﻿566 Telephonic Measurement of Electromotive Force, 



in all of which a current at least as small as 10 -8 amperes is 

 pronounced audible, would indicate, not to mention the more 

 striking results of Tait and Preece. They agree in order 

 with the data of Lord Rayleigh for frequencies between 300 

 and 500. Perhaps, however, it would be more fitting to 

 regard the extent of the silent interval as the sensitiveness of 

 the telephone, in which case my results are of the order of 

 those found by Lord Rayleigh for the 4-foot octave. 



Throughout my work, it will be remembered, the telephone 

 w T as excited by isolated taps, and pains were taken to assist 

 the ears by aid of sounding-tubes. One therefore expects 

 a difference of behaviour in the two cases. 



In relation to ihe measurement of electromotive force, 

 therefore, the telephone does not quite come up to the 

 requirements. By using the shades of audibility at the mar- 

 gins of the silent interval for the determination, it is possible 

 to define electromotive forces of the order of one volt with an 

 accuracy of one in one thousand. For thermoelectric forces 

 like those occurring in pyrometry, the telephonic method is 

 available only in rough work. Thus a single iridio-platinum 

 couple (20 per cent, iridium in the alloy), with its hot junc- 

 tion kept at low red heat, showed the following values of 

 apparent electromotive force, e, at the margins of the silent 

 field :— 



Noise. e= 9800 microvolt. ) Mean Temperature = 837° C. ) 



? 9752 „ / 9480 834° / Mean 



? 9212 „ f micro- 795° f 815° C. 



Noise. 9170 „ ) volt. 792° J 



The temperatures corresponding to the two apparent values 

 of thermoelectromotive force on each side of the silent field 

 have been added. Hence the interval of silence corresponds 

 to about 40° Centigrade ; but the sensitiveness at the margins 

 of the interval is such that the mean temperature deduced 

 is in error only 2° or 3°. However, close measurement like 

 this can be made only under conditions of exceptional silence 

 in the surroundings. In a noisy room increments of less than 

 10° at 1000° will escape detection, though the results will be 

 more favourable in proportion as temperature increases. 



In endeavouring to account for the occurrence of the silent 

 interval, one may note first that the thickness of the dia- 

 phragm has little influence within the limits of '015 centim. 

 to "030 centim. employed. The range of silence does decrease, 

 however, when the telephone is made more sensitive either by 

 modifications of the key or, again, by moving the armatures 

 and coils closer to the diaphragm when other things are left 

 unchanged. In such cases the forces actuating the plate are 



