170 MAGNETISM 



In 1600 Dr. Gilbert, of Colchester, published his great book 

 De Magnete, containing an account of experiments which founded 

 the science of Magnetism and Electricity, and in this work he 

 pointed out that the behaviour of the compass needle could be 

 explained on the supposition that the earth is a great magnet, 

 somewhat irregularly magnetised so that it cannot be supposed to 

 have poles merely coincident with the earth's poles. He used a 

 spherical lode-stone as a model, and showed that near the surface 

 a small needle exhibited the tendency to point towards the pole. 

 He showed further that it dipped in one wav in one hemisphere 

 and in the other way in the other hemisphere, that it was perpen- 

 dicular to the surface at the two poles and parallel to the surface 

 round the equator. 



It must not be supposed that there are definite magnetic poles 

 in the earth, but rather that there is a region of South-seeking 

 polarity in the Northern Hemisphere towards which the NSP of a 

 compass points, and a region of North-seeking polarily in the 

 Southern Hemisphere. These regions are not round but n< ar the 

 geographical poles. There is one point in eacli hemisphere at 

 which a magnetised needle free to move about its rent re of gravits 

 dips vertically downwards. That in the Northern Elemisplu re has 

 been visited several times and is situated about 70 N latitude and 

 97 W longitude, and there the NSP is downwards. That in the 

 Southern Hemisphere was visited in 190!) and is Mtuated about 

 S latitude and 155 E longitude. 'Die positions of HUM- points 

 show that the earth is not magnetised symmetrically. 



It is very important to observe that the earth's action on 

 magnets is directive only. The two poles ofanv ordinary magnet 

 may be regarded as practically at the same 1 distance from a pole <>t 

 the earth, and therefore they will he acted on by equal and opposite 

 forces by that pole. The resultant action of the two poles of tin- 

 earth thus being a couple is diivctive and does not tend to move 

 the centre of gravity of a magnet. 



Magnetisation induced by the earth. Not only is the 

 magnetisation of the earth sufficient to direct magnetised 

 but it produces quite considerable induced magnetisation. If in 

 this part of the world a poker be held upright and struck sharply 

 the lower end becomes a NSP and the upper a SSP, the polarity 

 acting strongly on a compass needle. If the poker be re\vi M d 

 and again struck, the magnetisation is quickly reversed, and what 

 is now the lower end becomes a NSP. Any iron lying more or 

 in the general direction of the dipping needle is thus magnetised 

 by induction, as, for instance, the front plates of iron fire-grates 

 or iron gas-pendants. Iron ships are magnetised by the earth and 

 their compasses are affected seriously by this magnetisation. The 

 deviation of the compass thus produced must be allowed for. 



Methods of magnetisation. Various methods have been 

 devised to magnetise bars of steel by stroking them with magnets. 



