Deflection magnetometer consists of a small magnetic needle pivoted on a sharp support such that it is free to rotate in a horizontal plane. A light, thin, long aluminium pointer is fixed perpendicular to the magnetic needle. The pointer also rotates along with the needle (Fig.).
There is a circular scale divided into four quadrants and each quadrant is graduated from 0o to 90o. A plane mirror fixed below the scale ensures, reading without parallax error, as the image of the pointer is made to coincide exactly with pointer itself. The needle, aluminium pointer and the scale are enclosed in a box with a glass top. There are two arms graduated in centimetre and their zeroes coincide at the centre of the magnetic needle.
Broad?side on (or) Tan B position - Deflection magnetometer
The magnetic field at a point along the equatorial line of a bar magnet is perpendicular to the horizontal component of Earth?s magnetic field. If the magnetometer and a bar magnet are placed in such way that this condition is satisfied, then this arrangement is called Tan B position.
To achieve this, the arms of the deflection magnetometer are placed along the North ? South direction (i.e) along the magnetic meridian. The magnet is placed along East - West direction (i.e) parallel to the aluminium pointer as shown in the Fig..
When a bar magnet of magnetic moment M and length 2l is placed at a distance d from the centre of the magnetic needle, the needle gets deflected through an angle θ due to the action of the following two magnetic fields.
(i) The field B due to the bar magnet along its equatorial line
(ii)The horizontal component of Earth?s magnetic field Bh.
The magnetic field at a distance d along the equatorial line of the bar magnet,
B=?0/4 π . M/(d2-l2) 3/2
According to tangent law
B = Bh tan θ
?0/4 π . M/(d2-l2) 3/2 = Bh tan θ
If the magnet is short, l is small compared to d and hence l 2 is neglected.
?0/4 π . M/d3 = Bh tan θ
Comparison of magnetic moments of two bar magnets
(i) Deflection method
The deflection magnetometer is placed in Tan B position. A bar magnet of magnetic moment M1 and length 2l 1 is placed at a distance d1 from the centre of the magnetic needle, on one side of the compass box (Fig.). Since, the sensitivity of the magnetometer is more at 45o, the distance
of the bar magnet should be chosen such that the deflection lies between 30o and 60o. The readings corresponding to the ends of the aluminium pointer are noted as θ1 and θ2. The magnet is reversed pole to pole and kept at the same distance. Two more readings θ3 and θ4 are noted. By placing the magnet on the other side of the compass box at the same distance, four more readings θ5, θ6, θ7 and θ8 are noted as above. The mean of the eight readings gives a value θI.
The experiment is repeated as above for the second bar magnet of magnetic moment M2 and length 2l2 by placing at a distance d2. Now the mean of the eight readings gives a value of θII.
Applying tangent law, for the first magnet,
?0/4 π . M1/(d12-l12) 3/2 = Bh tan θI ????.(1)
and for the second magnet
?0/4 π . M1/(d22-l22) 3/2 = Bh tan θII ????.(2)
From the above equations (1) and (2), we get
M1/M2 = (d12-l12) 3/2 tan θI / (d22-l22) 3/2 tan θII
If the magnets are placed at the same distance, then d1 = d2 = d
M1/M2 = tan θI / tan θII
(ii) Null deflection method
The deflection magnetometer is placed in Tan B position (Fig.). A bar magnet of magnetic moment M1 and length 2l1 is placed on one side of the compass box at a distance d1 from the centre of the magnetic needle. The second bar magnet of magnetic moment M2 and length 2l2 is placed on the other side of the compass box such that like poles of the magnets face in the opposite direction. The second magnet is adjusted so that the deflection due to the first magnet is nullified and the aluminium pointer reads 0o - 0o. The distance of the second magnet is x1. The first magnet is reversed pole to pole and placed at the same distance d1. The second magnet is also reversed and adjusted such that the aluminium pointer reads 0o - 0o. The distance of the second magnet is x2.
The experiment is repeated by interchanging the magnets. Two more distances x3 and x4 are noted. The mean of x1, x2, x3 and x4 is taken as d2.
Since the magnetic fields due to the two bar magnets at the centre of the magnetic needle are equal in magnitude but opposite in direction.
B1 = B2
M1/M2 = [ (d12 ? l12)2 d2 ] / [ (d22 ? l22)2 d1 ]
M1/M2 = (d12 ? l12)3/2 / (d22 ? l22)3/2
If the bar magnets are short, l1 and l2 are negligible compared to the distance d1 and d2
M1/M2 = d13 / d13
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