. The Bell System technical journal . ribed in some detail elsewhere.^--^^^^ Inone form, a directly heated thermistor is connected into the feedback circuitof the amplifier in such a way that the amount of feedback voltage is variedto compensate for any change in the output signal. By this arrangement,the gain of each amplifier in the transmission system is continually adjustedto correct for variations in overall loss due to weather conditions and otherfactors, so that constant transmission is obtained over the channel at alltimes. In the Type K2 carrier sj^stem now in extensive use, the syste
. The Bell System technical journal . ribed in some detail elsewhere.^--^^^^ Inone form, a directly heated thermistor is connected into the feedback circuitof the amplifier in such a way that the amount of feedback voltage is variedto compensate for any change in the output signal. By this arrangement,the gain of each amplifier in the transmission system is continually adjustedto correct for variations in overall loss due to weather conditions and otherfactors, so that constant transmission is obtained over the channel at alltimes. In the Type K2 carrier sj^stem now in extensive use, the systemgain is regulated principally in this way. In this system the transmissionloss variations due to temperature are not the same in all parts of the passband. The loss is corrected at certain repeater points along the transmissionline by two additional thermistor gain controls: slope, proportional to fre- H EATER T^PE/THERMISTOR constantI /;t\ ipRi I^CCt^ to CURRENTS (^) ^f^2 (Nif) CONTROLLED SOURCE T Vp^t I rV^W CIRCUIT DISCTHERMISTOR. HEATER THERMISTOR Fig. 21.—Circuit employing an auxiliary disc thermistor to compensate for effect ofvarying ambient temperature on a control thermistor. quency, and bulge, with a maximum at one frequency. These thermistorsare indirectly heated, with their heaters actuated by energy dependent uponthe amplitude of the separate pilot carriers which are introduced at the send-ing end for the purpose. In this type of application, the thermistor will react to the ambient tem-perature to which it is exposed, as well as to the current passing through this is important, the reaction to ambient temperature can be elimi-nated by the use of a heater type thermistor as shown in Fig. 21. Theheater is connected to an auxiliary circuit containing a temperature com-pensating thermistor. This circuit is so arranged that the power fed intothe heater of the gain control thermistor is just sufficient at any ambienttemperature to give a controlled and const
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