construction and working of moving coil galvanometer

Moving magnet or tangent galvanometer

Construction: It consists of a circular coil of insulated copper wire wound on a circular non magnetic frame. The frame is mounted vertically on a horizontal base provided with leveling screws on the base. The coil can be rotated on a vertical axis passing through its centre. A compass box is mounted horizontally at the centre of a circular scale. The compass box is circular in shape. It consists of a tiny, powerful magnetic needle pivoted at the centre of the coil. The magnetic needle is free to rotate in the horizontal plane. The circular scale is divided into four quadrants. Each quadrant is graduated from 00 to 900. A long thin aluminium pointer is attached to the needle at its centre and at right angle to it. To avoid errors due to parallax a plane mirror is mounted below the compass needle.

Working: The current to be measured is now sent through the coil, and produces a magnetic field, perpendicular to the plane of the coil, and directly proportional to the current. The magnitude of the magnetic field produced by the coil is B; the magnitude of the horizontal component the earth's magnetic field is BH. The compass needle aligns itself along the vector sum of B and BH after rotating through an angle theta from its original orientation, such that, tan theta = B/BH. Since the magnetic field of the earth is constant, and B depends directly on the current, the current is thus proportional to the tangent of the angle through which the needle has turned.

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