550 



BELL SYSTEM TECHNICAL JOURNAL 



With the switches in Fig. 8 in position "^," the alternating current 

 input to the thermophone and the amphfier gain are adjusted to 

 obtain a convenient voltmeter deflection {Vvm)- Then: 



VvM = IiRPEtA. 



Operating the switches to position "5" and manipulating the attenu- 

 ator to attain the same voltmeter deflection as before we obtain 



Then : 



WvM = VAX W-^-^'"^^ = I.RiA X W-'-o-^^K 

 Ri X 10(^-«^^) 



Et — 



RF 



The moving coil receivers are also calibrated in a sealed chamber in 

 an atmosphere of hydrogen by an arrangement shown in cross-section 



ATTENUATOR 



OSCILLATOR 



y 



? ? A 



■AAA/WWV- 



^ 



o\B 



V///J//K 



TRANS. 

 AMPLIFIER 



» TUNED 



V.T 

 VOLTMETER 



Fig. 9 — Circuit for calibrating receiver. 



in Fig. 7. A condenser transmitter, calibrated in the above manner, 

 is actuated by the sound output of the receiver under test driven by 

 the oscillator. The process involved is as follows: Referring to 

 Fig. 9. 



VvM — Voltage across voltmeter. 

 Fo = Voltage across attenuator input. 

 V R — Voltage across receiver terminals 



Rk + Rf 



X 10*^^"-''^^''^ 



