42 



MAGNETISM. 



motion is a source of magnetism, and 

 that strong grounds exist for the belief, 

 that even the magnetism of the earth has 

 its origin in electric currents circulating 

 round the equatorial regions. 



In giving an account of the methods 

 of procuring artificial magnets, we shall 

 begin with those which depend solely on 

 this source of magnetic power, and which 

 would enable us to obtain them if we 

 were unprovided with any instrument 

 previously magnetized. 



(169.) The success of every plan that 

 can be put in practice for obtaining 

 magnets must depend on two circum- 

 stances : first, the efficacy of the induc- 

 tion ; and, secondly, the fixation of the 

 magnetism that has been induced. That 

 quality and temperament of steel which 

 is most favourable to the former, is least 

 favourable to the latter of these objects ; 

 but various methods may be devised 

 which shall answer both these intentions. 

 The particular purposes intended to be 

 answered by the magnetic instrument we 

 are constructing, will frequently deter- 

 mine our preference of one or other of 

 these methods, as well as guide us in the 

 choice of the material to be used, and of 

 the form and dimensions to be given 

 to it. 



(170.) Magnetism is most readily com- 

 municated to an unmagnetic bar of iron 

 or steel by means of certain combina- 

 tions of steel bars already magnetized 

 to saturation, which combinations may 

 be regarded as highly-charged magnetic 

 batteries ready for action, and capable 

 of exerting a powerful influence, in in- 

 ducing magnetism, on all the iron in 

 their vicinity. But an apparatus of this 

 kind cannot at all times be commanded, 

 nor can it at once be constructed : it 

 must be the result of a long preliminary 

 process, of which the object is to impart 

 to each single bar additional quantities 

 of magnetism, until it has acquired, by 

 gradual steps, the full measure it is 

 capable of retaining. We shall first, 

 then, point out the methods of mag- 

 netizing those bars which are to com- 

 pose the apparatus or battery just men- 

 tioned. 



2. Method by Percussion. 



(171.) The most Advantageous form 

 for the steel bars thai are to be employed 

 for this purpose, is taat of a rectangular 

 prism, of which the length is about ten 

 times the breadth, and about twenty 

 times the thickness. Six or eight bars 

 of this kind, and t of equal size in every 



respect, should be provided. We have 

 already seen that a certain degree of 

 magnetism may be given to each of these 

 bars by a few blows with a hammer, 

 while they are held in a vertical position 

 ($ 6). This effect results, as we have also 

 seen ( 107), from the direct inductive 

 power of the earth. But the efficacy of 

 this power will be very considerably in- 

 creased, if it be combined with the in- 

 ductive influence of other masses of iron 

 placed near it, or in contact with it: not- 

 withstanding the iron itself, which thus 

 adds to the effect, derives its own power 

 from the same source, namely, the mag- 

 netism of the earth. Thus, Mr. Scoresby 

 found that a steel bar which acquired a 

 feeble magnetism by being hammered 

 vertically when resting upon stone or 

 pewter, received a considerable accession 

 of power when subjected to the same 

 degree of hammering while it was placed 

 upon a parlour poker, also kept in a ver- 

 tical position. The poker, under these 

 circumstances, became strongly mag- 

 netic, and in this state exerted upon the 

 bar a much more powerful inductive in- 

 fluence than the earth alone could have 

 done. Hence the magnetism of an iron 

 bar, although temporary and dependent 

 on position alone, may serve as a very 

 important auxiliary in the development 

 of the magnetism in steel bars, which is 

 capable of being permanently retained. 

 This is, in fact, the great principle on 

 which the art of making artificial mag; 

 nets of high power is founded. 



(172.) The effect of the auxiliary iron 

 bar, or of the poker used in the above 

 experiment, is greater in proportion as 

 it is longer ; but as it would not be con- 

 venient to employ a bar of iron beyond 

 a certain length, the magnetizing process 

 may be continued by the aid of a still 

 more powerful auxiliary, namely, very 

 long bars of soft steel. Mr. Scoresby 

 provided himself with two bars of this 

 description, thirty inches long, and one 

 inch broad ; and also with a large bar of 

 soft iron. This iron bar was first ham- 

 mered in a vertical position. It was 

 then laid on the ground, with its acquired 

 south pole towards the south, and upon 

 that end of it the large steel bars were 

 made to rest while they were hammered; 

 they were also hammered upon each 

 other. On the summit of one of the 

 large steel bars, each of the small bars 

 (which were eight inches long, and half 

 an inch broad), held also vertically, was 

 hammered in succession. In a few mi- 

 nutes they had all received considerable 



