﻿Measurement of Electromotive Force. 



563 



quantities just specified, give under hi' the smallest incre- 

 ment of i' in microamperes just audible on either side of the 

 interval of silence, and under Ai' the extent of this interval. 

 In other words, if//, i 2 , —ij, — i/ are two pairs of currents 

 just distinguishable in loudness, one pair at each margin of 

 the interval of silence, then 



Sound therefore just ceases (marked ?) for currents i 2 ' and i/ 

 on either side of zero, and is just audible for currents // and 

 i( on either side of zero. Inasmuch as the values i{ i 2 and 

 ij if may be considered symmetrical with respect to i' = 0, 

 the value of e/E can be found by taking the mean of the 

 measurements for i 2 and i 3 ' with an accuracy of one half the 

 sensitiveness of the telephone at the margins of the silent 

 field. It is for this reason that the inertness of the telephone 

 is no serious drawback, apart from the time lost in making an 

 extra observation. Thus, for instance, the following four 

 values of e/E correspond to t/, i 2 ', ij, «V> 



*/E = 4990, 4996, 5028, 5034, 

 whence, by taking symmetrical terms, e/E = 5012. 



Table showing Sensitiveness of Telephones. 



Signal. 



i'. 



Si'. 



Ai'. 



di'/Ai'. 



E. 



e/E. 

 R. 

 r. 



Remarks. 



Noise. 



? 



? 

 Noise. 



-29 

 -3-8 

 +3-8 

 +2-8 



•9 

 76 

 9 



2 

 •503 

 4950 

 5000 



Double telephone* 

 (fig. 2). Fine- 

 toothed key. 



Noise. 

 ? 



? 



Noise. 



-24 

 -19 

 4-2-0 

 -k24 



4 

 39 

 9 



2 

 490 

 10400 

 10000 



Do.* 



Noise. 



? 



? 

 Noise. 



Noise. 



? 



? 

 Noise. 



-2-2 

 -20 

 4-1-9 

 +22 



•2 

 39 

 15 



2 



492 

 20700 

 20000 



Do.* 



-1-5 

 -1-3 

 +1-2 



+ 15 



•2 



2-5 

 10 



2 



•501 

 20500 

 20000 



Do* 

 With new sound- 

 chamber. 



* Coils with 1200 turns each, 1-25 centim. long, -8 centim. internal diameter, 

 1'4 centim. external diameter. 



I 



