Moving Coil Galvanometer

Electrical Instruments Electrical instruments are of fundamental importance in the field of measurements. They are almost based on the magnetic effects associated with current. Some of the common electrical instruments are () moving coil galvanometer (ii) ammeter (iii) voltmeter, and (iv) the multimeter.

 Moving Coil Galvanometer

 Moving Coil Galvanometer - A galvanometer is a device used for indicating the flow of current in a circuit. There are two types of galvanometers namely (1) moving magnet, and (ii) moving coil type. The most commonly used galvanometer is the moving coil type.

The moving coil galvanometer was first devised by Lord Kelvin and later modified by D'Arsonval. The principle on which the galvanometer works is that when a conductor carrying current is placed in a magnetic field, experiences a mechanical force given by Fleming's left hand rule.

It consists of a rectangular coil of fine copper wire suspended by a thin phosphor bronze e P) Between the poles pieces of a horse-shoe magnet. P carries a small concave mirror (M) to measure small deflections of the coil with a lamp and scale arrangement (Fig. 2.18). The phosphor bronze wired provides the controlling couple for the moving coil. The current enters and leaves the coil via suitable terminals connected to P and the fine spring Q. The pole pieces of the horse shoe magnets are curved inside and a soft iron piece is placed inside the coil without touching it. This makes the lines of magnetic force to be concentric towards the center of the space between the poles and hence render radial magnetic field. When current (l) is passed through the coil, the coil gets deflected through an angle (0)

The relation between 1 and is given by 1 = Ge where G=C/nBA, which is known as galvanometer constant.

C - Is the torsional couple per unit twist

n - Is the number of turns in the coil

B - is the magnetic induction

A - is the area of the coil

from this it is clear that the defection of the coil is directly proportional to the current passing through the coil. The scale is calibrated and it can give directly the magnitude of current

Shunt Sometimes the galvanometer is connected in a circuit in which large current is flowing. If the whole current is allowed to pass through the galvanometer, it will spoil the coil. To avoid this, a' low resistance is connected in parallel with the galvanometer so as to bypass a major part of the total current through this resistance. As a result, only a very small fraction of total current passes through the galvanometer. This parallel low resistance is called a shunt