TELEPHONE AND MICROPHONE. 523 



The amplitude A' is at first inversely as the period, so long 

 as the oscillations are not rapid ; for a very short period we shall 



have 



A'_M 



A~L'" 



This problem corresponds to the case of an induction coil, 

 the inducing current of which was sinusoidal. 



545. TELEPHONE AND MICROPHONE. For a circuit placed in 

 a variable magnetic field, equation (10) of (518) becomes 



(43) -dt dt dt 



Let us suppose that the circuit contains a constant electromotive 

 force, that the factor L is constant, and that the coefficient Q 

 varies periodically. This is the case, for instance, of an electro- 

 magnet, in front of which a magnet is made to oscillate. We 

 may write, in that case, 



. 27T/ 



1 ~ 7 F' 



The result is the same as if the electromotive force of the 

 battery experienced a periodical variation equal to Qj cos 21? . 



When the permanent state is attained, the induced current 

 oscillates according to the same period, and may still be represented 

 by the expression 



I = L + A sin 27r ( 



\ m 



Substituting this value in equation (43), we deduce from it 



27I-L 



tan 



TR 



4^! QL 



^2 



T 2 



If the electromotive force E is zero, the current transmitted 

 is periodical ; this is the case of the telephone. 



