744 ACTION OF CURRENTS ON MAGNETS. 



which the bar must be held for magnetisation its raised end cannot 

 be well brought near the south pole of the test magnet even if the 

 support upon which it swings is placed near the edge of the table. 

 If an iron bar is to be used again for the same experiment, it should 

 not be used for any other purpose, and must be kept in a horizontal 

 position, one end pointing a little to the north of east, the other a 

 little to the south of west, that is in a direction at right angles to 

 the position of a freely suspended magnet. Any piece of iron or 

 steel which is kept for a time in such a position that one end of it 

 points more to one of the magnetic poles of the earth than the 

 other end, acquires a feeble permanent magnetism, especially if the 

 piece of steel or iron has been at the same time subject to mechani- 

 cal concussion of its particles by blows, twists, etc. ; it is in this way 

 that magnetism is frequently developed in steel and iron instruments, 

 tools, lightning conductors, etc. 



The iron bar used for the experiment on the inductive action of 

 the earth must obviously be quite free from every kind of magnetism. 

 It should be placed horizontally in the approximate east-west 

 direction previously described, and either pole of a small magnetic 

 needle should be successively brought near each end of the bar ; if 

 in each of these four trials, attraction takes place uniformly, the bar 

 is free from magnetism. 



If the current of a galvanic battery passes near to 

 a freely movable magnetic needle, the needle is de- 

 flected from its former position, unless the direction of 

 the current is the same as that of those portions of the 

 Amperian currents of the needle which are nearest to the 

 current. The deflection is the greater the more intense 

 the current, and with a very strong current the needle 

 may even assume a position almost at right angles to its 

 original north-south direction. If the wire through 

 which the current passes is above the magnetic needle, 

 and the current flows from north to south, the north 

 end of the needle is deflected towards the east. 



The direction in which the north pole of the needle 

 is deflected depends on the relative positions of the 

 needle and the wire through which the current flows, 



