TELEPHONE DIAPHRAGMS. 



461 



therefore, that sudden variations of barometric pressure, within the 

 normal range, might produce perceptible changes in the free imped- 

 ance, near resonance. 



Fig. 22. — Motional-Impedance Circles of Receiver 

 C IN Air and in Vacuo. 



Effects of Variations in Mechanical Resistance. 



Since changes in atmospheric pressure had been shown to effect a 

 marked influence upon the mechanical resistance r of the diaphragm, 

 some tests were made to ascertain the mechanical resistance offered by 

 small circular aluminum vanes of different diameters, fastened by a 

 small metallic tie-rod to the center of the diaphragm. The small 

 vane was in each case held at its center, coaxial with the diaphragm, 

 and with its plane parallel to that of the diaphragm (see Fig. 23). 

 In changing from one size of vane to another, the mass of the tie-rod 

 was so altered as to maintain the resonant frequency of the loaded 

 diaphragm substantially unchanged, and therefore, likewise, the total 

 equivalent mass. The results are given in the following Table. 



