OP ARTS AND SCIENCES. 257 



XIII. 



CONTRIBUTIONS FROM THE PHYSICAL LABORATORY OF THE 

 MASSACHUSETTS INSTITUTE OF TECHNOLOGY. 



XXIV. — EXPERIMENTS WITH THE THERMAL 

 TELEPHONE. 



By Charles R. Cross. 



Communicated October 14, 1885. 



In the Telegraphic Journal for 1880, p. 204, Mr. William H. Preece 

 describes some very interesting experiments with a new form of tele- 

 phonic receiver, consisting of a fine stretched wire, one end of which 

 is attached to the middle of a diaphragm and the other to an adjust- 

 able support. The wire is placed in circuit with a voltaic battery and 

 a microphone. When the microphone is actuated by the voice, the 

 electrical undulations thereby produced in the battery circuit cause 

 corresponding variations in the temperature, and hence in the tension 

 of the wire, and thus the diaphragm is set into vibration, so that speech 

 is reproduced. 



Mr. Preece used six " bichromate of potash " cells. He does not 

 state the amount of the heating effect on the wire, but from what is 

 said it is evident that the temperature of the wire was but slightly 

 elevated. 



The statement of results in the paper referred to is quite brief; and 

 the experiments detailed below were undertaken in order to ascertain 

 to what extent speech can be reproduced by such a receiver, both 

 as to loudness and intensity ; to study somewhat further the changes 

 due to variation in the length and diameter of wire of different mate- 

 rials, under varying conditions of current strength ; and, especially, to 

 observe the effect of varying the temperature of the wire. Most of 

 the experimental work has been carried on, under my direction, by 

 INIessrs. AV. J. Hopkins and J. O. EUinger, students in the Laboratory 

 of the Institute. 



We first sought to find under what conditions we could realize the 

 maximum loudness of speech using phitiuum wire, which had given 



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