176 PROCEEDINGS OP THE AMERICAN ACADEMY 



XV. 



CONTRIBUTIONS FROM THE JEFFERSON PHYSICAL 

 LABORATORY, HARVARD UNIVERSITY. 



NEUTRALIZATION OF INDUCTION. 

 By John Trowbridge and Samuel Sheldon. 



Presented May 28, 1889 



The invention of the telephone drew attention to the extraordinary 

 sensitiveness of Faraday's electrotonic state, and immediate attempts 

 were made to construct induction balances, so called, which might serve 

 for quantitative measurements. Thus we have Hughes's induction 

 balance, which had its prototype in the balance described in Max- 

 well's " Electricity and Magnetism," Vol. II. § 636, due to Felici,* 

 and which differs from Hughes's balance merely in the employment 

 of a galvanometer instead of a telephone. By substituting the lat- 

 ter instrument, Hughes showed that great sensitiveness could be 

 obtained, and even proposed to adopt an instrument for measuring 

 minute amounts of impurities in coins arising from alloys. 



The great difficulty, however, in the employment of Hughes's induc- 

 tion balance in quantitative work arises from the difficulty of getting 

 a good minimum of tone in the telephone. The method that Hughes 

 employed was, briefly, to employ four coils, — two in a circuit through 

 which an alternating current or an interrupted current was passed, 

 and two other coils placed contiguous to the coils which were in the 

 interrupted circuit, but in another circuit.' By interposing a telephone 

 in the last mentioned circuit, and by properly placing the coils in this 

 circuit with reference to those in the circuit through which the inter- 

 rupted current was passed, a balance could be obtained, or an imperfect 

 minimum of sound in the telephone, when the induction between the 

 sets of coils was neutralized. In order 'to obtain a standard, Hughes 

 employed a wedge of zinc, which was thrust between one of the coils 

 in the interrupted circuit and one of the coils in the telephone circuit, 

 in order that the mutual induction between these coils might balance 



* Nuovo Cimento, vol. ix. p. 345, 1859. 



