The temperature coefficient TC (symbol [α] = K^-1) of the electrical resistance indicates the change of electrical resistance of a (resistor) conductor under the influence of heat. The change of the resistance value per kelvin [1/K] is the temperature coefficient. It is often also indicated in ppm (parts per million). In the case of metals, the resistance mostly rises (positive temperature coefficient). Carbon exhibits a negative temperature coefficient. Light bulbs with a tungsten filament have a low resistance when switched on and only reach their nominal resistance when hot. Initially, they take on a high starting current that may result in tripping the circuit breaker in the case of high loads. The carbon filament lamps commonly used in the beginnings of electrical engineering only reached their full illuminating power when the wire was hot and had reached its lowest value. In the case of load resistors the TC is unwanted because it changes the characteristic values of the resistor – something that has to be considered for the design. Some resistor materials exhibit particularly low TC values and are therefore used for precision and measuring resistors. Special metal oxides with a particularly large positive (PTC) or negative TC (NTC) are used as current limiting (PTC) or voltage limiting (varistors) resistors or serve as temperature sensors in various applications.