﻿On a Quantitative Theory of the Telephone. 295 



also a scraping contact-apparatus B, as indicated in the figure. 

 The adjustment was made by varying the resistance in C until 

 the sound was just easily audible in the telephone under trial. 

 Experiments conducted upon this plan showed that T 1 was 

 only about five times as sensitive to current as T 2 . It was 



C 



1000 



D 



noticed, however, that the sounds, though as equal as could 

 be estimated, were not of the same quality, and in this 

 probably lies the explanation of the discrepancy between the 

 two methods of experimenting. In the latter the original 

 sound is composite, and the telephone selects the most 

 favourable elements — that is, those nearly in agreement with 

 the natural pitch of its own plate. In this way the loudness 

 of the selected sound becomes a question of the freedom of 

 vibration of the plate, an element which is almost without 

 influence when the sound is of pitch far removed from that of 

 the proper tone of the telephone. There was independent 

 reason for the suspicion that 1\ had not so well defined a 

 proper pitch as was met with in the case of some other 

 telephones. 



P.S. — Measurements with the electro-dynamometer have 

 been made by Cross and Page* of the currents used in 

 practical telephony. The experiments were varied by the 

 employment of several transmitters, and various vowel sounds 

 were investigated. The currents found were of the order 

 2 X lO -4 amperes. 



XXXIV. An Attempt at a Quantitative Theory of the 

 Telephone. By Lord Rayleigh, Sec. R.S.f 



THE theory of the telephone cannot be said to be under- 

 stood, in any but the most general manner, until it is 

 possible to estimate from the data of construction what its 

 sensitiveness should be, at least so far as to connect the 

 magnitude of the vibratory current with the resulting con- 

 densations and rarefactions in the external ear-passage. 



* Electrical Review, Nov. 14, 1885. I owe this reference to Mr. Swin- 

 burne. 



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

 Meeting of the British Association. 



