OF ARTS AND SCIENCES. 69 



CONTRIBUTIONS FROM THE PHYSICAL LABORATORY OF THE 

 MASSACHUSETTS INSTITUTE OF TECHNOLOGY. 



XXXV. — ON THE EXTENT OF THE EXCURSION 



OF THE ELECTRODES OF A MICROPHONE 



TRANSMITTER. 



By Charles R. Cross. 



Presented AprU 9, 1890. 



The character and extent of the motions of the electrodes of a micro- 

 phone transmitter, when actuated by sound-waves of different degrees 

 of intensity, is a subject in telephony of by no means slight impor- 

 tance, but to which very little study has been given. The present pa- 

 per describes the results of some observations relating to this subject, 

 which have been made at various times during the past two years. 



Several years ago an attempt was made by Mr. W. W. Jacques 

 and the writer to gain some knowledge as to the amplitude of the 

 vibrations of the hammer electrode of a microphone, by observing it 

 with a microscope while in operation, and noting the extent of the 

 blurred portion of the image. The results, though giving all that could 

 be expected from so crude a method, were not very satisfactory so far 

 as definite measurement was concerned. 



It afterwards occurred to the writer that the matter might be studied 

 more completely by the use of the stroboscopic method, and an ar- 

 rangement of apparatus was adopted by which the motions of the 

 electrodes could readily be observed. 



This was done in the following manner. The microphone to be 

 studied was placed in the field of a microscope, whose line of coUimation 

 was horizontal. Behind the microphone, at a suitable distance, was 

 placed a Helmholtz mercury interrupter, with a tuning-fork making 

 128 vibrations per second. The extra current due to the electromag- 

 nets of the interrupter was quite large, so that a brilliant spark was 

 obtained at each rupture of the circuit, as the platinum style of the 

 interrupter left the mercury. The interrupter being properly placed, 



