A thermistor is a resistor whose resistance changes significantly with a change in temperature. The bead is generally manufactured from doped titanium which is stable in most applications. There are two types, NTC which is negative temperature coefficient and the resistance decreases with temperature or a PTC which is a positive temperature coefficient and the resistance increases with temperature.
PTC Thermistors for the thermal protection of electrical machines, is a temperature dependent component. The rated operating temperature (ROT) corresponds to the curie point temperature of the ceramic. The resistance, of the PTC thermistor, rises very steeply with relatively small increases in temperature, thus triggering the switching function.