GENERAL ACCOUNT OF MAGNETIC ACTIONS 169 



magnetisation by induction, the needle pole inducing in the near 

 part of the iron an unlike pole by which it is attracted, the like 

 pole by which it is repelled being at a greater distance and there- 

 fore not having so much effect. 



Retentivity and permanent magnetism. With a soft 

 iron bar the induced magnetisation usually disappears on with- 

 drawal of the inducing magnet, but if the bar is of steel more or 

 less magnetisation remains permanently. This quality of retaining 

 magnetisation after the removal of the inducing magnet is termed 

 retentivity^ and the magnetisation so retained is termed permanent 

 magnetism. In general the harder the steel the greater is its 

 retentivity, while the more nearly it approaches soft, pure iron the 

 less is its retentivity. The highest degree of retentivity appears 

 to be possessed by steel containing a certain proportion of tungsten, 

 and called magnet steel. 



If we wish to determine whether a given bar possesses permanent 

 magnetism, i.e. whether it is a magnet or not, we may bring up 

 the two ends in succession near to one pole of a suspended compass 

 needle. If the bar is magnetised in the usual way, one end or the 

 other will be a pole like that of the needle to which it is presented, 

 so that in one case repulsion will ensue. If, however, it is un- 

 magnetised both ends will attract the same pole of the compass 

 needle, for by induction an unlike pole will in each case be formed 

 in the nearer part of the bar, and by this it will be attracted. 

 Frequently with a weakly magnetised bar we may observe repulsion 

 of a pole by one end of the bar when at some distance from the pole, 

 and this changes to attraction at a smaller distance. We may ascribe 

 this change to induction. At a small distance the pole of the 

 compass needle acts by induction, forming in the nearer part of the 

 bar a pole unlike itself and so strong as to mask the effect of the 

 like pole by which it was previously repelled. 



The earth is a great magnet. The tendency of a 

 horizontally suspended magnet to point towards the North led to 

 the invention of the compass, which was introduced into Europe 

 about the twelfth century, probably from China, where it had been 

 used for many hundreds of years. At first it was supposed to 

 point towards the Pole Star, but as early as 1269 Adsiger dis- 

 covered that the needle did not point exactly North.* Columbus 

 in voyaging across the Atlantic in 1492 again discovered that the 

 compass needle was deviated from the geographical North, and by 

 different amounts in different longitudes. This deviation is termed 

 the "variation"" or "declination. 11 In 1576 Norman discovered 

 the "dip. 11 He found that if a needle was suspended through its 

 centre of gravity it dipped with its North end downwards through 

 an angle of 7.1 50' with the horizontal at London. f 



* Encyc. Met. Magnetism, p. 737. 



f Hartuiann had previously discovered the tendency to dip, but did not 

 publish the discovery. See Dove's Jfepertorium dcr Physik, ii. (1838). 



