the moving coil, which gives the instrument its name, is composed of the copper wire wound on a thin rectangular aluminium former. the former is mounted centrally on hard steel spindles and can rotate around a fixed cylindrical soft-iron core. the core is placed between the pole pieces in such a manner that an annunal gap is formed between it and the pole piece. a pointer is attached to the former and traverses a linear scale. the spindles which bear the moving coil are mounted on jeweled bearings. two spiral hair springs ảe attached to the spindles. they are wound in opposition and are adjusted so that they balance when the pointer is at the zero mark on the scale.2. operationthis instrument operates on the principle that when a current carrying conductor is placed in a magnetic field, a force is exerted on the conductor which causes it to move.when the meter is inserted in a live circuit, current flows through the control springs into the coil. thí sets up a magnetic field around the coil which reacts with the radial magnetic field of the permanent magnet. the reaction produces a torque which tends to rotate the coil. since the strength of permanent magnet's field is uniform, this torque is directly proportional to the current flowing in the coil. as the coil rotates, the control springs tighten thus opposing the motion of the coil. when the deflecting force of the coil is balanced by the controlling force of the springs, the coil comes to rest. the extent of the coil's movement, and hence the size of the current flowing through the coil, is indicated on the scale by the pointer.
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