﻿294 On the Minimum Current audible in the Telephone. 



of manifesting itself. If we bring together the numbers* 

 derived with the revolving magnet and with the forks, we 

 obtain in the case of 1\ : — 



Pitch. Source. Current in 10~ 8 amperes. 



128 .... Fork 2800 



192 .... Revolving magnet . . 250 



256 Pork 83 



307 .... Revolving magnet . . 49 



320 .... Fork 32 



384 ... . ' . ; 15 



512 7 



640 4-4 



768 10 



It would appear that the maximum sensitiveness to current 

 occurs in the region of frequency 640 ; but observations at 

 still higher frequencies would be needed to establish this 

 conclusion beyond doubt. Attention must be paid to the fact 

 that the sounds were not the least that could be heard, and 

 that before a comparison is made with the numbers given by 

 other experimenters there should be a division by 2, if not 

 by 3. But this consideration does not fully explain the 

 difference between the above table and that of Ferraris 

 already quoted, from which it appears that in his experiments 

 a current of 5 X 10 -9 amperes was audible. 



It is interesting to note that the sensitiveness of the 

 telephone to periodic currents is of the same order as that of 

 the galvanometer of equal resistance to steady ■ currents f, 

 viz. that the currents (at pitch 512; just audible in the 

 telephone would, on commutation, be just easily visible by 

 a deflexion in the latter instrument. But there is probably 

 more room for further refinements in the galvanometer than 

 in the telephone. 



If we compare the performances of the two telephones T ± 

 and T 2 , we find ratios of sensitiveness to current ranging from 

 13 to 30 : so that T 2 shows itself inferior in a degree beyond 

 what may be accounted for by the resistances. It is singular 

 that an experiment of another kind led to the opposite con- 

 clusion. The circuit of a Daniell cell A was permanently 

 closed through resistance-coils of 5 ohms and of 1000 ohms. 

 The two telephones in series with one another and with a 

 resistance-box C were placed in a derived circuit where was 



* The observations recorded were made with my own ears. Mr. Gordon 

 obtained very similar numbers when he took my place. 



t See, for example, Ayrton, Mather, and Sumpner, Phil. Mag. vol. xxx. 

 p. 90, 1890, " On Galvanometers." 



