To magnetize a steel bar by an electrical method
The best method of making magnets is to use the magnetic effect of an electric current. A cylindrical coil wound with 500 or more turns of insulated copper wire is connected in series with a 6 or 12 V electric battery and switch (Fig. 30.4). A coil of
switched on and off. On removing and testing the bar it is found to be magnetized. It is unnecessary to leave the current on for any length of time, as the bar will not become magnetized any more strongly and the coil may be damaged through overheating. The polarity of the magnet depends on the direction of flow of the current. If, on looking at the end of the bar, the current is flowing in a clockwise direction, that end will be a S pole; if anticlockwise, it will be a N pole. This may be easily be remembered by the symbols C\) and S as in Fig. 30.4. The magnetic effect of an electric current is discussed in further detail in chapter 36.
Commercially, C- and U-shaped magnets are made by linking them with one very thick copper conductor through which an enormous surge of current is passed for a fraction of a second (Fig. 30.5).