(i) Need for a radial magnetic field :
The relation between the current (i) flowing through the galvanometer coil, and the angular deflection (ϕ) of the coil (from its equilibrium position), is
where θ is the angle between the magnetic field vector B and the equivalent magnetic moment vector μm of the current carrying coil.
Thus I is not directly proportional to ϕ . We can ensure this proportionality by having θ = 90°. This is possible only when the magnetic field vector B, is a radial magnetic field. In such a field, the plane of the rotating coil is always parallel to vector B .
To get a radial magnetic field, the pole pieces of the magnet, are made concave in shape. Also a soft iron cylinder is used as the core.
where R' = Resistance required to change the range from 0 to V/2