Q.State the advantages and disadvantages of a moving coil galvanometer. A moving coil galvanometer (M.C.G.) has 10 turns each of length 12 cm and breadth 8 cm. The coil of M.C.G. carries a current of 125 $\mu$A and is kept perpendicular to the uniform magnetic field of induction $10^{-2}$ T. The twist constant of phosphor bronze fibre is $12 \times 10^{-9}$ Nm/degree. Calculate the deflection produced.

MCG: coil + torsion suspension + radial field, described in words; separately, use W = PΔV for the isobaric compression.

Suspended coil M.C.G. (described): a rectangular coil of many turns of fine insulated wire, wound on a light non-magnetic (aluminium) frame, is suspended by a fine phosphor-bronze fibre between the concave pole pieces of a strong permanent magnet. A soft-iron cylindrical core is placed within the coil, and together with the concave poles this produces a strong radial magnetic field, so the coil's plane always stays parallel to the field, giving a torque and hence a deflection proportional to the current, i.e. a linear scale. A mirror attached to the suspension, together with a lamp-and-scale arrangement, is used to read the deflection. The lower end of the coil connects to a hair-spring (or a second fibre) that also carries current back and provides part of the restoring torque.

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Advantages: high sensitivity, measures small currents accurately. Disadvantages: cannot measure AC directly, delicate. Deflection from NIAB = kθ.

Advantages: a moving coil galvanometer (M.C.G.) is highly sensitive and can detect and measure very small currents; using a radial magnetic field, its scale is linear (uniform); it can be converted into an ammeter or voltmeter of any range by suitable shunt/series resistance.

Disadvantages: it responds only to DC, not AC (the average deflecting torque over a cycle of AC would be zero); it is delicate (suspension can be damaged by rough handling or excess current); it requires shielding/careful calibration as external magnetic fields can affect the reading; zero error can develop due to ageing of the suspension.

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