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.


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.