Question

To measure the magnetic moment of a bar magnet, one may use
(a) a tangent galvanometer
(b) a deflection galvanometer if the earth's horizontal field is known
(c) an oscillation magnetometer if the earth's horizontal field is known
(d) both deflection and oscillation magnetometer if the earth's horizontal field is not known

Answer 1

To measure the magnetic moment of a bar magnet, one may use  a deflection galvanometer if the earth's horizontal field is known , an oscillation magnetometer if the earth's horizontal field is known  and both deflection and oscillation magnetometer if the earth's horizontal field is not known.

Explanation:

Denial of (a):

Tangent galvanometer is a device used to measure electrical current; it cannot be used to measure a bar magnet's magnetic moment.

Reasons for (b) and (c):

$\frac{M}{B_{H}}$ of a permanent bar magnet is measured using deflection magnetometer.

Furthermore, the magnetometer of oscillation is used to measure the size of a magnet bar. So, if you know the horizontal field of Earth, $M B_{H}$ then you can calculate the magnetic dipole of a magnet bar, $B_{H}$.

Reason for  (d):

Using magnetometer for deflection and oscillation, $\frac{M}{B_{H}}$ and $M B_{H}$can be measured, respectively. So if we subtract the result obtained from both instruments, $B_H$ cancels as

$\frac{M}{B_{H}} \times M B_{H}=M^{2}$

Therefore, it is not appropriate to have the value of collision $M B_{H}$.

Therefore, if the horizontal field of the earth is not know, we can use both deflection and oscillation magnetometers.