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.