MAGNETIC FIELD ALONG THE AXIS OF A CIRCULAR COIL-DETERMINATION OF BH
To determine the horizontal component of Earth’s magnetic field using current carrying circular coil and deflection magnetometer.
Circular coil apparatus, compass box, rheostat, battery or power supply, ammeter, commutator, key and connecting wires.
BH → Horizontal component of Earth’s magnetic field (T)
µo → Permeability of the free space (4π x 10-7 H m-1)
n → Number of turns included in the circuit (No unit)
I → Current flowing through the coil (A)
r → Radius of the circular coil (m)
x → Distance between center of compass box and centre of the coil (m)
· The preliminary adjustments are carried out as follows.
i. The leveling screws are adjusted so that the circular coil is vertical.
ii. The wooden bench is adjusted to be along the magnetic east-west direction i.e., along aluminium pointer.
iii. The circular coil is rotated so that its plane is in magnetic meridian i.e., along the north-south direction.
iv. A compass box is placed with its centre coinciding with the axis of the coil.
v. The compass box alone is rotated till the aluminium pointer reads 0° − 0°
· Electrical connections are made as shown in the circuit diagram.
· The compass box is placed along its axis, with its centre at a distance x from the centre of the coil on one side.
· A suitable current (1A) is passed through the coil by adjusting rheostat so that the deflection of the aluminium pointer lies between 30 º and 60 º.
· The value of the current I is noted from ammeter.
· Two readings θ1 and θ2 corresponding to two ends of the pointer are noted.
· Now the direction of the current is reversed using commutator, two more readings θ3 and θ4 are noted.
· Now the compass box is taken to the other side and is kept at the same distance x.
· Four more readings θ5, θ6, θ7 and θ8 are taken as done before.
· These eight readings and their average value are tabulated.
· The experiment is repeated for another value of current, say 1.5 A by keeping the compass box at the same distance x.
· The radius of the circular coil is found by measuring the circumference of the coil using a thread around the coil.
· The number of turns n of the coil is noted.
· From the values of n, r, x and I/tanθ, the horizontal component of Earth’s magnetic field is now found using the formula.
Number of turns in the coil n=
Circumference of the coil (2πr) =
Radius of the coil r =
To find horizontal component of the Earth’s magnetic field
Horizontal component of the Earth’s magnetic field at a place = _________T