124 Dr. 0. J. Lodge on Intermittent Currents 



ifr 

 that is, its indications give the value - \ i 2 dt, where again r } 



the total duration of the current, is small. 



But the telephone-plate has such a very small period of 

 swing that the same t, which is vanishing compared with the 

 time of oscillation of a needle, may be many times greater than 

 that of the telephone-plate. Moreover the plate is not limited 

 to one mode of vibration, but can have minor vibrations su- 

 perposed on the fundamental ; so that it can enter into the 

 changes going on, and render minute fluctuations audibly ap- 

 parent which would in more slowly moving detectors be simply 

 merged in the total effect. 



Thus a rapidly alternating current (such as the telephone 

 itself produces), which is totally unfelt by a galvanometer, is 

 appreciated by a telephone-plate in its variability — the pitch 

 of the note indicating the number of vibrations, even if they 

 are so rapid as to produce a shrill whistle. The telephone, in 

 fact, does not integrate the current, but gives all its fluctua- 

 tions within certain limits. 



The complete theory of the telephone, setting forth precisely 

 on what the loudness of its indications depends, would be a 

 most interesting and important investigation ; but if it has 

 been attacked, I am ignorant of it. It seems probable that the 

 loudness of the sounds will be found to depend upon the am- 

 plitude of the vibrations of the plate and upon their velocity 

 conjointly — in other words, both on the total change of the 

 current and on the rate at which the change takes place; i. e., 

 in the symbols hereafter to be used, that the loudness is a direct 



function ofj -p I shall not, however, assume anything of 

 etc 



this sort, but shall content myself with simply finding the 



value of the current j as a function of the time, leaving the 



rest to be done subsequently. 



It is quite true that the telephone is only an indicator and 

 not a measuring-instrument ; but so many null methods can 

 be devised which permit measurements to be made with a 

 simple detector, that it is probable that it will have important 

 applications in this capacity also. And hence I think the 

 general theory of intermittent currents, or of currents in the 

 variable stage, will come into more prominence than hitherto. 



The induction-balance furnishes an illustration. The thing 

 itself is old and well known : Dove made experiments with 

 it; and Felici established the laws of current induction by its 

 aid: but its power as an instrument of research was never ap- 

 preciated till Prof. Hughes applied to it an intermittent cur- 

 rent and a telephone. 



