How does the resistance of a thermistor varies with temperature?
Conductivity in a thermistor increases(resistance decreases) very sharply as the the temperature is raised.This is because the conductivity is mainly by flow of electrons.In the cold state the electrons are mainly confined to bound states in the atoms and so cannot move,so no current.Rising temperatures cause increasing numbers of electrons to break free of the bound state so you get rapidly increasing conduction.In the temperature ranges where they are most useful they are amongst the most sensitive ways of following small temperature changes.
