A temperature sensor (16;36;50;60;70), for example for a heating tape, or other heating means, characterised in that first and second electrical conductors (20a,20b; 37,38; 51,52; 61.62; 71.72) thereof each have a substance such as salt in electrical contact therewith, said substance having a significantly different electrical resistivity when the substance is at a temperature below a transition temperature range than when the substance is at a temperature above said transition temperature range.