lELEl'UONE RECFJV'ERf; AT LOW fOllT.R IXPl'TS 



40.1 



wiri> lirst inadf of the clanipc*! impidatuc r»f six instruinenls at a 

 fri'<|iK'iuv of 1,()(K) cycles for a widi- ranRi- of input currt-nt, and later 

 till' work was i-xti'iidod to the measurement of the vibratory charac- 

 teristics. A l)rid^;e network was used for measuring the current 

 supplied to the impeilance bridge and from the circuit constants the 

 current through llie receiver under test could lie calculated. TheVe- 



Controlling Nthvtjrk Impedanct B''idfl« 



Oscillator 



Htod Receivers 



Fig. 



sistances in the various arn)s of the controlling bridge network were 

 chosen so as to furnish an essentially constant current through the 

 receiver under test, although its impedance might vary through a rather 

 wide range. With the extremely small values of currents involved, 

 it was necessar>' to amplify the power to the bridge balancing receivers 

 approximately 100 TU. For this amount of amplification, it was 

 obviously necessary to take extreme precautions in grounding and 

 shielding the apparatus, in order to reduce to inaudibility the effect 

 of stray fields from the source of current supply. This was success- 



