For most conductors, the electrical resistance increases when heated. This is because the increased temperature causes the atoms in the conductor to vibrate more vigorously, which interferes with the orderly flow of electrons. The greater the temperature increase, the greater the resistance becomes. This phenomenon is known as positive temperature coefficient of resistance (PTC).
However, a few materials, such as carbon, experience a decrease in resistance when heated. This is known as negative temperature coefficient of resistance (NTC).
