MAGNETISM 195 



159. Poles of the Magnet. If a lodestone be dipped into 

 iron filings, it will be found that there are two places on its 

 surface where the filings seem to be held with greater force 

 than elsewhere, and this condition is also found to exist in all 

 artificial magnets. When any magnet is hung up in such a 

 way that it is free to turn in any direction, it soon assumes a 

 general north and south position and, when pushed out of 

 this position, it returns to it as soon as released. This shows 

 that the position assumed is not accidental, and indicates the 

 existence of some sort of force affecting it. The end of the 

 magnet which thus invariably turns toward the north is called 

 the north-seeking pole, or simply the north 



pole, while the opposite end is the south 

 pole. On many magnets, the poles are in- 

 dicated by the letters " N " and " S " stamped 

 upon them. The poles of a magnet agree 

 with the points on its surface which attract 

 iron most strongly. FlG 67 _ A mag . 



160. The Earth a Magnet. The earth netoscope. (Tower, 

 itself appears to be a great magnet with one ' 



pole near the geographical north pole, and another in the 

 southern hemisphere on the opposite side of the earth. It is 

 to this magnetic north pole, rather than to the geographical 

 north pole, that the magnet turns. This pole is west of 

 Baffins Bay, but its position varies somewhat. In consequence, 

 there are many places in the United States where magnets do 

 not point due north and south if allowed to swing free. In 

 surveying and other work depending on the use of the com- 

 pass, allowance must therefore be made for this difference. 



161. The Compass. The compass is simply a magnetized 

 needle so mounted as to move freely in a horizontal circle 

 and thus indicate by its position the direction of the north 

 magnetic pole. A true north and south line is then easy to 

 establish by making proper allowance for the difference be- 



