Neutral points are the positions where the magnetic field of a magnet is completely neutralised by the horizontal component of earth’s magnetic field. The position & number of neutral points depends upon the position of magnet.
If a magnet is placed vertically as shown then we get only one neutral point situated at point L at left side of magnet if magnet’s north is down. On reversing the magnet neutral point will be at right side of magnet at point R.
Consider a magnet of dipole moment M, length 2L placed horizontally. Following drawings show the top view of the situation
Horizontal component of earth’s field is towards . When the magnet’s north (n) is towards earth’s south (S), then magnetic field of the magnet will be towards opposite to that of BH & both get cancelled at a point somewhere on the axial line of magnet & thus two neutral points (shown by cross marks) are obtained on the axial line (i.e. towards geographic north south line). If neutral points are situated at distance r from the centre of the magnet then at these points we can write
On reversing the magnet i.e. when magnet’s north (n) is towards earth’s north (N), two neutral points are obtained on the equatorial line (i.e. towards east – west line). At these points we can write