There are the following different methods.
(a) Touch method,
(b) Electrical method,
(c) Other methods.
Touch method. The touch method can be further categorised into
(i) Single touch method. In this method a permanent magnet is rubbed with the soft iron or steel specimen, to be magnetised. The direction of the movement is shown in the Fig. 11.6. The direction is uniform. The process is continued for long time, so this method is not frequently used.
(ii) Double touch method. In this method two magnets are taken and are rubbed over the specimen to be magnetised as shown in Fig. 11.7.
Fig. 11.6. Single touch method.
Fig. 11.7. Double touch method.
(iii) Divide and touch method. In this method two more magnets are used and the specimen is placed as shown in Fig. 11.8. In this case the specimen is magnetised in less time. Both magnets are moved as shown by dotted lines. In this method a non-magnetic piece is kept between the magnets to avoid the possibility of attraction and sticking the magnetic poles.
- Fig. 11.8. Divide and touch method.Disadvantages. There are the following disadvantages of the touch method:l. It requires more time.2. The magnet cannot be Of the required strength.3. The specimen cannot have more strength than the magnet used for magnetisation.Uses. This method is not used for practical purposes.
(b) Electrical method. In this case the coil of insulated wire is placed over the soft iron piece to be magnetised. The direct current is passed through the coil to energise it. The magnetic flux is produced having two poles north and south as shown in Fig. 11.9. The nature and polarity depends upon the specimen, if a soft iron is used than the magnetism will last as long as the current is flowing, if hard steel then the magnetism will retain for a long time.
Fig. 11.9. Electrical method.
Advantages. There are the following advantages of electromagnet:
l. It requires less time.
2. The specimen can be magnetised in any Of the polarities. The polarity can be changed by simply changing the direction of the current in the coil.
3. It requires less labour.
4. The electromagnets can be made Of any shape, size and polarity also.
Uses. This method is used in most of the electrical machines and appliances viz., generators and transformers, electrical bell, buzzer etc.
(c) Other methods. The other methods of magnetisation are:
- If you pound a higher retentivity substance with a hammer while the substance is placed in the direction of earth’s magnetic field, the substance becomes a permanent magnet.
- A high retentivity substance can be heated and subsequently cooled while it is placed in the direction of the earth magnetic field, the substance thereby formed into a permanent magnet.