TELEPHONE DIAPHRAGMS. 473 



alent mass of the diaphragm by the pole or poles of the receiver, 

 when sinusoidal alternating current, i = I^e^"' absamperes, passes 

 through the winding. A is a constant of the receiver, depending upon 

 its structure. It represents the force exerted on the diaphragm per 

 unit current. 



It may be defined by the expressions: — 



dynes/absampere (34) 



. ^ N§So_ 



in a monopolar receiver, and 



A = " = 2N3g.o5 dynes/absampere (35) 



in a bipolar receiver, where N is the number of turns in the winding, 

 including all coils, 3Jo is the mean flux density in the air-gap at the 

 normal position of rest, due to the permanent-magnet system, in 

 the absence of electric current excitation, and 31 is the reluctance of 

 the magnetic circuit to alternating mmfs. 2 = l/si, is the permeance 

 of the circuit to such mmfs. Consequently, from a magnetic point of 

 view, the strength of the receiver is A/N dynes per absampere per 

 turn of exciting winding. This is equal to the product of ^o"!, the 

 permanent normal flux-density and the permeance of the a.c. mmf. 

 The maximum cyclic pull on the diaphragm will be A I^ dynes, and 

 the rms. pull A I^/ V2 dynes. 



Since, however, owing to the effects of hysteresis and eddy-currents, 

 the alternating flux in the air-gap or gaps of the receiver will lag 

 behind the exciting alternating current by some angle ^]°, the instan- 

 taneous pull will not be in phase with the current, taken at standard 

 phase, but will be expressed by 



fi = A.i \J, dynes Z (36) 



Consequently, to current as standard phase, the velocity x in (6) will 

 be 



X = — \^i -— Z (37) 



z sec. 



The alternating emf. induced in the winding by the rate of alteration 

 of air-gap, and flux in the permanent magnetic circuit, will be 



e^ = A.x \^2 ab volts Z (38) 



