Explain the construction and working of moving coil instrument.

Explain the construction and working of moving coil instrument.
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1 Answer

Answer :

Construction: The moving part is a coil wound on a light aluminium frame, mounted on a shaft made of stainless steel which is pivoted at either ends on jewel bearing made of sapphire. The coil is placed between poles of a permanent magnet. Moving system is made light as far as possible to have high torque and weight ratio. There are two phosphor bronze springs of very less resistance. This is used as terminals for the current to pass through the coil and out of coil. It is also serves the purpose of control force. Aluminium coil former acts as a damping device by eddy current damping. There is a knife edge pointer and a counter weight to avoid its sagging. 

Working: When ever a current carrying conductor is placed in a magnetic field a force is experiences by the conductor. Moving coil meters work on this principle.

Force = BINL

As the BNL is constant, force (F) is directly proportional to current (I).

That is FαI. Where B is magnetic flux, N is number of turns and L is length of coil. 


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