The operation of electrothermic
instruments (or thermal instruments) depends on variation in the
properties of the circuit element due to the flow of current to be
measured. The main significance of such instruments is that they can be
used to measure current at very high frequencies without much errors. So
obviously these instruments are used for current measurements at
frequencies well above the range of moving iron and electrodynamometer
type instruments. These instruments can be used for precision voltage
measurement at moderate frequencies.
1.1 Classification of Electrothermic Instruments
The electrothermic
instruments are based on various thermo-electric effects. which are
used for measurement of current. There are three main types of electrothermic instruments given as follow,
i) Hot wire instruments : Such instruments are based on the property that when current passes through wire, the wire gets heated and expands.
ii) Thermocouple instruments :
Such instruments are based on the principle of thermocouple. When
current passes through a junction of two dissimilar metals, the junction
gets heated and e.m.f. is developed.
iii) Bolometers : Such
instruments are based on the fact that when current passes through
circuit element, it gets heated. Due this dimension changes and
resistance of circuit element changes.
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