Question

To measure the magnetic moment of a bar magnet, one may not use.

• A. a tangent galvanometer.
• B. a deflection magnetometer if $B_H$ is known.
• C. a vibration magnetometer if $B_H$ is known.
• D. a deflection and vibration magnetometer. If $B_H$ is not known.

Answer 1

The correct option is A a tangent galvanometer.

Tangent Galvanometer is a type of Galvanometer used to measure the current.
When a a freely suspended magnet is disturbed from its mean position in a uniform magnetic field(Here, earth's magnetic field), it starts vibrating about the mean position. Vibration Magnetometer is used to find period of oscillations which given by the relation, $T=2\pi \sqrt{\frac{I}{m{B}_H}}$.
$I=$Moment of Inertia, $B_H=$Earth's horizontal Magnetic Field, $m=$Magnetic moment.
Deflection Magnetometer uses Tangent law, which states when a bar magnet is suspended in two Magnetic fields $B$ and $B_H$ , it comes to rest making an angle $/theta$ with the direction of $B_H$. In equilibrium state, $B=B_H\tan \theta$.
One can measure the magnetic moment using both deflection magnetometer and vibration magnetometer, given the value of $B_H$. One can also use both without knowing earth's horizontal magnetic field.