When a coil of insulated wire is placed around a magnetic substance and electric current is passed through the coil, the substance becomes an electro-magnet and produces flux. The magnetic substance is called ‘magnetic core’, the coil around it is called ‘magnetising coil’ and the current flowing through the coil is called ‘exciting current’.
The polarity developed at any end of an electro-magnet is determined by the direction of the flow of current through the magnetising coil. If on looking at one end of the magnet the current appears to flow anti-clockwise, the end behaves as a north pole and the other end where the current appears to flow clockwise behaves as a south pole.
In order to change the polarities of an electro-magnet the direction of the flow of current through the magnetising coil is to be reversed. This is done by reversing the connections of the coil with the supply lines, the positive terminal of the coil being connected with the negative line and the negative terminal with the positive line.
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As a result current also will flow through the coil in the reversed direction. At that end of the magnet where the current was previously flowing anti-clockwise through the coil will now flow clockwise, and at the other end where the current was flowing clockwise will now flow anti-clockwise.
Due to this reversal in the direction of the flow of current the polarities of the magnet will be reversed, the end so long behaving as a north pole will now behave as a south pole and the other end will behave as a north pole. This is clearly shown in figures 53(a) and 53(b) Where the polarities of a bar magnet have been reversed. In case of a horse-shoe magnet also the polarities of the magnet can be reversed by adopting the same procedure, i.e. by reversing the connections of the magnetising coil with the supply lines. This is shown in figures 54(a) and 54(b).
