REFERENCE SYSTEM FOR TELEPHONE TRANSMISSION 549 



thermal elements, the direct current being sufficiently large to make 

 negligible the double-frequency effect resulting when alternating 

 current is supplied to the thermophone. The alternating current 

 passing through the thermal-elements (gold leaf being selected because 

 of its low heat capacity) causes variations in their temperature. 

 Periodic expansions and contractions of the surrounding gas, resulting 

 from the varying heat transfer occasioned by the periodic, temperature 

 changes of the thermal elements, constitute sound waves of precisely 

 determinable pressure. These sound waves actuate the transmitter 

 and the ratio of the voltage output to the sound pressure gives the 

 transmitter calibration. The process involved is as follows: Referring 

 to Fig. 8. 



A 



ATTENUATOR 

 Fig. 8 — Circuit for calibrating condenser transmitter. 



Et = Voltage generated by condenser transmitter per bar (one 



dyne/cm.-), 

 /i = Alternating current through thermophone and attenuator. 

 P = Pressure developed by thermophone per volt alternating 

 current across it. 

 VvM = Voltage across voltmeter. 



A = Ratio of voltage delivered to 600-ohm load by the transmitter 

 amplifier to voltage impressed in series with condenser 

 transmitter. 

 Vo = Voltage across attenuator input. 

 Ri = Input impedance of attenuator. 

 R = Thermophone resistance. 

 N = Attenuator setting in db. 

 TP = Thermophone. 

 CT = Condenser transmitter. 



