﻿Measurement of Electromotive Force. 



5«5 



5. Inferences. — The chief result in these series of data (and 

 more might be added) is the occurrence of the silent field, 

 whose extent, A/', is fully 5 to even 15 times the interval, 67', 

 within which change of sound-intensity is appreciable when 

 the limits of the field in question are exceeded. Thus the 

 phenomenon remained unchanged in character when different 

 resistances r and R were introduced, when the coils of the 

 double telephone were replaced by others, or when the sen- 

 sitiveness was changed j n marked degree. This was also the 

 case with the Bell telephone for different coils, diaphragms, 

 keys, &c. With allowances for the difficulty of estimating 

 faint sounds, the extent Ai' of the silent interval decreases as 

 the sensitiveness of the telephone increases, a result to be 

 anticipated ; but the case is always one involving a true dis- 

 continuity. Thus if the intensity of the current in the helix 

 be laid off as abscissa and the sound-intensity as ordinate, 

 one would expect to find a curve like a b c (fig. 3), where the 



Ksr. 3. 



equidistant points 1, 2, 3, . . . represent appreciable differences 

 of loudness, and the symmetrical branches converge at the 

 origin of coordinates. The curve actually found has the form 

 a! b' b" c". Sound ceases before the otherwise audible current 

 increments are quite expended, so that the part b' d V of the 

 converging curve is truncated. 



The sensitiveness of the telephones was found to be largely 

 dependent on the construction of the contact-key, a statement 

 already made in § 3. However, under my best conditions 

 shades of intensity of less than 10~ r ampere were acoustically 

 indistinguishable, not to mention the much larger interval of 

 silence which intervened. The results thus obtained are, 

 therefore, less encouraging than the data given in experi- 

 ments of De La Rue, Brougb, Pellat, Ferraris, and others*, 



• I have stated that my experiments were made at the incentive of the 

 earlier researches. 



