Thermocouple Ammeter
The working principle of Thermocouple Ammeter is based on the Seebeck effect, which was discovered in 1821. A thermocouple, made of two dissimilar metals (usually bismuth and antimony) is used in the construction of this ammeter. The hot junction of the thermocouple is welded to a heater wire AB, both of which are kept in vacuum as shown in Figure (A). The cold junction of the thermocouple is connected to a moving-coil ammeter
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When the current to be measured is passed through the heater wire AB, heat is generated, which raises the temperature of the thermocouple junction J. As the junction temperature rises, the generated thermoelectric EMF increases and derives a greater current through the moving-coil ammeter. The amount of deflection on the MC ammeter scale depend on the heating effect, since the amount of heat produced is directly proportional to the square of the current. The ammeter scale is non-linear so that, it is cramped at the low end and open at the high end as shown in Figure (B).
This type of “current squared: ammeter is suitable for reading both direct and alternating currents. It is particularly suitable for measuring radio-frequency currents such as those which occur in antenna system of broadcast transmitters. Once calibrated properly, the calibration of this ammeter remains accurate from dc upto very high frequency currents.
Read article – moving-coil ammeter
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