TELEPHONE DIAPHRAGMS. 



447 



the double maximum amplitude of the luminous band, divided by the 

 magnification factor, as found for the setting, should give the true 

 maximum cyclic amplitude ,t^ of the diaphragm from its normal 

 position of rest. 



The amplitude measurer is then removed from the receiver T, and a 

 damping device is applied to the diaphragm. A corresponding series 

 of simultaneous inductance and resistance measurements is then made 

 over the same range of frequency as before, with the telephone dia- 

 phragm damped, so as to suppress its vibrations. The application 

 ■of a finger to the center of the diaphragm will serve to damp out the 

 vibrations; but this is not a satisfactory damping method; ])ecause, 



Fig. 13. — Method of Damping Telepho>k Diaphragm 



first, the pressure is not uniform during the period of testing, and 

 secondly; because any pressure on the diaphragm, by diminishing the 

 air-gap, alters, to some extent, the magnetic characteristics of the in- 

 strument. A convenient form of damping device is indicated in Fig- 

 ure 13. It virtually applies a relatively large mass to the diaphragm, 

 without imposing any appreciable mechanical pressure; so that in- 

 ertia is depended upon for extinguishing the vibration. The receiver 

 is securely supported in a horizontal position. A small threaded 

 metallic disk K, is attached, by means of shellac, to the center of the 

 diaphragm. Thick shellac varnish appears to be a convenient material 

 for cementing a load to the diaphragm. The cylindrical metallic 

 mass M (4 cm. long and 2 cms. in diameter) is then connected mechani- 



