effective and rapid heating of the sensor and which is easily controlled by the electronics the heating element could also be a peltier device these semiconductor devices will generate heat if direct current is passed through them in one direction and will absorb heat i e cool if heat is passed through them in the opposite direction this cooling capability might be useful in enabling the sensor to reach equilibrium with the environment and thus reduce the time needed for the measurement cycle the heater and temperature sensor may be connected to one another electrically as well as mechanically and thermally but in the preferred embodiment they are electrically isolated this eliminates the possibility of electrical cross talk between the heater and the sensor and allows for the use of ac in one part of the circuit if desired in the heater for example while dc is used in another the preferred electronic circuitry fig4 consists of a ramp generator 14 feeding a comparator 16 that is also fed from circuitry connected to the output of the temperature sensor 11 the comparator output drives the amplifier 18 that delivers power to the heater 12 in this arrangement a difference between the ramp value and the value fed back from the sensor is amplified and applied to the heater so as to minimize that difference thus power delivered to the heater varies as required to cause the temperature of the sensor to move up steadily at the rate determined by the ramp generator the level of power driving the heater at any time.