With the aid of a labelled diagram, describe the structure and mode of operation of a moving coil galvanometer.

The structure and mode of operation of a moving coil galvanometer.

Structure

• It consist of a rectangular coil of fine insulated copper wire wound on an aluminium frame to provide electromagnetic damping.
• The coil together with the frame of aluminium are mounted over a soft iron cylindrical core and freely pivoted on jeweled bearing to minimize friction at contact.
• The suspension torsion wire suspending the coil is attached to a pair of control hair springs T₁ and T₂ for feeding current in and out of the coil and control rotation of the coil.
• The coil is then suspended between concave pieces of a strong magnet to provide magnetic field.

Mode of action

• Current I to be measured is passed into the coil via hair spring T₁.
• The current then causes the coil to experience a deflection torque, r = NABI due to a couple force causing rotation in a radial magnetic field.
• The coil turns with the pointer through angle θ until stopped by restoring torque, r = kθ provided by a pair of hair springs T₁ and T₂.
• At equilibrium, NABI = Kθ
• current I = (k/NAB)θ
• I θ, hence the instrument has a linear scale

Where          B = magnetic field strength between the poles of the magnets

A = area of the plane of the coil

N = number of turns of the coil

k = torsion constant of suspension wire

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