44 



MAGNETISM. 



pointed out. But we have seen reason 

 to conclude that this theory can be ap- 

 plicable only to the minutest individual 

 particles of which magnetic bodies are 

 composed, and that a magnet should 

 really be viewed as an aggregate of an 

 indefinite number of minute magnets. 

 This consideration cannot but have an 

 important influence on the art of impart- 

 ing magnetism to such an aggregate; 

 as it will lead us to apply our means to 

 effect, as far as it is in our power, a 

 change in the magnetic state of each 

 portion of the aggregate. The greater 

 the proximity of the pole of the magnet 

 which is to effect this change, to the 

 part in which the change is to be pro- 

 duced, the greater will be the effect 

 produced. Agreeably to this view of 

 the subject, we shall succeed in con- 

 verting a steel bar into a magnet of 

 greatest power, by subjecting every part 

 of the surface of the bar successively to 

 the contact of the magnetizing pole. 

 Let us trace the consequences of the 

 practical application of this principle. 



4. Method by the Single Touch. 



(178.) One of the earliest methods 

 which was employed for giving magnet- 

 ism to a bar C (fig. 45), was to lay it flat 



A C B 



on a table, and placing an artificial mag- 

 net, M, on one of its ends, A, and at right 

 angles to it, to slide it along the surface 

 of the bar till it arrived at the other end 

 B ; and then, lifting it cautiously to a suffi- 

 cient height to render its inductive influ- 

 ence insensible, to bring it down again 

 to its former situation, and renew the 

 operation. This was repeated several 

 times on each of the surfaces of the bar, 

 the pole of the magnet being always 

 passed in the same direction, and the 

 same pole employed. 



It is evident that when the mag- 

 net is first applied to the end of the 

 bar, it will induce in that end a po- 

 larity of the opposite kind to that of 

 the pole N of the magnet which is in 

 contact with it. Let us suppose, for the 

 convenience of explanation, that this is 

 a north pole ; the end A of the bar to 



which it is first applied will first become 

 a south pole, and the portions at a little 

 distance from that end will acquire an 

 equal degree of northern polarity. But 

 as the magnet advances along the bar, a 

 similar change will be induced in each 

 successive particle of the surface which 

 it approaches and touches ; that is, 

 each particle will now be converted into 

 a south pole, although it had before been 

 rendered a north pole. In as far as this 

 takes place, therefore, the advance of the 

 magnet reverses the effect it had at first 

 produced. In like manner, the mag- 

 net has no sooner quitted the end to 

 which it was first applied, than it tends to 

 induce in it the northern polarity, at the 

 same- time that it renders the part which 

 it then touches, a south pole. The same 

 succession of changes, and reversal of the 

 magnetism of each part, takes place 

 during the whole of the progress of the 

 magnet along the bar, with the excep- 

 tion of the end which it touches last. It 

 leaves this end of the bar in the state of 

 a south pole, while the other end re- 

 mains a north pole. The intermediate 

 parts may be considered as constituting 

 a series of small magnets, with all their 

 north poles turned towards A, and their 

 south poles towards B. 



(179.) However conformable to the- 

 ory this method of magnetizing may 

 appear to be, experience shows that it is 

 very little superior to that by simple con- 

 tact. It has also, like 1hat method, the 

 disadvantage of frequently producing 

 consecutive poles ; and these more es- 

 pecially occur when the bar to be mag- 

 netized is of some length, or consists of 

 very hard steel. They are also very 

 readily produced if care be not taken to 

 prevent the magnet from resting for a 

 longer time on some portions of the bar 

 than on the rest ; for in this case the 

 poles are multiplied very much in the 

 manner stated in a former chapter 

 ($ 3 0, 3 1 ) ; a pole of one kind being formed 

 at the point where the contact has been 

 too long protracted, and two others of 

 the contrary denomination in the imme- 

 diate vicinity. 



(180.) A singular circumstance cha- 

 racterizes this method of magnetizing by 

 touching, as it is called. If, after a bar 

 has been impregnated with as much mag- 

 netism as it is capable of receiving by 

 this method from a strong magnet, an 

 attempt be made to increase the effect 

 by renewing the same operation upon 

 the bar with a weaker magnet than 

 the one that was first employed, the 



