Doc: Moving Coil Galvanometer Class 12 Notes | EduRev

Physics Class 12

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Class 12 : Doc: Moving Coil Galvanometer Class 12 Notes | EduRev

The document Doc: Moving Coil Galvanometer Class 12 Notes | EduRev is a part of the Class 12 Course Physics Class 12.
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18. MOVING-COIL GALVANOMETER:

Doc: Moving Coil Galvanometer Class 12 Notes | EduRevFig: Moving coil galvanometer

The main parts of a moving-coil galvanometer are shown in figure. A rectangular coil of several turns is wound over a soft-iron core. The wire of the coil is coated with an insulating material so that each turn is insulated from the other and from the iron core. The coil is suspended between the two pole pieces of a strong permanent magnet.

The current to be measured is passed through the galvanometer. As the coil is in the magnetic field Doc: Moving Coil Galvanometer Class 12 Notes | EduRev of the permanent magnet, a torque Doc: Moving Coil Galvanometer Class 12 Notes | EduRev acts on the coil. Here n = number of turns, i = current in the coil Doc: Moving Coil Galvanometer Class 12 Notes | EduRev = area-vector of the coil and Doc: Moving Coil Galvanometer Class 12 Notes | EduRev = magnetic field at the site of the coil. This torque deflects the coil from its equilibrium position.

Doc: Moving Coil Galvanometer Class 12 Notes | EduRevFig: Radial magnetic field

The pole pieces are made cylindrical. As a result, the magnetic field at the arms of the coil remains parallel to the plane of the coil everywhere even as the coil rotates. The deflecting torque is then Doc: Moving Coil Galvanometer Class 12 Notes | EduRev. As the upper end of the suspension strip W is fixed, the strip gets twisted when the coil rotates. 

This produces a restoring torque acting on the coil. If the deflection of the coil is q and the torsional constant of the suspension strip is k, the restoring torque is kq. The coil will stay at a deflection q where:

niAB = kθ or, Doc: Moving Coil Galvanometer Class 12 Notes | EduRev

Hence, the current is proportional to the deflection. The constant Doc: Moving Coil Galvanometer Class 12 Notes | EduRev is called the galvanometer constant, and may be found by passing a known current, measuring the deflection q and putting these values in equation.

Sensitivity:

The sensitivity of a moving-coil galvanometer is defined as θ/i. From equation, the sensitivity is Doc: Moving Coil Galvanometer Class 12 Notes | EduRev. For large sensitivity, the field B should be large. The presence of soft-core increases the magnetic field.

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