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Question
Explain the construction and working of the Moving coil Galvanometer.
Solution
Construction:
- M.C.G. consists of a coil of several turns mounted (suspended or pivoted) in such a way that it can freely rotate about a fixed axis, in a radial uniform magnetic field.
- A soft iron cylindrical core makes the field radial and strong.
Moving coil galvanometer
Working:
- The coil rotates due to a torque acting on it as the current flows through it. The torque acting on the current-carrying coil is τ = NIAB sinθ.
Here θ = 90° as the field is radial.
∴ τ = NIAB
where A is the area of the coil,
B is the strength of the magnetic field,
N is the number of turns of the coil,
and I the current in the coil. - This torque is counterbalanced by a torque due to a spring fitted at the bottom so that a fixed steady current I in the coil produces a steady angular deflection Φ.
- The larger the current is, the larger is the deflection and the larger is the torque due to the spring. If the deflection is Φ, the restoring torque due to the spring is equal to KΦ where K is the torsional constant of the spring. Thus, KΦ= NIAB, and the deflection Φ = `("NAB"/"K")"I"`
This means the deflection Φ is proportional to the current I i.e., Φ ∝ I.
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