Schematic variation of the electrical resistance of a platinum
resistance thermometer (PRT) and a thermistor. Notice that
the resistance of a PRT increases gradually with increasing
temperature. In contrast, the resistance of a thermistor
falls dramatically as the temperature is increased.

A thermistor is simply an electrical resistor whose resistance changes rapidly with temperature. Usually thermistors are made from small beads of complex materials and although they are not as reproducible as platinum resistance thermometers, they are much more sensitive. Commonly the resistance falls exponentially with increasing temperature and such devices are said to have a negative temperature coefficient (NTC). The strong change in temperature is useful for establishing sensitive temperature control of a system.

As a consequence of the high sensitivity, the range of use of an individual thermistor is comparatively narrow, and a variety of thermistors are needed to suit different ranges and applications. They are popular in digital thermometers for everyday, medical and industrial use. The following graph compares the characteristic of a low-temperature thermistor with that of a PRT.