thermistor

Resistor whose resistance changes significantly when its temperature changes. The resistance of a semiconductor thermistor decreases with increase of temperature. As temperature rises, the resistance of the material decreases so that the current passing through a thermistor increases rapidly. They are used to electronically detect changes in temperature (electrical thermometers), as in fire alarms and thermostats. They also are used in lamp filaments and electric motors to stop large currents flowing through them when they are initially turned on.

When a lamp is switched on, the thermistor is cold initially and conducts poorly and has a high resistance. As the bulb heats up, the intrinsic conduction of the thermistor improves because the increasing thermal energy of vibrating atoms liberates electrons. These electrons flow more freely as current as the resistance of the lamp's thermistor decreases. The resistance of metals increases with temperature. When a metal is heated, the atoms vibrate more, thus impeding the flow of electrons.