Name: Conversion of a Galvanometer Into a Voltmeter
Uploaded: 2019-07-19
Duration: 7 min 18 s
Description: This video helps us understand the working of a galvanometer. It also teaches us how to convert it into a voltmeter of the desired range both theoretically a...

Question

Describe with principle, the construction and working of a moving coil galvanometer.

Accepted Answer

Principle of working of moving coil Galvanometer:

Moving coil Galvanometer works on a principle that a current carrying coil is kept in a magnetic field, it experiences torque.

Construction of a moving coil Galvanometer:

The moving Coil Galvanometer consists of a coil $C$ wound and a non-metallic frame (usually brass). The coil is suspended between two poles of a permanent magnet which are cylindrical shape. The coil is suspended with a movable torsion head $H$ by a strip or wire made of phosphor bronze which acts as path for the current to the coil also. End of the wire is connected to the terminal $T_{2}$ of the galvanometer. The other end of the coil is connected to a light spring $(S)$ which is finally connected to the terminal $T_{1}$ . The spring exerts a very small restoring torque on the coil.

A soft iron is placed within the frame of the coil. A plane circular mirror $(M)$ is attached on the wire or strip to note the deflection of the coil using lamp and scale arrangement.

Here, the area of the loop is $A$ , the magnetic field intensity is $B$ , the number of turns in the coil is $N$ and the current in the coil is $I$ .
The torque acting on the coil will be equal to, $τ = N I A B \sin θ$

Here, $θ$ is is the angle made by the normal to the plane of the coil with the direction of the magnetic field.

When cylindrical poles of permanent magnet are used then it produces radial magnetic field, then $θ = 90 °$ .

Then, $τ = N I A B$ this is referred to be deflecting torque.

As the coil gets deflected, the spring produces restoring toque that is $K θ$ .

Then, $K θ = N I A B \Rightarrow I = \frac{K θ}{N A B}$

Since, $I = G θ$ then, $G = \frac{K}{N A B}$ this is referred to be galvanometer constant.

Thus, the deflection in the coil is directly proportional to the current flowing through it.

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