# Moving Coil Galvanometer

It is an electrical device used to detect the presence of current in a circuit and to find the direction of current. It can also measure small amount of current.

Working Principle: When a rectangular wire carrying current is placed in a uniform magnetic field, it experiences torque.

Construction:

Moving coil galvanometer

It consist of a rectangular coil wrapped in a non-magnetic metallic frame and suspended between two strong curved poles of a magnet as shown in figure. The coil is suspended by phosphorus bronze strip and supported by a hair spring by next side.

Working:
Let N, A and I be the number of turns in coil, area of plane and amount of current passing through the coil respectively. Let ‘B’ be the uniform magnetic field strength(flux density) between the poles of magnet. As we pass the current from the coil, the deflecting torque is produced and is given by:
τ= BINAcosθ
Here, ‘θ’ be the angle between the magnetic lines of force and plane of coil. Due to the radial effect, produced by the magnet, θ=0º.
∴τ= BINA ……….(i)

Due to the deflecting torque, restoring torque will be produced in the string and is given by:
τ= kθ,

Here, ‘k’ is called terminal constant and ‘θ’ is the angular displacement of the coil. In equilibrium:

τ= τd

or, kθ =BINA

$\theta = \frac{BNA}{k}.I$

Here, B, N, A, and k are constant.
∴ θ ∝ I.

Since the deflection of the coil is directly proportional to the current flowing through it, we can linear deflection in the galvanometer to detect the current flowing through it.

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