The temperature of a thermometer will fall, other things being equal, if its net rate of energy loss decreases. However, if its temperature is kept constant using heater, it is possible to infer changes in its net power loss/gain from changes in the power that has to be supplied with the heater. This principle can be used to measure any type of energy flux that is absorbed - to known extent - by the thermometer.
The hot-wire anemometer (circuit 11) increases V
OUT until the power dissipated in the wire sensing element, and hence its temperature and resistance, has risen to the point where the bridge at equilibrium. Air movement past the wire would cool it, but V
OUT increases compensate for the increased dissipation restoring the wire to its equilibrium temperature. The
equilibrium behaviour of the system is independent of the heat capacity of the wire. However, the
dynamic response, and noise figure, are both improved by minimising the size of the wire.
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