Insulators and semi-conductors behave in a different way when the temperature increases, because their resistivity decreases. That is: the resistance of an insulator and of a semi-conductor decreases with temperature increase, (their resistance-temperature coefficient is negative!). This feature can be used to advantage as the following example shows.
One example of this effect occurs in a thermistor, which is a thermally sensitive resistor whose resistance alters with temperature; a negative temperature coefficient (n.t.c.) thermistor is one whose resistance reduces with increase in temperature. A thermistor is used in the cooling-water temperature-measuring circuit of a car or lorry; it is inserted in the cooling water and connected in series with the battery and temperature gauge. As the water temperature rises, the resistance of the n.t.c. thermistor falls and allows more current to flow through the temperature gauge; this causes the gauge to indicate variations in water temperature.