TELEPHONE DIAPHRAGMS. 431 



The above data provide only three equations, (1), (2) and (3), 

 whereas there are four unknown quantities to be determined; namely, 

 A, VI, r and s. A fourth independent equation, containing an addi- 

 tional experimental relation, is needed, in order to determine these 

 four constants. 



For supplying the missing fourth equation, it should be sufficient, 

 theoretically, either to measure any one of the four constants directly, 

 by an independent method ; or to alter any one of the three constants 

 m, r, and s, to a known extent, without changing the others, and then 

 to repeat the circle diagram. From the difference between the circle 

 diagrams, taken before and after the change, a fourth and independ- 

 ent equation should be forthcoming. 



Object of the New Research here Reported. 



The object of the research here described, was to ascertain how the 

 missing fourth equation might best be obtained for evaluating a, 

 m, r, and s. Several methods have been tried, and one, in particular, 

 is recommended. Certainly by this preferred method, or perhaps by 

 one of the other methods, these four constants can now be measured 

 for any telephone receiver. Numerical values of these constants are 

 tabulated for certain receivers. 



Attention has also been given to the effects of variations in struc- 

 ture and environment, upon the motional-impedance circle diagram, 

 and on the characteristic constants. Nearly one hundred circle 

 diagrams, in all, on various receivers, have been made and examined 

 during the course of the work. 



Evaluation of the Telephone-Receiver Characteristic Con- 

 stants .1, m, r, AND s. 



The following methods have either been tried or suggested, at differ- 

 ent times, for evaluating the characteristic constants of a telephone 

 receiver. 



(1) By the direct measurement of a. 

 " " " " s. 



m. 



(4) By loading the diaphragm with a known mass at the center. 



(5) By varying the elastic constant s. 



(6) By measuring the amplitudes of vibration over the air-gap. 



(2) 

 (3) 



