420 WELLS'S NATURAL PHILOSOPHY. 



Recent investigations hare shown that tlie influence of magnetism, which 

 was once supposed to be wholly restricted to iron and its compounds, is al- 

 most as pervading and wide-extended as that of electricity. The emerald, 

 the ruby, and other precious stones, the oxygen of the air, glass, chalk, bone, 

 wood, and many other substances, are more or less susceptible to magnetic 

 influence. This influence, however, is perceptible only by the nicest tests, 

 and under peculiar circumstances. 



Artificial masnets of iron or steel mav be of any required 

 In what form ,. , ^ ,. . „" , 



are artificial form, or of almost any dunensions. Jbor general purposes, 



matmcts con- ^hev are limited to straight bars, 

 ■tructed ? - ° 



"When a piece of iron not magnetic is brought in contact 



Fig. 349. with a common magnet, it will be attracted by either 



pole ; but the most powerful attraction takes place when 



both poles can be applied to the surface of the piece of 



iron at once. The magnetic bars are for this purpose 



bent somewhat into the shape of the letter U, and are 



termed horse-shoe magnets. 



Several of these are frequently joined together with 



their similar poles in contact ; they then constitute a 



compound magnet, and are very powerful, either for 



lifting weights or charging other magnets. 



For the purpose of distinguishing between the two 



poles of an artificial magnet, the end of the bar which" is designated as the 



north pole is generally marked with a -)- or with the letter N. 



ifwe break an ^^ "^^ break a magnet across the middle, 

 n^t'^ what"ol^ ^^^^ fragment becomes converted into a pcr- 

 curs? ^'gg^ magnet ; the part which originally had a 



north pole acquires a south pole at the fractured end, and 

 the part which originally had a south pole, gets a north pole. 



Thus, if the bar N S, Fig. 350, be 

 broken in the center, each of the fractured 

 ends will exhibit a polar state, as perfect 

 as the entire magnet ; the fractional end 5 ^ 

 becoming a south and n a north pole, al- 

 though at this middle point, where n and 

 . e join, no magnetism could, before the breaking, have been detected. 



If we divide up a magnet to the extreme degree of mechanical fineness 

 possible, each particle, however small, will be a perfect magnet. 



The properties of a magnet arc not at all affected by 

 the presence or absence of air ; but its influence is as 

 great in a vacuum as in any other situation. 



Heat weakens the power of a magnet, and a white heat 

 destroys it entirely. 



