Aim: how To determine the magnetic field intensity at the center and along the axis of a circular coil carrying current and hence find the earth’s horizontal magnetic field by deflection method.
Apparatus: Deflection magnetometer, spirit level, commutator, ammeter, variable power supply and connecting wires.
Formula: (i)The magnetic field intensity of coil,
B=(μ_0 n I)/2 a^2/(a^2+x^2 )^(3/2) T
(ii) Horizontal component of earth’s magnetic field,
B_H=B/(tan θ) T
where, B - the magnetic field intensity at the centre of a circular coil (T),
n - number of turns in the TG coil,
a - radius of the coil (m)
x - distance between the centre of the coil and pointer in the
compass box (m)
 - permeability of free space = 4π x 10-7 Hm-1.
I - the current through the coil (A)
 - mean deflection in TG