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Abstract

An apparatus for measuring the differential between a plurality of physical quantities, such as temperature, includes transducers 10 and 12 each positioned to sense the physical quantity. The transducers are preferably of the resistive or nonreactive variety. A single, controlled current power source 15 is connected sequentially to the transducers through a switching circuit 20. Each transducer is connected to the switching circuit and to an amplifier 40 through four-conductor cables 35, 45. The amplifier is therefore connected continuously to the transducers to sense both the voltage developed thereacross when the transducer is connected to the source of current through the switching circuit and also to sense continuously any other voltages, such as those that might be produced by thermocouple effects. The output of the amplifier 40 is applied to a linearizing circuit 50 whose output is used to control the magnitude of the output current of the current source 15 in response to the absolute resistance of each transducer. The switching circuit 20 is controlled by a clock 30 the output of which also controls a synchronous demodulator 55 which is connected to the output of the amplifier means for sampling the peak-to-peak output of the amplifier after all transients have decayed.

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