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BELL SYSTEM TECHNICAL JOURNAL 



roughly, 40 ohms resistance. The low side of the other transformer 

 operates through a tuned circuit into a similar thermocouple heater. 



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Fig. 1- 



-Circuit Used for the Analysis of Speech. (The Usual Details of the Three- 

 Stage Amplifier Are Not Shown) 



The diagram of Fig. 1 exhibits the essential features of the electrical 

 circuits just described. 



When the diaphragm of the condenser transmitter is set in vibration 

 by speech a current made up of a range of frequencies flows in the 

 heater of thermocouple I., while the heater of thermocouple II is 

 traversed only by such a band of frequencies as the resonant circuit 

 allows. Fig. 2 shows a number of typical resonance curves obtained 



Fig. 2 — Resonance Curves Showing the Resolving Power of Apparatus 



in the course of calibrating this apparatus. These curves are such 

 that the tuned circuit functions as a filter transmitter only a narrow 

 region of frequencies. One side of the twin amplifier transmits the 



