Direction of magnetic flux
Fig. 31.1 shows a bar magnet NS resting on the edge of a glass trough containing water, while a magnetized steel knitting needle ns is pushed through a large cork and floats on the water with its N pole uppermost. When the needle is held near the N pole of the magnet and then released it is repelled and travels towards the S pole along
a curved path which represents the direction of the magnetic flux. If the experiment is repeated with the S pole of the needle uppermost the needle travels in the reverse direction. Clearly, the direction of travel depends on which pole of the needle is uppermost and one of these directions has to be chosen as a standard direction. It is conventional to choose the direction of the force on a N pole. Consequently, the direction of the magnetic flux at any point is defined as the direction of the force on a N pole placed at the point.