458 KENNELLY AND AFFEL. 



degrees of twist of the cap as abscissas, an approximate scale of cor- 

 responding imposed torque being added. 



It is known that a considerable proportion of the mechanical re- 

 sistance r offered by a receiver diaphragm, is due to the braking effect 

 of eddy currents set up in the diaphragm, during its vibration in a 

 strong magnetic field. It is reasonable, therefore, to suppose that 

 the diminution in r, which was found to accompany the increase in 

 screwing-on torque, and in air-gap, may have been due to a reduction 

 in eddy-current damping action, since the variations of magnetic field 

 within the diaphragm would be diminished. It may be safely inferred 

 that the adjustment of a telephone-receiver cap is able to affect the 

 receiver constants considerably. In this particular receiver, the most 

 sensitive and satisfactory setting was with the cap screwed on tightly. 



Effect of Variations in Temperature. 



In order to ascertain the influence of temperature upon the char- 

 acteristics of a receiver, a large electric oven ^^ was used, in which 

 the receiver was placed, at a conveniently controlled temperature. 

 A number of motional-impedance circles were observed, at oven tem- 

 peratures from 16° C. to 50° C, all other conditions being maintained 

 constant. It was found that two effects were produced with rise of 

 temperature; namely: 



(1) A reduction in resonant frequency, amounting to about 2.5 

 cycles per second, per degree C. rise. 



(2) A slight reduction in circle diameter, which, however, was not 

 always noticed. 



The results obtained are given in the following Table. They are 

 given in the sequence of observation. 



The reasons for the above indicated effects of temperature on the 

 receiver characteristics have not been analysed. They might be 

 attributed to temperature changes in the mechanical elasticity con- 

 stants of the diaphragm; or to expansional effects in the structure; 

 or to both causes. 



It is evident, therefore, that, judging from this particular instru- 

 ment, the influence of temperature on a telephone receiver's character- 

 istics are very appreciable. Care should be taken to maintain the 

 temperature of the instrument constant during any set of observations. 

 With this object in view, it was customary to conduct the tests with 

 the instrument inside the closed oven, and with the heat shut off. 



18 Bibliography, No. 18. 



