90 PROCEEDINGS OP THE AMERICAN ACADEMY 



VIII. 



CONTRIBUTIONS FROM THE PHYSICAL LABORATORY OF THE 

 MASSACHUSETTS INSTITUTE OF TECHNOLOGY. 



XXXI.— THE STRENGTH OF THE MICROPHONE CURRENT 

 AS INFLUENCED BY VARIATIONS IN NORMAL PRESS- 

 URE AND MASS OF THE ELECTRODES. 



By Annie W. Sabine. 



Presented by Charles R. Cross, Norember 14, 1888. 



The experiments described in the present paper, undertaken at the 

 suggestion of Professor Cross, constitute a study of the variation of 

 the current in the secondary circuit of a microphone transmitter, as 

 related to variations in the normal pressure and in the mass of the 

 electrodes of the microphone. They form a continuation of work of 

 a similar character which has been prosecuted in the Rogers Labo- 

 ratory during the past few years, and a portion of which has already 

 been presented to the Academy.* 



The instruments used were similar to those previously employed by 

 Messrs. Patterson and Tucker, and described in the latter of the two 

 articles just referred to. The electrodynamometer was calibrated by 

 means of a Hartmann galvanometer, using reversals in the ordinary 

 manner so as by four readings to eliminate the effect of the earth's 

 magnetism on the suspended coil. 



The microphone contact was set into vibration by the sound of a 

 stopped organ-pipe (C 4 of 512 vibrations) kept as constant as possible 

 by means of an air-blast furnished with a regulating air-chamber. 

 Weights were gradually added to the upper (anvil) electrode, so that 

 the mass of this and its pressure on the lower electrode were thereby 

 increased by measured amounts. The weights added were usually in 

 the form of thin copper washers weighing t 8 q of a gram each, though 

 fractions of this weight were used in some cases. One chromic acid 

 cell was used as a battery. 



* See Proc. Am. Acad., Vol. XXI. p. 248 ; Ibid., Vol. XXIII. p. 228. 



