Two magnets are held together in a vibration magnetometer and are allowed to oscillate in the earth’s magnetic field. With like poles of magnets together, $12\, oscillations$ per minute are made but for unlike poles together only $4\, oscillations$ per minute are executed. The ratio of their magnetic moments is

The time period of a vibration magnetometer is $T_0$. Its magnet is replaced by another magnet whose moment of inertia is $3$ times and magnetic moment is $1/3$ of the initial magnet. The time period now will be

The radius of the coil of a Tangent galvanometer. which has $ 10 $ $turns$ is $0.1\,m. $ The current required to produce a deflection of $60°$  $({B_H} = 4 \times {10^{ - 5}}\,T)$ is.....$A$

A short bar magnet placed in a horizontal plane has its axis aligned along the magnetic north-south direction. Null points are found on the axis of the magnet at $14\; cm$ from the centre of the magnet. The earth's magnetic field at the place is $0.36\; G$ and the angle of $dip$ is zero.If the bar magnet is turned around by $180^o$, where will the new null points (in $cm$) be located?

A tangent galvanometer has $80$ turns of wire. The internal and external diameters of the coil are $19 \,cm$ and $21 \,cm$ respectively. The reduction factor of the galvanometer at a place where $H=0.32$ oersted will be $(1$ oersted $=80 \,A / m )$

A long vertical wire in which a current is flowing produces a neutral point with the earth's magnetic field at a distance of $5\, cm$ from the wire. If the horizontal component of the earth's magnetic induction is $0.18\, gauss$, then the curent in the wire is......$A$