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The D Arsonval Moving Coil Galvanometer

Principle. It is an instrument used for the measurement of current. Its action is based on the torque acting on a current-carrying coil placed in a magnetic field.

Construction – It consists of a rectangular coil (10 to 20 turns) of insulated fine Cu-wire wound on a light brass or aluminum frame. The coil is suspended between the cylindrical pole pieces of a permanent horse-shoe magnet NS by means of a thin phosphor-bronze strip. The upper end of the strip is attached to a torsion head. The lower end of the coil is attached to a spring of phosphor-bronze. A soft iron cylindrical core C is placed symmetrically within the coil without touching it. It concentrates the lines of force and thus makes the magnetic field between the pole-pieces strong. A small mirror is attached to the lower portion of the suspension strip. The mirror is used to reflect a beam of light on to a (translucent) scale.

The current to be measured enters at one terminal T1 and passes through the suspension, coil and spring and finally leaves at the second terminal T2.
Moving Coil Galvanometer

Theory. Let B be the magnitude of the magnetic field induction due to the permanent magnet, A the area of the coil, and N the number of turns in it. The magnitude of the torque acting on the entire coil when carrying a current i is.
            τ = NiBA sin θ
Here, θ is the angle which the normal to the plane of the coil makes with the direction of B. The magnitude field B is made ‘radial’ by making the pole-pieces cylindrical. In this case the normal to the plane of the coils is always at right angles to the magnetic field B so that θ = 90o. Hence the torque acting on the coil is
            τ = NiBA sin 90o = NiBA.
Let Φ be the steady angular deflection of the coil. Then, the elastic torque developed in the suspension is cΦ, where c is the ‘torsional constant’ of the suspension. Therefore, we have
            τ = NiBA = c Φ
or            i = c/NBA Φ
or            i = k Φ
Here, k = c/NBA = galvanometer constant.
Thus the deflection angle of the galvanometer is proportional to the current through it.

Moving Coil Galvanometer1

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