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The resistance of a micromachined polysilicon thermistor sensor has been measured under excitation by both a constant current and constant power step. The time‐varying and steady‐state response of the polysilicon resistor at ambient pressures of 1 atm and 10-4 Torr were observed under various step amplitudes. The increase of resistance above its room‐temperature value changed exponentially with time; the time constant varied inversely with the square of the current step amplitude but was constant with power step amplitude. The steady‐state change of resistance varied linearly with the applied power under both modes of excitation. The time‐varying and steady‐state responses were as predicted by a simple analytic model developed. The steady‐state operation of these devices at the two pressures permitted us to directly determine the energy losses by heat conduction through the surrounding gas and conduction through the support arms. At 1 atm, up to 90% of the heat loss is by conduction through the gas. The operation of the device was also simulated numerically and results were obtained in reasonably close agreement with experiment. The simulation also allows us to predict the amount of heat flow through the various support arms; 70% of the heat conducted by the support arms flows through 8 of the 20 support arms.