202 BELL SYSTEM TECH MCA L JOURNAL 



Continuous operation tests of these tliermistors indicate very satisfactory 

 stability with an indelinitcly long life. A grouj) of eight power meter ther- 

 mistors, normally operated at 10 milliwatts and having a maximum rating 

 of 20 milliwatts, were o])erated for over 3000 hours at a power input of 30 

 milliwatts. During this lime the room temperature resistance remained 

 within 1.5 per cent of its initial value, and the power sensitivity, which is the 

 significant characteristic, changed by less than 0.5 per cent. 



When power measuring test sets are intended for use with wide ambient 

 temjierature variations, it is necessary to temperature compensate the ther- 

 mistor. This is accomplished conventionally by the introduction of two 

 other thermistors into the bridge circuit. These units are designed to be 

 insensitive to bridge currents but responsive to ambient temperature. One 

 of the compensators maintains the zero point and the other holds the meter 

 scale calibration independent of the effect of temperature change on the 

 measuring thermistor characteristics. 



Automatic Oscillator Amplitude Control 



Meacham, and Shepherd and Wise" have described the use of thermis- 

 tors to provide an effective method of amplitude stabilization of both low 

 and high frequency oscillators. These circuits oscillate because of positive 

 feedback around the vacuum tube. The feedback circuit is a bridge with 

 at least one arm containing a thermistor which is heated by the oscillator 

 output. Through this arrangement, the feedback depends in phase and 

 magnitude upon the output, and there is one value of thermistor resistance 

 which if attained would balance the bridge and cause the oscillation ampli- 

 tude to vanish. Obviously this condition can never be exactly attained, 

 and the operating point is just enough different to keep the bridge slightly 

 unbalanced and produce a predetermined steady value of oscillation output. 

 Such oscillators in which the amplitude is determined by thermistor non- 

 linearity have manifold advantages over those whose amplitude is limited 

 by vacuum tube nonlinearity. The harmonic content in the output is 

 smaller, and the performance is much less dependent upon the individual 

 vacuum tube and upon variations of the supply voltages. It is necessary 

 that the thermal inertia of the thermistor be sufficient to prevent it from 

 varying in resistance at the oscillation frequency. This is easily satisfied 

 for all frequencies down to a small fraction of a cycle per second. Figure 20 

 shows a thermistor frequently used for oscillator control together with its 

 static electrical characteristic. This thermistor is satisfactory in oscillators 

 for frequencies above approximately 100 cycles per second. Similar types 

 have been developed with response characteristics suited to lower frequencies 

 and for other resistance values and powers. 



