A vibration magnetometer consists of two identical bar magnets placed one over the other such that they are perpendicular and bisect each other. The time period of oscillation in a horizontal magnetic field is 25/4 second. One of the magnets is removed and if the other magnet oscillates in the same field, then the time period in second is

The arrangement of magnets is shown in fig. (6).It is clear from figure that distance between free north and south pole is 2 l. Therefore, the dipole moment of the combination = 2 M.The total moment of inertia of the combination = I + I = 2IThe time period of vibration magnetometer in this combination is given byT′=2πI′M′BHor T′∝I′M′ ∴T′∝2I(2M) ......(1)when one magnet is removed, thenT∝IM  ......(2)From eqs. (1) and (2), we getTT′=121/4 or T=T′21/4 ∴T=25/421/4=2sec.