23i PROCEEDINGS OP THE AMERICAN ACADEMY 



XVI. 



CONTRIBUTIONS FROM THE PHYSICAL LABORATORY OF 

 THE MASSACHUSETTS INSTITUTE OF TECHNOLOGY. 



XLI. — ON THE EXCURSION OF THE DIAPHRAGM OF 

 A TELEPHONE RECEIVER. 



By Charles R. Cross and Henry M. Phillips. 



Presented January 11, 1893. 



The present investigation is a continuation of one upon the same 

 subject by Messrs. Cross and Mansfield, the results of which have 

 already been published in these Proceedings.* 



In the study of the excursion of the telephone diaphragm described 

 in the paper referred to, the current employed was an alternating cur- 

 rent from a dynamo-electric machine, the magnitude of which current 

 was considerably above that of the telephone currents employed in 

 practice. In the present investigation we have employed the currents 

 furnished by a powerful long-distance microphone transmitter to 

 actuate the receiver, and have measured the amplitude of the vibra- 

 tion of the diaphragm of the receiver under such curi-ents when the 

 strength of the polarizing magnet was i^^aried. 



The apparatus employed was substantially indentical with that de- 

 scribed in the paper referred to. A more sensitive electro-dynamom- 

 eter with a unifilar suspension was used, and the support holding the 

 disk of the receiving telephone was made somewhat more rigid, as it 

 was found that it yielded slightly under the strong pull of the polar- 

 izing magnet when this was strongly magnetized, and in close proxim- 

 ity to the diaphragm. The microphone, actuated by a Cg stopped 

 organ -pipe of large scale, blown by a carefully regulated blast, was 

 placed in the primary circuit of an induction coil, such as is ordinarily 

 employed with the long-distance transmitter. In the secondary circuit 

 was placed the receiving telephone, which was to be studied, and also 

 the electro-dynamometer and an adjustible water resistance. In a few 

 experiments the ordinary hand magneto-receiver was used, but in most 



* Ante, page 93. 



