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#### Question

Why is it necessary to introduce a cylindrical soft iron core inside the coil of a galvanometer?

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Why does a galvanometer show a momentary deflection at the time of charging or discharging a capacitor? Write the necessary expression to explain this observation.

**a)** A circular coil of 30 turns and radius 8.0 cm carrying a current of 6.0 A is suspended vertically in a uniform horizontal magnetic field of magnitude 1.0 T. The field lines make an angle of 60º with the normal of the coil. Calculate the magnitude of the counter torque that must be applied to prevent the coil from turning.

**(b)** Would your answer change, if the circular coil in (a) were replaced by a planar coil of some irregular shape that encloses the same area? (All other particulars are also unaltered.)

A galvanometer of resistance *G* is converted into a voltmeter to measure upto *V* volts by connecting a resistance *R*_{1} in series with the coil. If a resistance *R*_{2} is connected in series with it, then it can measures upto *V*/2 volts. Find the resistance, in terms of *R*_{1} and *R*_{2}, required to be connected to convert it into a voltmeter that can read upto 2 V. Also find the resistance *G* of the galvanometer in terms of *R*_{1} and *R*_{2}

Two moving coil meters, M_{1} and M_{2} have the following particulars:

*R*_{1} = 10 Ω, *N*_{1} = 30,

*A*_{1} = 3.6 × 10^{–3} m^{2}*, **B*_{1} = 0.25 T

*R*_{2} = 14 Ω, *N*_{2} = 42,

*A*_{2} = 1.8 × 10^{–3} m^{2}, *B*_{2} = 0.50 T

(The spring constants are identical for the two meters).

Determine the ratio of (a) current sensitivity and (b) voltage sensitivity of M_{2} and M_{1}.

Increasing the current sensitivity of a galvanometer may not necessarily increase its voltage sensitivity. Explain, giving reason.