94 PROCEEDINGS OP THE AMERICAN ACADEMY 



IX. 



CONTRIBUTIONS FROM THE PHYSICAL LABORATORY OF THE 

 MASSACHUSETTS INSTITUTE OF TECHNOLOGY. 



XXXIL — RESEARCHES ON MICROPHONE CURRENTS. 



By Charles R. Cross and Annie W. Sabine. 

 Presented November 14, 1888. 



In coutinuation of studies described in former papers from the Rogers 

 Laboratory, the authors have observed the variations in the secondary- 

 current which occur under different circumstances as to mass and 

 normal pressure when the material of the electrodes of a microphone 

 is varied. The methods and apparatus employed were identical with 

 those described in the immediately preceding paper in these Proceed- 

 ings (page 89). 



In the first series of experiments both electrodes were made of the 

 same material, the hammer electrode being a very small button, and 

 the anvil electrode a large one, as in the Blake transmitter. As the 

 mass and pressure upon the anvil electrode were varied, the currents 

 in the secondary at first increased up to a maximum, and afterwards 

 diminished, as shown in the article just referred to, as well as in the 

 curves given with the present paper. Tables I. to XVI. give the 

 numerical results obtained, and Figures 1 to 14 are curves showing 

 the relations between the variables, the loads being abscissas, and the 

 corresponding currents in milliamperes the ordinates. The unit of 

 weight adopted was ■^^- of a gram. The remarks in the third column 

 of the tables indicate the nature of the sound heard in a receiving tele- 

 phone placed in the secondary circuit. The current was in all cases 

 zero when the load was zero. 



TABLE L 



Electrodes. — Iron, Iron. 

 Load. Current. Remarks. 



1 .19 Sound rough and unsteady. 



2 .24 Shrill, clear, metallic, unsteady. 



3 .16 " " " " 



