Question

Describe what the D’Arsonval Galvanometer consists of. (b) Show that the angle of deviation of the pointer is directly provided by the Current in the coil.

I will like if correct

Answer 1

Sol: (a) The D’Arsonval Galvanometer is also called Permanent Magnet Moving Coil Galvanometer (PMMC).

It consists of-

1) Permanent Magnet: It is a pare of two opposite poles separated by some distance to produce a uniform magnetic field in the gap.

2) Moving Coil: It is the current carrying element. It is either rectangular or circular in shape and consists of number of turns of fine wire. This coil is suspended so that it is free to turn about its vertical axis of symmetry.

3) Iron core: The iron core is spherical in shape if the coil is circular but is cylindrical if the coil is rectangular. It makes magnetic field  radial in the gap. The iron core is used to provide a flux path of low reluctance and therefore to provide strong magnetic field for the coil to move in this increases the deflecting torque and hence the sensitivity of the galvanometer.

3) Suspension: The coil is supported by a flat ribbon suspension which also carries current to the coil. The other current connection in a sensitive galvanometer is a coiled wire. This is called the lower suspension and has a negligible torque effect.

4) Indication: The suspension carries a small mirror upon which a beam of light is cast. The beam of light is reflected on a scale upon which the deflection is measured.

5) Zero Setting: A torsion head is provided for adjusting the position of the coil and also for zero setting.

(b) Here if number of turn in coil is N area of coil is A magnetic field is B and current through the coil is I then deflecting torque is given as-

Here NBA is constant for a perticular galvanometer suppose it is G then-

Now if deflection is then restoring torque produced by suspension spring (c)-

where c is constant

and here  

hence

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