Thermistor

This is a type of resistor is made from a sintered semiconductor material which has a resistance that changes with temperature. There are two types of thermistor:
(i) negative temperature coefficient (NTC) – the resistance of the thermistor falls with increasing temperature

(ii) positive temperature coefficient (PTC) - the resistance of the thermistor rises with increasing temperature

The NTC is the type most often used in schools so I will just consider that type in detail.
The increasing temperature produces more free electrons and so the resistance falls. These electrons are able to 'jump' from the valence band to the conduction band. This increase in conduction electrons masks the effect due to the increase thermal motion of the atoms and electrons.

At low temperature its resistance is large (thousands of ohms), at high temperature its resistance is small (tens of ohms). Very little current will flow through it when it is cold. This means that its resistance increases as the temperature falls.

A graph for the thermistor of resistance against its temperature is shown in Figure 1.

You can investigate this property practically by doing the experiment described in the 14-16 Experiment guides called Thermistor resistance.

Uses of thermistors (NTC)

Thermistors are used as temperature sensors in thermostats in ovens and irons, in fire alarms and on the wing of a plane to detect when the temperature falls low enough for ice to form. They are also in use in premature baby units to detect when a baby may have stopped breathing, current limiting devices and thermometers.

(Sintering is a process where particles of the material are heated below their melting point until they stick together)