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The thermistor is a temperature-sensitive resistor much like the RTD. The major difference between the thermistor and the RTD is that the thermistor generally has a negative temperature coefficient and the thermistor exhibits a greater change of resistance for each degree of temperature change. A negative temperature coefficient means that the resistance of the thermistor decreases as its temperature increases. It should be noted that some thermistors are available that have a positive temperature coefficient, but the vast majority used as temperature sensors will have a negative temperature coefficient. The main feature that makes the thermistor so useful is that it's far more sensitive than the thermocouple or the RTD. It's important to understand that the resistance of the thermistor is not linear.

Thermistors are generally manufactured in two broad categories. The first uses semiconductor material that is formed into a particular shape such as a beaded head, probe shape, or rod shape, and covered with glass. The second uses the semiconductor material that is manufactured into the shape of a wafer, disk, or washer and then is coated with a metal cover. The shape of these thermistors makes them useful in industrial applications where they may be mounted directly to nuts and bolts so that they are held in place. ill. 1 shows examples of these types of thermistors.

Examples of thermistors used as temperature sensors.
Above: ill. 1 Examples of thermistors used as temperature sensors.

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Linearity of Thermistors