The thermistors are usually made of

These are devices whose resistance varies quite markedly with temperature mean having high-temperature coefficient of resistivity. [Their name are derived from thermal resistors]. Depending on their composition they can have either negative temperature coefficient or positive temperature coefficient or positive temperature coefficient or positive temperature coefficient characteristics. The negative temperature coefficient type consists of a mixture of oxides of iron, nickel, and cobalt with small amounts of other substances. The positive temperature coefficient types are based on barium titanate.