50 



MAGNETISM. 



come immediately in contact with the 

 bar to be magnetized, when the com- 

 pound magnet is applied to it with the 

 proper inclination, and the whole will 

 powerfully conspire in producing the 

 same effect. 



(199.) The parallelograms of steel bars 

 and soft iron should be kept firm by 

 wedges, in the manner of printers' types, 

 and the extremities of the magnetic bars 

 should be perfectly cleaned. In rubbing 

 the bars, it is recommended by some 

 authors to apply considerable pressure ; 

 but Captain Kater found increased pres- 

 sure rather injurious than beneficial. 

 Dr. Robison conceived that by wetting 

 their extremities he obtained a greater 

 effect ; but he found that the least drop of 

 oil between the bars greatly obstructed 

 the operation of the magnets, as was 

 also the case when the smallest piece of 

 the thinnest gold leaf intervened. He 

 found that bars which were rough re- 

 ceived a more powerful magnetism 

 than those which were moderately po- 

 lished ; but that, if moderately rough, 

 they acquired the first degrees of mag- 

 netism more expeditiously than smooth 

 bars, but did not ultimately receive so 

 strong an impregnation as the latter. 



10. Comparative Advantages of the 

 different Processes. 



(200.) An account of Coulomb's ex- 

 periments on the comparative advan- 

 tages of the different, methods of mag- 

 netizing bars of different thicknesses, 

 lengths, and forms, is given by Biot in 

 his " Traite de Physique." The strength 

 of each magnet was estimated by the 

 number of oscillations which it per- 

 formed in a given time on each side of 

 the magnetic meridian by the influence 

 of terrestrial magnetism. The following 

 are some of the results of his inquiry. 



He found that steel wires of small dia- 

 meter may be rendered magnetic to an 

 equal degree, whether touched by the 

 method of Duhamel or of yEpinus, or 

 even when simply rubbed with the single 

 pole of a strong magnet, for in all these 

 cases they became magnetized to satura- 

 tion. AVhen a plate of unannealed steel 

 of greater width than the wires, but 



of equal length and thickness, was sub- 

 jected to experiment, a slight difference 

 was perceptible in the degree of magne- 

 tism resulting from the different methods, 

 those of Duhamel and ^Epinus being the 

 most efficacious. The difference was 

 more perceptible when the steel was 

 made of a harder temper, and increased 

 still more when thicker plates of steel w ere 

 tried. The processes of Duhamel and 

 of .yEpinus, when applied to plates of 

 which the thickness is less than one 

 twelfth of an inch were nearly of equal 

 power ; but when they exceed this thick- 

 ness, that of ^Epinus was decidedly the 

 most efficacious. In the case of bars 

 sixteen inches long, one inch broad, and 

 about one third of an inch thick, the 

 comparative intensities of the magne- 

 tism produced by the methods of JEpi- 

 nus and Duhamel were nearly in the 

 proportion of nine to eight. 



(201.) Captain Kater also made a 

 series of valuable experiments on the 

 effects of different methods of magnet- 

 izing, and on the influence of extent of 

 surface, independently of the mass, on 

 the directive force.* The directive force 

 was estimated by means of the balance 

 of torsion of Coulomb. This instrument 

 consists of a fine wire terminated above 

 by an index at right angles to it, which 

 is moveable round a circle divided into 

 degrees. To the lower end of the wire a 

 cradle is attached for receiving the needle 

 which is the subject of experiment. The 

 instrument being adjusted so that the 

 needle was in the magnetic meridian 

 when the wire had no torsion, the index 

 was turned, and the wire consequently 

 twisted, until the needle was made to 

 deviate 60 from its original position. 

 The number of degrees passed over by 

 the index would then be the measure of 

 the directive force of the needle. 



The needles to be magnetized were 

 right-angled parallelograms, five inches 

 long ; the one seven-tenths of an inch 

 broad, and the other of half this 

 breadth. The broadest was reduced in 

 thickness till it was of the same weight 

 as the other, namely, one hundred and 

 forty-two grains. The magnets em- 

 ployed were first placed perpendicularly 

 on the centre of the needle, their oppo- 

 site poles being joined; their lower ex- 

 tremities were then separated and kept 

 asunder by placing between them apiece 

 of wood a quarter of an inch thick, Iheir 

 upper extremities remaining in contact. 



* Philosophical Transactions for 1821, p. 104, 



