﻿On tlie Minimum ('tin-cut audible in the Telephone. 285 



alcohol an increase oi '1)08 dyne per centimetre. Correcting 



the former result for dissociation, the result is a rise corre- 

 sponding to *953 dyne for a non-dissociated substance in 

 water ; and the number *968 for solution in alcohol would 

 undergo but a slight diminution on account of the very small 

 amount of dissociation in alcoholic solution. 



Thus it appears that the rise, after this correction has 

 been applied, is almost the same for these two solvents, as it 

 ought to be in accordance with the fourth of the theoretical 

 deductions stated above. 



XXXIII. On the Minimum Current audible in the Telephone. 

 By Lord Rayleigh, Sec.R.S* 



''PHE estimates which have been put forward of the 

 minimum current perceptible in the Bell telephone vary 

 largely. Mr. Preece gives 6 x 10 -13 ampere f ; Prof. Tait, 

 for a current reversed 500 times per second, 2 X 10~ 12 ampere {. 

 De la Rue gives 1 X 10 -8 ampere, and the same figure is 

 recorded by Brough § as applicable to the strongest current 

 with which the instrument is worked. Various methods, 

 more or less worthy of confidence, have been employed, but 

 the only experimenter who has described his procedure with 

 detail sufficient to allow of criticism is Prof. Ferraris || , whose 

 results may be thus expressed : — 



Pitch. 



Do 3 



*a 3 



La 3 



Do 4 



Ke 4 



The currents were from a make-and-break apparatus, and 

 in each case are reckoned as if only the first periodic term of 



* Communicated by the Author, having been read at the Oxford 

 Meeting of the British Association. 



t Brit. Assoc. Report, Manchester, 1887, p. 611. 



X Edin. Proc. vol. ix. p. 551 (1878). Prof. Tait speaks of a billion 

 B.A. units, and, as he kindly informs me, a billion here means 10 12 . 



§ Proceedings of the Asiatic Society of Bengal, 1877, p. 255. 



|| Atti della B, Accad. d. Sci. di Torino, vol. xiii. p. 1024 (1877). 





Minimum current 





in amperes. 



264 



23x10-9 



352 



17 x 10-9 



440 



10 x 10-9 



528 



7x10-9 



594 



5 x 10-9 



