Thermistors are electrical resistors whose value is dependent on the temperature of their material. Metals, ceramic materials or silicon can be used as the resistive material. Thermistors contain either an NTC resistor or a PTC resistor and can thus be tailored to suit a very specific application. Components with a PTC resistor are used for instance to protect electric motors. If the motor becomes too hot the current is either limited or switched off. Varistors are electrical resistors whose value depends on the voltage applied across them. This voltage-dependent characteristic is used for protection of devices and circuits against overvoltage.