TELEPHONE DIAPHRAGMS. 437 



Method of loading the Diaphragm with a Central Mass. 



It is shown in Appendix I, at formula (19), that the natural or reso- 

 nant angular velocity of vibration of a diaphragm, considered as a 



simple vibrator, is expressed bv the relation ojq = — • If, therefore, 



a known mass mi grams, is applied at the center of the diaphragm,^ 

 and the resonant angular velocity, under load, determined, we should 

 expect it to be: 



't-'oi = ■vl radians per sec. (4) 



^w+mi 



provided that the vibratory behavior of the loaded diaphragm is the 

 same as in the original unloaded condition, except in regard to reso- 

 nant frequency. By combining (19) of App. I, with (4) immediately 

 above, it should be possible to evaluate both s and m 'in terms of wq, 

 wi and nil. 



Effect of Adding a Mass ini to the Center of the Diaphragm. 



The effect of adding a small metallic cylindrical mass mi grams, 

 to the center of the diaphragm, so as to obtain equation (4), was in- 

 vestigated by Messrs. H. A. Affel and O. C. Hall, in a thesis for 

 the Massachusetts Institute of Technology, on "Telephone-Receiver 

 Characteristics," in 1914. 



A standard bipolar Bell telephone receiver was used with the fol- 

 lowing dimensions: 



Area of each pole in cm. X cm 1.4X0. 225 



Distance separating poles in cm 0.85 



External diameter of diaphragm in cm 5 . 40 



Diameter of clamping circle in cm 4.94 



Thickness of diaphragm in cm . 031 



Total weight of diaphragm in gms 4. 181 



Direct-current resistance of coils, ohms at 20°C .... 73 . 



The electrical connections employed were the same as those in Fig. 1 

 of the 1912 paper, above referred to. The electrical measurements 

 were all taken at constant voltage (0.42 volt r. m. s.) across the tele- 



8 Bibliography, No. 13. 



