406 BELL SYSTEM TECHNICAL JOURNAL 



Fig. 3 shows impedance characteristics, with their associated circles, 

 of the same receiver with currents of 2X10~' and 2X10"' amperes. 

 It will be seen that the differences between these curves are insignifi- 

 cant when one considers the low precision of motional impedance data 

 in the absence of extreme precautions with regard to constancy of 

 temperature, etc. Moreover, other impedance analyses at inter- 

 mediate values of current agree with the abo\e within the precision 

 of the measurements. 



To summarize the results, it may be said that the characteristics 

 of receivers remain substantially unaltered as the current is reduced to 

 the point of minimum audibility. In taking impedance measurements, 

 it is well to use a current which is low enough to be on the flat part 

 of the curve. This can usually be done without the use of ampli- 

 fiers between the impedance bridge and balancing receivers. The 

 fact that the vibratory characteristics of the receiver remain un- 

 altered as the power input is reduced to the threshold of audibility 

 throws an interesting light on the behavior of the diapihragm material 

 iiiulur very small motions. Calculations of the minimum audible 

 <implitude near resonance, based on the fact that the constants of the 

 material remain unchanged, show it to be of the order of 10~^ centi- 

 meters. This motion is less than the mean molecular diameter of 

 the diaphragm material. 



