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66 F~JNL~AMENTALS OF' THE:RMC>ME:TRY PART VI THE:RMISTC>R THERMC>ME:TE:RS by Henry E. Sostmann and Philip D. Mets ABSTRACT Thermistors are a useful and important class of electrical ther- mometer, with a transfer function of resistance versus temperature. The resistance-temperature relationship is usually highly non-linear, and much effort has been given to the invention of linearizing circuitry. In the most commonly used types, the coefficient of resistance change is negative; that is, higher temperatures result in higher conduction (lower resistance). (We will ignore positive temperature coefficient thermistors, which have little metrological signifigance). Quality of performance varies widely, in terms of interchangeability, stability, temperature range, and other characteristics. Consideration here will be given only to classes of thermistor which are of interest to the metrologist, but it is worth mentioning that there is a very large number of cruder types, and the number of thermistors produced by all producers in a given year is perhaps in the millions. We will also not consider here, except in a discussion of applications, the many configurations of sheathed, encapsulated, etc., thermistor temperature probes which are available in the marketplace. THEORY OF OPERATION Figure 1 illustrates the property of intrinsic conductivity in a semiconducting