442 KENNELLY AND AFFEL. 



It will be seen that the equivalent mass of the diaphragm alone, as 

 deduced from observations unloaded and with some one load, vary from 

 1.88 to 1.42 gms., whereas the equivalent mass, as deduced from obser- 

 vations with different loads, only varies between 1.15 and 1.18 gms. 



Owing to the possibility of introducing a change in the equivalent 

 mass, or other constants of the unloaded diaphragm by the addition 

 of a load, the method of determining m by adding loads should be 

 considered as open to criticism, unless sufficiently checked. 



Method of Maximltm Resonant Amplitude Measurement over 



AN Air-Gap. 



It is shown at the end of Appendix I, that if the maximum c^-clic 

 amplitude of vibration is known at a point on the diaphragm over an 

 air-gap, and also the maximum cyclic alternating-current strength 

 producing this vibration, the missing fourth relation is arrived at; 

 so that with the aid of the motional-impedance circle, all four char- 

 acteristic constants are determinable. This method should have the 

 advantages, that it need not interfere appreciably with either the 

 structure, or the normal mode of operation, of the telephone receiver. 

 It also has the advantage that the amplitude of vibration, which is 

 usually so small for the feeble currents of normal operating strength, 

 becomes very appreciable at the resonant frequency, and is ordinaril\' 

 about 5 microns per milliampere of testing current. Strictly speaking, 

 the amplitude should be measured over the air-gap, whose cyclic 

 variation induces the motional emf. ; but ordinarily, it suffices to 

 measure the amplitude at or near to the center of the diaphragm. 

 The method was evolved from the experimental exploration of ^■ibra- 

 tion in diaphragms, ^^ and the apparatus used is a simplified form of 

 such an explorer, for measurements at the center, instead of at any 

 or all points on the surface. 



Amplitude Measurer. 



The amplitude measurer is illustrated in Figs. 10 and 11. The 

 brass clamping frame f can be attached to the cap of any ordinary 

 receiver, and serves to support the working parts of the instrument. 



12 Bibliography, No. 17. 



