26 



MAGNETISM. 



responds in its properties to the north 

 pole of a common magnet. 



(105.) It may be necessary to remark 

 that this circumstance of the south pole 

 of the terrestrial magnet being situated 

 near the north pole of the earth, and vice 

 versa, has occasionally created a confu- 

 sion of terms. Some authors have taken 

 a fancy to reverse the names we have 

 hitherto given to the magnetic polari- 

 ties : assuming that it is more correct to 

 set out by calling that property which 

 distinguishes the pole of the terrestrial 

 magnet situated in the northern regions, 

 the northern polarity : and consequently 

 to give the name of the south pole to that 

 pole of the compass, or ordinary magnet, 

 which is attracted towards it, and which 

 of course has the opposite polarity. For 

 the same reason they would call the ant- 

 arctic pole of the magnet of the earth, the 

 south pole, and that end of the needle 

 which is turned towards it, the north 

 pole. Mr. Savery * endeavoured to avoid 

 this confusion of terms by using the 

 word end, in contradistinction to that of 

 pole ; and this phraseology is adopted 

 by Mr. Christie in his papers in the Phi- 

 losophical Transactions, as appears from 

 the following passage : " To prevent 

 any ambiguity, I must here state, that 

 by the south pole of a magnet, I under- 

 stand always the end which, when the 

 magnet is freely suspended, points to- 

 wards the north pole of the earth ; so 

 that the north end is the south pole, and 

 the south end the north pole of a mag- 

 netic needle ." It matters little which 

 set of terms are used, provided they are 

 clearly denned, and all persons agree to 

 abide, by these definitions. But where- 

 ever a diversity of practice exists, it is 

 then best to adhere to that which most 

 generally prevails : in the present case 

 the authorities in favour of the nomen- 

 clature we have adopted are much the 

 most numerous. 



(106.) Some have attempted to avoid 

 the confusion which the changes just 

 mentioned would lead to, by the intro- 

 duction of the terms Boreal and Austral 

 instead of north and south : the former 

 set of terms having reference to the na- 

 tural magnetism of the earth, the latter 

 to that of the needle, or artificial mag- 

 net : that is, they would express what 

 we have all along called the northern 

 polarity, by the term Austral polarity ; 

 and the southern polarity, they would 

 translate by the expression Boreal po- 



* Phil. Trans, for 1730, p. 295. 

 t Ibid, for 1823, p. 344. 



larity. We should not have dwelt upon 

 this comparatively unimportant topic, 

 were it not that this change of language 

 is sanctioned and adopted by Biot and 

 most of the Continental writers on mag- 

 netism. 



(107.) Assuming it, then, as an hypo- 

 thesis, that the earth contains in its axis, 

 or near it, a powerful magnet, let us 

 note the consequences which follow from 

 it, and compare them with the facts. 

 We shall begin with those that relate to 

 the inductive power of the earth's mag- 

 net : a power which will be exerted in 

 the direction which a magnetised needle, 

 at perfect liberty to move, would assume 

 in consequence of the action of terrestrial 

 magnetism; that is, in the direction of the 

 magnetic position. In this part of the 

 globe this position is, as we have seen, 

 not very far from the perpendicular to the 

 horizon. A bar of unmagnetised iron 

 placed in the vertical position, or near it, 

 ought therefore to become magnetic 

 from the influence of the earth, and 

 merely in consequence of its position. 

 Its lower end should exhibit the proper- 

 ties of a north pole, and its upper end 

 those of a south pole. All this agrees 

 perfectly with experience. An iron bar 

 held nearly upright will be found, at its 

 upper end, to attract the north pole of 

 a compass needle, and repel the south 



?ole : it is, therefore, itself a south pole, 

 ts lower end, on the contrary, will at- 

 tract the south, and repel the north pole 

 of the compass : and has therefore a 

 northern polarity. That these properties 

 of the ends of the bar depend altogether 

 on the position of the bar itself, is proved 

 by reversing its position ; when the two 

 ends will be found to have exchanged 

 polarities merely by their change of 

 situation : the upper end being always a 

 south, and the lower end a north pole. 

 On the other hand, if the bar be placed 

 in a position at right angles to the mag- 

 netic position, (for example, horizontally, 

 and with the ends directed to the east 

 and west,) it will not exhibit any cha- 

 racteristic magnetism. 



(108.) The magnetism which a soft 

 bar of iron derives from its position with 

 relation to the earth, is, as we have just 

 seen, of a transitory kind ; immediately 

 lost on turning the bar so that it makes 

 a right angle with the magnetic position ; 

 and again acquired, but with contrary 

 poles, when its position is reversed. But 

 this is not the case with harder bars, for, 

 by remaining for a considerable time in 

 a vertical position, they are found to ac- 



