Galvanometer

A device used to detect current in any circuit precisely is called a galvanometer. It works on the principle that a current carrying coil, when placed in a magnetic field, experiences a torque.

A galvanometer consists of a coil wound on a non-magnetic frame. A soft iron cylinder is placed inside the coil. The assembly is supported on two pivots attached to springs with a pointer. This is placed between the pole pieces of a horse shoe magnet providing radial field.

When a current is passed through the coil, it will rotate due to the torque acting on it. The spring sets up a restoring force and hence, a restoring torque. If α is the angle of twist and k is the restoring torque per unit twist or torsional constant, then

NBIA sinθ = k α

For θ = 90º, sin θ= 1. So, in the instant case

NBIA = kα

INBA/k = α

I = kα/NBA

where k/NBA is called galvanometer constant. From this you can conclude that

α ∝ I

Deflection produced in a galvanometer is proportional to the current flowing through it provided N, B A and k are constant. The ratio α/I is known as current sensitivity of the galvanometer. It is defined as the deflection of the coil per unit current.

The more the current stronger the torque and the coil turns more. Galvanometer can be constructed to respond to very small currents (of the order of 0.1µA).

Sensitivity of a Galvanometer

In order to have a more sensitive galvanometer,

• N should be large
• B should be large, uniform and radial
• area of the coil should be large
• torsional constant should be small

The values of N and A cannot be increased beyond a certain limit. Large values of N and A will increase the electrical and inertial resistance and the size of the galvanometer. B can be increased using a strong horse shoe magnet and by mounting the coil on a soft iron core. The value of k can be decreased by the use of materials such as quartz or phospher bronze.