448 KEXXELLY AND AFFEL. 



cally with the disk K by means of a screw rod. The weight of the 

 cyhnder M is carried by the flexible metallic strip s, and counter- 

 balanced on the beam bb'^ by the weight w. When carefully applied, 

 the receiver diaphragm D at rest, remains damped without being 

 stressed. 



The vector differences between the free and damped impedances, ^^ 

 over the range of impressed frequency, will enable the motional- 

 impedance circle to be drawn. 



From the data thus obtained, all four constants of the instrument 

 may be computed as indicated in Appendix I. The circle diagram 

 supplies ojo, A, and Z^. The amplitude measurer gives .t^. The con- 

 ditions of the Rayleigh bridge, and its voltmeter, give i^. 



Data Obtaixed on Particular Receivers by IMeans of Ampli- 

 tude Measurements. 



The following are the results of tests on four telephone receivers, 

 marked A, B, C and D respectively, using the amplitude method, 

 in conjunction with the motional-impedance circle diagram. Two of 

 these, A and B, were tested in a vibration explorer ^^ previously 

 referred to, and the other two — C and D, — ■ were tested with the 

 amplitude measurer above described. Although the tests with the 

 vibration explorer interfered with the normal structure of the instru- 

 ment, by reason of the removal of the receiver cap, and are therefore 

 not completely representative of the normal operating conditions; yet 

 it was deemed important to secure, from the vibration explorer, a 

 check on the amplitude method, through an independent determina- 

 tion of the equivalent mass. 



The Taljle is divided into three parts. The first gives the Data 

 obtained from the dimensions of the poles and of the particular dia- 

 phragm used; also the d. c. resistance of the winding at 20°C, by 

 Wheatstone Bridge. The telephone receiver D, was a "1000-ohm'' 

 instrument. The others were of the usual "75-ohm" type. 



15 It has been suggested by the Engineering Department of the Western 

 Electric Co., that if two receivers are available, with sufficiently similar con- 

 stants, one (free) may be inserted in the arm bd, and the other (damped) in 

 the arm cd. A single series of inductance and resistance measurements, over 

 the range of impressed frequency, will then be sufficient to determine the- 

 motional impedance directly. 



16 Bibliography, No. 17. 



