VIBRATING TELEPHONE DIAPHRAGMS. 457 



Summary. 



1. The depression angles ^^^ and (S.,^ of the diameter of a tele- 

 phone receiver's motional-impedance circle are not closely connected, 

 and are differently affected by impressed frequency. In some cases 

 (3j° was found to be the greater, and in others ^/. Increase in fre- 

 quency increased ftj°. Neither angle was markedly affected by 

 changes in air-gap. The relations between ^i" and ^o" ^'^^Y be con- 

 veniently studied by means of Lissajous figures. 



2. Both the vibration galvanometer, and the oscillograph, have a 

 motional-impedance circle, and a corresponding useful series of mo- 

 tional constants A, m, r and .y. Tests were made on well-known 

 types of these instruments, and their theory is outlined in Appen- 

 dix I. 



3. The motional-impedance circle of a telephone receiver may 

 sometimes reveal a distortion, accompanied by an absorption and a 

 suppression of power. The distortion is ordinarily a reentrant loop. 

 It may also be a general shrinking of velocity, over a considerable 

 range of frequency, accompanying a flattening of the impedance 

 circle. 



4. A distortion in the form of a reentrant loop is attributable to 

 the existence of a secondary or dependent vibratory system, having 

 its own motional constants and resonant frequency. The invading 

 loop may in particular cases be so large as almost to bring the mo- 

 tional impedance to zero near the main diameter. In such a case, 

 there will be two frequencies of markedly large amplitude, one on 

 each side of the frequency of greatest absorption. 



5. A distortion in the form of a general flattening of the imped- 

 ance circle might be accounted for by the existence of secondary vi- 

 bration in a dependent attached system, not having a definite natural 

 frequency. 



6. The dissymmetrical clamping of an amplitude measurer to the 

 cap of a telephone receiver may introduce such deformation of the 

 clamping circle as will give rise to a reentrant loop or loops. Care 

 should therefore be taken to avoid introducing dissymmetrical 

 stresses when applying such an instrument to a receiver. 



7. A dependent motional system, consisting of a short strip spring 



