MAGNETISM. 



viously been rendered powerfully mag- 

 netic. He next placed the two compound 

 magnets end to end, with their dissimilar 

 poles adjoining each other, but separated 

 by a small piece of wood (fig. 49), 



Fig. 49. 



which kept them asunder for a short 

 space; and then, inclining them so that 

 they formed a very obtuse angle with each 

 other, he placed them on the middle of 

 one of the steel bars, and, without sepa- 

 rating them, made them slide backwards 

 and forwards along the surface of the 

 bar ; repeating the operation, with the 

 usual precautions as to the direction of 

 the poles, on the other bar, and on both 

 sides of each. 



With regard to the position of the 

 magnets, it is evident that this process 

 is analogous to that of Duhamel; but 

 as the magnets, during the whole time 

 they are rubbed upon the bars, are kept 

 in the same relative situation with re- 

 spect to each other, their operation de- 

 pends upon the principle of the double 

 touch peculiar to Mitchell's process. 

 JEpinus tried different angles of inclina- 

 tion for the magnets, with a view to 

 discover that which gave the greatest 

 effect ; and concluded that the maximum 

 of effect was obtained when the mag- 

 nets made angles of fifteen or twenty 

 degrees on each side with the steel bar 

 on which they were to act. 



(193.) When an inquiry was insti- 

 tuted as to the comparative efficacy of the 

 methods of Duhamel and of yEpinus, the 

 latter was found to possess this advan- 

 tage that it enabled the experimenter 

 to magnetize bars of considerable length 

 and thickness by means of bars which 

 themselves possess no great power, 

 which was not the case with the process 

 of Duhamel. At the same time the 

 method of ./Epinus is liable to many in- 

 conveniences ; in the first place, we 

 scarcely ever obtain, by its means, an 

 equal degree of magnetic power in the 

 two ends of the bars to which it is ap- 

 plied. This will appear by placing any 

 one of these bars on a table, and laying 

 on it a sheet of paper, on which are 

 strewed some very fine iron filings; 



when it will be seen, by the manner in 

 which the filings arrange themselves, 

 that the neutral point of the bar does not 

 occupy the exact middle of the bar, but 

 is sensibly nearer to that end to which 

 the magnets used in the operation of 

 touching it had been last applied. 



In the second place, magnets formed 

 by the process of .^Epinus are much more 

 liable to have consecutive poles than 

 those obtained by Duhamel's process ; 

 and this is especially the case if the 

 magnets are of some length. These 

 consecutive poles, which are irregularly 

 formed in various parts of the magnet, 

 are, it is true, in general extremely 

 feeble ; but still they must always im- 

 pair very considerably the directive 

 force, which becomes a very serious 

 objection when the magnets are intended 

 for compass-needles. The inequality of 

 strength or of diffusion of the two 

 principal poles is also disadvantageous 

 with a view to the same object. Hence 

 the process of Duhamel will always be 

 found preferable for the construction of 

 compass-needles; while that of-^pinus 

 is more serviceable when it is wished to 

 obtain a very considerable magnetic 

 power in large bars, for the purpose of 

 batteries, or other magnetic combina- 

 tions, where it imports little whether 

 the neutral points be exactly coincident 

 with the centres of each individual piece. 



9. Coulomb's Process. 



(194.) The attention of M. Coulomb, 

 already distinguished by his researches 

 in electricity, was engaged for a con- 

 siderable period in perfecting the art of 

 making magnets; and his numerous 

 communications to the French Academy 

 and Institute contain a great mass of 

 valuable observations on this subject. 

 Some of the results of his experiments 

 are given by Biot, in his "Traite tie 

 Physique."* 



(195.) The magnetic apparatus for 

 impregnating a steel bar consists, as we 

 have seen, of two parts ; the first is that 

 which is fixed, and applied to the bars 

 in such a manner as to act by its con- 

 tinued inductive operation ; this includes 

 the armatures of soft iron, as well as the 

 fixed magnets that may be substituted 

 for them : and the second is the moveable 

 magnetic bars, or combinations of bars, 

 which are made to slide and rub over 

 the bar to be magnetized. For the 

 construction of the fixed part of the 

 apparatus, Coulomb employed bars of 



