

 type would be best secured by the employment of a revolving 

 magnet in the proximity of an inductor-coil of known con- 

 struction. The electromotive force thus generated operates 

 in a circuit of known resistance ; and, if the self-induction 

 can be neglected, the calculation of the current presents no 

 difficulty. The sound as heard in the telephone may be re- 

 duced to the required point either by varying the distance 

 ( B) between the magnet and the inductor, or by increasing 

 the resistance (R) of the circuit. In fact both these quan- 

 tities may be varied ; and the agreement of results obtained 

 with widely different values of R constitutes an effective test 

 of the legitimacy of neglecting self-induction. When R is 

 too much reduced, the time-constant of the circuit becomes 

 comparable with the period of vibration, and the current is 

 no longer increased in proportion to the reduction of R. 

 This complication is most likely to occur when the pitch is 

 high. < 



In order to keep as clear as possible of the complication 

 due to self-induction, I employed in the earlier experiments a 

 resistance-coil of 100,000 ohms, constructed as usual of wire 

 doubled upon itself. But it soon appeared that in avoiding 

 Scylla I had fallen upon Charybdis. The first suspicion of 

 something wrong arose from the observation that the sound 

 was nearly as loud when the 100,000 ohms was included as 

 when a 10,000-ohm coil was substituted for it. The first 

 explanation that suggested itself was that the sound was being- 

 conveyed mechanically instead of electrically, as is indeed 

 quite possible under certain conditions of experiment. But a 

 careful observation of the effect of breakiug the continuity of 

 the leads, one at a time, proved that the propagation was 

 really electrical. Subsequent inquiry showed that the anomaly 

 was due to a condenser, or leyden, like action of the doubled 

 wire of the 100,000-ohm coil. When the junction at the 

 middle was unsoldered, so as to interrupt the metallic conti- 

 nuity, the sounds heard in the telephone were nearly as loud 

 as before. In this condition the resistance should have been 



