MAGNETISATION. 7 37 



time before the first piece of steel will be capable of at- 

 tracting a second, and still longer before a third will be 

 attracted by the second. But the magnetism thus mani- 

 fested by the small steel bars does not disappear when 

 they are removed from the electro-magnet ; each of 

 them is now a permanent though very feeble magnet. 



A bar of steel may be made magnetic more completely 

 and in a shorter time than by placing it simply in con- 

 tact with a magnet, if the magnet, or better, electro- 

 magnet, is moved repeatedly, and in a definite manner, 

 along the steel bar to be magnetised. One pole of the 

 magnet is placed upon the middle of the straight or 

 horse-shoe bar to be magnetised, and repeatedly drawn 

 from the middle to one end. The second pole of the 

 magnet is then placed upon the middle, and similarly 

 drawn to the opposite end of the bar an equal num- 

 ber of times ; during this process, either the bar to 

 be magnetised may be fixed and the magnet moved, 

 or, if more convenient, the latter may be at rest and 

 the bar moved relatively to it. In either case the 

 magnet and bar should not be at right angles to 

 each other, but the magnet and that portion of the 

 bar towards which the motion is directed should form 

 an acute angle. In fig. 371 A, e is an electro-magnet, 

 along one pole of which the bar s, which is to be mag- 

 netised, is moved in the direction of the arrow; in B, s 

 is a horse-shoe of steel, which is magnetised by drawing 

 the permanent magnet m along it. 



As with electro-magnets so with permanent magnets : 

 the weight which a horse -shoe magnet can support is 

 much more than double that which a single pole would 

 hold. 



3B 



