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BELL SYSTEM TECHNICAL JOURNAL 



oscillator. This may, for instance, be accomplished by replacing the 

 phone by a thermocouple connected to a standard recording galvan- 

 ometer. Such a recorder would probably record the average of the 

 current squared, but only for changes of energy received of not more 

 than fifty times. The daily variation of the energy level of static (at 

 60 kilocycles) is, however, generally at least 100 times and sometimes 

 even 10,000 times, so that this method of employing a receiving set 

 with fixed amplification is unsatisfactory. Besides it would be very 

 difficult to prevent overloading of the set if wx limit ourselves to the 

 use of 10- watt tubes. 



One important requirement of a static recorder is therefore that the 

 output level of the receiving set be kept constant, or more correctly, within 



TO MOTOR WHICH 



INCREASES GAIN 



OF SET \ 



TO MOTOR WHICH 

 DECREASES GAIN 

 ' OF SET 



METER RELAY 

 MOVING COIL 



TO LOW FREQ. 

 AMPLIFIER 



R 



Fig. 1 — Continuous recording system 



certain narrow limits. In the receiver here described, the amplifica- 

 tion of the receiving set is varied so as to satisfy this requirement, i.e., 

 the gain is automatically cut down when the output level reaches its 

 upper limit, and vice versa. 



The output level of the set meiy be kept constant either by a con- 

 tinuously or by a discontinuously working system. The first method 

 is mentioned here for comparison purposes only and is illustrated in 

 Fig. 1. The figure shows that the rectified output current is sent 

 through a moving coil relay w^hose pointer can only move between the 

 contact points b and c while its real zero position is at a. Nothing 

 happens if the pointer is between b and c but at the moment it touches 

 b a motor will start, to increase the gain of the receiving set and will 

 continue until the pointer is free again. Correspondingly the gain 

 will be decreased if the pointer touches c. The purpose of the large 

 resistance R and the condenser C is to prevent quick movements of 



