The process by which a magnetic substance is magnetised is known as magnetisation. This process can be carried out by:- 1. Mechanical Method 2. Electrical Method.
Essay # 1. Mechanical Method of Magnetisation:
By this method bar magnets are made from bars of magnetic substances by rubbing them mechanically with one or two permanent magnets.
There are three different methods of rubbing:
(i) Method of single touch:
ADVERTISEMENTS:
Place a bar AB of a magnetic substance on a table. Place one end, say N, of a strong bar magnet NS at one end, say A, of AB as shown in fig. 48(a). The bar magnet is drawn from A to B keeping it in an inclined position and always in contact with the magnetic substance. The magnet is then lifted and again its same end, i.e., North Pole is placed at the end A and it is again drawn from A to B.
The process is repeated several times and then the magnetic substance AB is turned over. Its opposite side is similarly rubbed several times with the north pole of the bar magnet always beginning from the end A and terminating at the end B. The bar AB will then be magnetised in the form of a permanent magnet. The end of A of the bar will have north polarity (i.e. same as that of the stroking pole) and the end B where rubbing ends will have south polarity.
(ii) Method of separate touch:
ADVERTISEMENTS:
The bar of the magnetic substance to be magnetised is placed on a table, its two ends being supported on two opposite poles of two other magnets as shown in fig. 48(b). Opposite poles of two strong bar magnets of same strength are placed together at middle of the magnetic substance and they are then drawn in opposite directions up to the end of the bar to be magnetised.
The contact of the magnets with the bar should always be maintained. The magnets are now lifted, placed together again at the middle of the bar and drawn in opposite directions up to the ends of the bar. The process is repeated several times.
The bar is now turned over and its other side is similarly rubbed several times using the same poles of the two magnets. The magnetic substance is then magnetised and it will be a permanent magnet.
ADVERTISEMENTS:
(iii) Method of double touch:
In this method also the bar of a magnetic substance to be magnetised is placed on a table with its two ends being supported by two opposite poles of two other magnets as shown in fig. 48(c).
Opposite poles of two strong bar magnets are placed at the middle of the bar to be magnetised with a piece of cork or wood in between the poles. The magnets are then moved together from the middle of the bar to one of its ends, then back to the other end and finally returned to the middle. The process is repeated several times. The bar is then turned over and its other side is then rubble in the same manner for a number of times.
ADVERTISEMENTS:
The magnetic substance is now magnetised and its two ends develop polarities opposite to that of the nearer stroking pole. It also becomes a permanent magnet.
Essay # 2. Electrical Method of Magnetisation:
A soft-iron core in the form of a bar or a horse-shoe is uniformly wound with an insulated copper or aluminium wire, and the two ends of this wire or coil is connected to a battery through a switch as shown in fig. 49. If the switch is closed and electric current is passed through the coil, the core is magnetised.
Such a magnet is called an electro-magnet. The soft-iron core will retain strong magnetism as long as current passes through the coil, but it will almost completely loose its magnetism at the instant the switch is opened and the flow of current is stopped.
In case the core is made of steel and its alloys, it will form a permanent magnet when once magnetised due to flow of current through the coil.
ADVERTISEMENTS:
The polarities developed by the ends of an electro-magnet depend on the directions of the flow of current through the coil. Viewed from an end if it is found that the current flows in anti-clockwise direction through the coil, the end will develop north polarity. The other end where the current flows in clockwise direction through the coil will develop south polarity.