8o ELEMENTARY LESSONS ON [CHAP. n. 



We now can see why a magnet should attract a not- 

 previously-magnetised piece of iron ; it first magnetises 

 it by induction and then attracts it : for the nearest end 

 will have the opposite kind of magnetism induced in it, 

 and will be attracted with a force exceeding that with 

 which the more distant end is repelled. But indiiction 

 precedes attraction. 



9O. Retention of Magnetisation. Not all mag- 

 netic substances can become magnets permanently. 

 Lodestone, steel, and nickel, retain permanently the 

 greater part of the magnetism imparted to them. Cast 

 iron and many impure qualities of wrought iron also 

 retain magnetism imperfectly. 

 Pure soft iron is, however, only 

 temporarily magnetic. The 

 following experiment illustrates 

 the matter : Let a few pieces 

 of iron rod, or a few soft iron 

 nails be taken. If one of these 

 (see Fig. 46) be placed in con- 

 tact with the pole of a perma- 

 nent steel magnet, it is attracted 

 to it, and becomes itself a tem- 

 Flg * 46 ' porary magnet. Another bit of 



iron may then be hung to it, and another, until a chain 

 of four or- five pieces is built up. But if the steel 

 magnet be removed from the top of the chain, all the 

 rest drop off, and are found to be no longer magnetic. 

 A similar chain of steel needles may be formed, but 

 they will retain their magnetism permanently. 



It will be found, however, that a steel needle is more 

 difficult to magnetise than an iron needle of the same 

 dimensions. It is harder to get the magnetism into 

 steel than into iron, and it is harder to get the magnetism 

 out of steel than out of iron ; for the steel retains the 

 magnetism once put into it. This power of resisting 

 magnetisation or demagnetisation, is sometimes called 



