Unit2.lesson2.(29)

The essential components of a moving-coil meter are: a permanent magnet, and a moving coil. The magnet is U-shaped or semi-circular and is made of a material such as Alcomax. Each pole terminates in a soft-iron pole piece, shaft and positioned in figure 2.2

The moving coil is composed of fine copper wires wound on a thin rectangular aluminum former. The former is mounted centrally on a hard steel spindle and can rotate around a fixed cylindrical soft-iron core. The core is placed between the pole pieces 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 are attached to the spindles. They are wound in position and are adjusted so that they balance when the pointer is at the zero mark on the scale.

A moving-coil meter operates on the principle that when a current-carrying conductor is placed in a magnetic field, a foce is exerted on it which causes it to move.

When the meter is inserted in a live circuit, current flows through the control springs into the coil. This set 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 the 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.