moving coil galvanometer construction, working and theory
Answers
The Moving Coil Galvanometer
Construction
It consists of a rectangular coil of a large number of turns of thinly insulated copper wire wound over a light metallic frame. The coil is suspended between the pole pieces of a horseshoe magnet by a fine phosphor – bronze strip from a movable torsion head. The lower end of the coil is connected to a hairspring of phosphor bronze having only a few turns.
The other end of the spring is connected to a binding screw. A soft iron cylinder is placed symmetrically inside the coil. The hemispherical magnetic poles produce a radial magnetic field in which the plane of the coil is parallel to the magnetic field in all its positions. A small plane mirror attached to the suspension wire is used along with a lamp and scale arrangement to measure the deflection of the coil.
Learn about Magnetic Field Due to Current Element, Biot-Savart Law
Working
Let PQRS be a single turn of the coil. A current I flows through the coil. In a radial magnetic field, the plane of the coil is always parallel to the magnetic field. Hence the sides QR and SP are always parallel to the field. So, they do not experience any force. The sides PQ and RS are always perpendicular to the field.
PQ = RS = l, length of the coil and PS = QR = b, breadth of the coil. Force on PQ, F = BI (PQ) = BIl. According to Fleming’s left-hand rule, this force is normal to the plane of the coil and acts outwards.
Force on RS, F = BI (RS) = BIl. This force is normal to the plane of the coil and acts inwards. These two equal, oppositely directed parallel forces having different lines of action constitute a couple and deflect the coil. If there are n turns in the coil, the moment of the deflecting couple = n BIl – b
Hence the moment of the deflecting couple = nBIA
When the coil deflects, the suspension wire is twisted. On account of elasticity, a restoring couple is set up in the wire. This couple is proportional to the twist. If θ is the angular twist, then, the moment of the restoring couple = Cθ, where C is the restoring couple per unit twist. At equilibrium, deflecting couple = restoring couple nBIA = Cθ
Hence we can write, nBIA = Cθ
I = (C / nBA) × θ where C is the torsional constant of the spring; i.e. the restoring torque per unit twist. The deflection θ is indicated on the scale by a pointer attached to the spring.
The sensitivity of Moving Coil Galvanometer
The sensitivity of a Moving Coil Galvanometer is defined as the ratio of the change in deflection of the galvanometer to the change in current. Therefore we write, Sensitivity = dθ/di. If a galvanometer gives a larger deflection for a small current it is said to be sensitive. The current in Moving Coil galvanometer is: I = (C/nBA) × θ
Therefore, θ = (nBA/C) × I. Differentiating on both sides wrt I, we have: dθ/di = (nBA/C). The sensitivity of Moving Coil Galvanometer increases by:
Increasing the no. of turns and the area of the coil,
Increasing the magnetic induction and
Decreasing the couple per unit twist of the suspension fibre.
Advantages and Disadvantages
Advantages
Sensitivity increases as the value of n, B, A increases and value of k decreases.
The eddy currents produced in the frame bring the coil to rest quickly, due to the coil wound over the metallic frame.
Disadvantages
Its sensitivity cannot be changed at will.
Overloading can damage any type of galvanometer.