Question

explain Galvanometer with its working process

Answer 1

Galvanometer is device used to detect current in a circuit

Principle: A current carrying coil placed in a magnetic field experiences a current dependent torque which tends to rotate the coil, and thus produces an angular deflection.

Construction :A Weston (pivoted-type)(used in laboratories) galvanometer consists of a rectangular coil of insulated copper wire wound on a light non magnetic metallic (Aluminium) frame. This frame has got an axle. Both the ends of this axle are pivoted by jeweled bearings. The motion of the coil is controlled by a pair of hair springs of phosphor- bronze on both the ends which provide the restoring torque for the coil. These springs also act as current leads. A light aluminium pointer is also attached to the coil which helps to measure the deflection of the coil on the measuring scale. The coil is symmetrically placed between two cylindrical horse shoe magnets or, pole pieces of a strong permanent horse-shoe magnet. A cylindrical soft iron-core is also mounted. Since it has been mounted symmetrically between the concave cylindrical poles, the magnetic lines of force are allowed to act along the radius of the iron core. Thus, we get a radial magnetic field. The soft iron core due to its high permeability, intensifies the magnetic field. This increases the sensitivity of the galvanometer.

Theory and working:
Let:
I = current through the coil
a,b = Sides of the coil where, b is the side facing the magnets
A = ab = Area of the rectangular coil
N = Number of turns in the coil

As the magnetic field is radial, at every instant of time, the plane of the coil would be parallel to the magnetic field. The side a will also remain parallel to the magnetic field. Thus, the side a will not experience any force by the magnetic field. However, the side “b” of the coil would remain perpendicular to the magnetic field and thus experience a force. From Fleming’s left hand rule, we see that the two opposite “b” sides of the rectangular coil would experience equal and opposite force. This will constitute a couple and this will produce a torque.
This torque,
t = Force x perpendicular distance between the lines of action of the two forces
= BIbN x a sin(theta) where theta is the angle between the area vector and the magnetic field which is 90 degrees

So, the torque = BIbN x a = BINA

This torque deflects the coil by an angle alpha. A restoring torque acts on the coil, by the hair springs such that, restoring torque = k.alpha

where k is the torsion constant of the spring

Now, in equilibrium position,
k.alpha = BINA
Thus, alpha is directly proportional to I.
Thus, alpha = (NBA/k).I
Or, I = (k/NBA).alpha
Or, I = G.alpha

G is a constant, called Galvanometer constant and is constant for a galvanometer.

Meanwhile, the current which produces a deflection of one scale division is termed as the Figure of Merit of the galvanometer.

Sensitivity of a galvanometer: A galvanometer is said to be sensitive if it shows a large scale deflection even when a small amount of current is passed through it, or a small amount of voltage is applied across it.

CURRENT SENSITIVITY: The amount of deflection shown by a galvanometer when a unit current is passed through it. I(s) = alpha/I

VOLTAGE SENSITIVITY: The amount of deflection shown by a galvanometer when a unit potential difference is applied across the ends of the galvanometer. V(s) = alpha/V =alpha/IR = I(s)/R

[where, alpha = (NBA/k).I ]

Thus the factors on which the sensitivity of a moving coil galvanometer depends on, are:
1. Number of turns of the coil, N
2. Strength of magnetic field, B
3. Area of the rectangular coil, A
4. Torsion constant of the spring and the suspension wires, k