454 



KENNELLY AND AFFEL. 



variations in temperature, (3), variations in air chamber between cap 

 and diaphragm, (4), variations in atmospheric pressure, (5), variations 

 in added mechanical resistance. 



Variations in the Clajviping Adjust:mext of the Cap under 

 Varying Torque. 



Tests of several telephone receivers, made before and after a removal 

 and replacement of the screw cap, were found, at times, to differ con- 

 siderably. This led to an investigation of the influence of screwing 

 on the receiver cap with varying degrees of tightness. The caps were 

 of the same molded composite material as the receiver cases. A lever 

 clamping device was designed and constructed as shown in Figure 16. 

 It consists of a brass rod AB, with a known sliding weight applied 

 at a measured horizontal radius arm r. The rod terminates in a 

 brass fork containing notches, which engage with pins p p screwed into 

 the cover. The receiver is clamped by its shell in a horizontal posi- 



FiG. 16. — Method or Applying Cap Torque. 



tion. The cap is then screwed on slackly, and the final screwing is 

 accomplished with the measured torque. The instrument is then 

 tested for motional impedance under these conditions. The torque 

 is expressed in gram-perpendicular-meters; i. e., in grams weight act- 

 ing vertically at a horizontal radius arm of one meter. 



The effects of varying the screwing-on torque upon the motional- 

 impedance circle of the particular receiver tested, (B with No. 36 

 diaphragm), are shown in Figure 17. It will be seen that with zero 

 imposed torque; i. e., with the cap laid on the diaphragm, but not 

 screwed, the motional-impedance circle has the smallest diameter, 

 and is nested within the others. The resonant frequency of the 

 diaphragm was 859 <^ . As the screwing-on torque is increased, the 



