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BELL SYSTEM TECHNICAL JOURNAL 



and as seen in Fig. 61 the resistance of the leak can be varied within 

 wide limits without an appreciable effect upon the performance of the 

 circuit. The source of plate potential may be either battery or 

 generator. The choke in the generator supply should be sufficiently 

 large to prevent an appreciable amount of oscillating current flowing 



cgf ? Rg 



Fig. 64 



Cgy 



Fig. 6; 



through the generator. In order to avoid short circuiting the generator 

 in the Hartley circuit, a large condenser C is interposed as shown in 

 Fig. 65. For high frequencies it is customary to make this con- 

 denser so large that it has very little effect upon the frequency of 

 oscillation. 



The power of the oscillating current may be removed either by 

 capacity or magnetic coupling, or it may be dissipated by resistance 

 within the oscillating circuit. Magnetic coupling is very satisfactory 

 for radio frequencies, but for the audio frequencies unless iron core 

 transformers are used, capacity coupling usually proves more con- 

 venient. In regard to the use of iron core inductance and transformers, 

 the reader should note the following Section. 



52. Frequency of Oscillation. It is frequently found that the tube 

 causes an appreciable deviation from the calculated frequency of 

 oscillation. This is not so much because the tube influences the 

 normal operation of the oscillating circuit as it is due to the fact that 

 the impedances of inductance coils (particularly when iron cored) and 

 of the condensers are liable to vary in value with the current amplitude. 

 Hence any change in applied plate potential, filament current, or out- 

 put resistance, because of its effect upon the amplitude of the oscillat- 

 ing current, will give rise to changes in frequency. In designing cir- 

 cuits to operate at a constant frequency regardless of slight changes 

 in tube constants, plate potential, filament current, etc., the most 

 important requirement is to provide inductances and condensers whose 

 impedances are independent of the currents they carry, and whose 

 resistance components are very small. 



