PROPERTIES AND USES OF THERMISTORS 



201 



for such applications as output amplitude controls for oscillators and am- 

 plifiers. Their nonlinear characteristics also fit thermistors for use as volt- 

 age regulators, volume controls, expandors, contactless switches and remote 

 control devices. To permit their use in these applications for d-c as well as 

 a-c circuits, nonpolarizing semiconductors alone are employed in thermistors 

 with the exception of two early types. 



Power Meter 



Thermistors have been used very extensively in the ultra and superhigh 

 frequency ranges in test sets as power measuring elements. The particular 

 advantages of thermistors for this use are that they can be made small in 

 size, have a small electrical capacity, can be severely overloaded without 



0.5 



ONE 



INCH 



Fig. 19. — Power measuring thermistors with different sized beads. 



change in calibration, and can easily be calibrated with direct-current or 

 low-frequency power. For this application the thermistor is used as a power 

 absorbing terminating resistance in the transmission line, which may be of 

 Lecher, coaxial or wave-guide form. Methods of mounting have been 

 worked out which reduce the reflection of high frequency energy from the 

 termination to negligible values and assure accurate measurement of the 

 power over broad bands in the frequency spectrum. Conventionally, the 

 thermistor is operated as one arm of a Wheatstone bridge, and is biased with 

 low frequency or d-c energy to a selected operating resistance value, for 

 instance 125 or 250 ohms in the absence of the power to be measured. The 

 application of the power to be measured further decreases the thermistor 

 resistance, the bridge becomes unbalanced and a deflection is obtained on 

 the bridge meter. A full scale power indication of one miUiwatt is customary 

 for the test set described, although values from 0.1 milliwatt to 200 milli- 

 watts have been employed using thermistors with different sized beads as 

 shown in Fig. 19. 



