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We are used to working with fixed resistance heaters such as MgO tubes or resistance wire. With fixed resistance heaters, the thermal coefficient of resistance (T^sub cr^) is quite low, changing minimally over a few hundred degrees temperature range.
PTC is an abbreviation for positive temperature coefficient. In a PTC or stone-heating element, the material's electrical resistance changes as a function of its temperature - the higher the temperature, the higher the resistance. This characteristic makes the stone element "self regulating." Peak wattage comes with lower stone temperature, with the wattage decreasing as the stone gets hotter. In many applications, there is no need for an overtemperature protection or limit switch. Check with UL or other agencies to see...