A positive temperature coefficient (PTC) refers to materials that experience an increase in electrical resistance when their temperature is raised. Materials which have useful engineering applications usually show a relatively rapid increase with temperature, i.e. a higher coefficient.

A negative temperature coefficient (NTC) refers to materials that experience a decrease in electrical resistance when their temperature is raised. The lower the coefficient, the greater a decrease in electrical resistance for a given temperature increase. Carbon is having negative temperature coefficient.

Mica is the example for negative temperature coefficient substance. Mica has negative temperature coefficient of resistance. So, resistance will decrease with increase in temperature.