The spind les which bear the moving coil are mounted on jeweled bearing two
spiral hair springs are attached to the spindles. They are wound in apposition
and are adjusted so that they balance whenthe pointer is at the zero mark.
Operation
This instrument operates on the principle that when a current carrying
conductor is placed ina 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 intothe coil. This 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 the permanent
magnet’s field is uniform, this torque isdirectly proportional to the current
flowing in the coil. As the coil rotates, the control springs tighten thus
opposing the motion of the coil. Whenthe 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 bythe pointer.
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