46 



MAGNETISM. 



soon found, however, that the attempt to 

 magnetize bars of a greater length by 

 this process was generally less successful, 

 or at least failed in giving to them all the 

 power of which they were susceptible. 

 Philosophers therefore renewed their ef- 

 forts to devise methods of greater and 

 more universal efficiency. M. Duhamel, 

 of the French Academy of Sciences, in 

 conjunction with M. Authcaume, at 

 length devised the following plan, which 

 was found to succeed even with bars of 

 considerable dimensions. 



(185.) He first laid the two bars of 

 steel intended to be magnetized, and 

 which were made of equal length, paral- 

 lel to each other, C D (see Jig. 47), and 

 Fig. 47. 



connected their extremities by two 

 shorter bars of soft iron, R r, so as to 

 form altogether a right-angled parallelo- 

 gram. Then taking two parcels of bars 

 already magnetized, M ra, the separate 

 bars of each parcel being placed with 

 their respective poles in the same direc- 

 tions, and firmly tied together, he brought 

 the poles of opposite kinds, N, S, into 

 contact over the middle of one of the 

 steel bars forming the parallelogram, 

 giving them a certain inclination to the 

 bars as seen in the figure. The angle 

 they formed with each bar was generally 

 about forty-five degrees, so that they 

 formed with each other a right angle. 

 Then separating them from each other, he 

 made them slide gently, and with an equa- 

 ble motion, towards the extremities of the 

 bar. This operation was repeated on the 

 same bar as often as appeared requisite. 

 The inclined parcels of magnets were 

 then taken to the opposite bar of the pa- 

 rallelogram, and applied to them in the 

 same manner ; taking care, however, to 

 reverse the disposition of the poles of the 

 magnets, so that the side on which the 

 north pole was placed in the one case, 

 was occupied by the south pole in the 

 other. After the bars had been rubbed 

 sufficiently on the one side, they were 

 turned on the other side, and the same 

 operations repeated on them in that situ- 

 ation. 



(186.) It is evident, that in as far as 

 the magnets exert their conjoined influ- 

 ence on the portions of the bars that lie 

 between them, and act only upon their 

 respective halves of the bars, the method 

 of Duhamel possesses all the advantages 

 of that of Dr. Knight. The combination 

 of many separate magnets in each bun- 

 dle, however, gives them greater power 

 in operating the requisite inductions a 

 power, indeed, which appears to be con- 

 siderably greater than that which a sin- 

 gle magnet of the same size as that of 

 the combined magnets would possess. 

 But the principal improvement in Du- 

 hamel's plan consists in the disposition 

 of the bars in a parallelogram in con- 

 junction with connecting pieces of soft 

 iron, which, acting as armatures, afford 

 an advantage of a similar kind to that 

 already explained in 176. In propor- 

 tion as the steel bars acquire magnetism, 

 these connecting pieces participate in the 

 acquisition of a similar power, and serve 

 to retain it in the bars themselves ; just as 

 the electricity which is imparted to the 

 inner coating of a Leyden jar is retained 

 by the reciprocal influence of the in- 

 duced and contrary electricity of the 

 outer coating. The magnetism of the 

 bars is retained by a similar influence, 

 and greater facility is thus afforded to 

 increase its amount by the subsequent 

 additions it is receiving from the action 

 of the magnets as they pass along the 

 surface. 



7, Method by Double Touch: 

 Process of Mitchell. 



(187.) While Duhamel was endea- 

 vouring to perfect his method in France, 

 the same object was occupying the at- 

 tention of experimental philosophers in 

 England ; and much about the same pe- 

 riod new processes for magnetizing bars 

 were invented by Mitchell and by Canton. 



(188.) Mr. Mitchell, of Cambridge, 

 published his improved method in 1750. 

 He employed two parcels of strongly 

 magnetized bars (M m, Jig. 48), joined 



Fig. 48. 



in a manner similar to those above de- 

 scribed, and placed them parallel to each 

 other, but with the poles of each parcel 

 reversed, leaving between the two par- 

 cels an interval of about a quarter or a 



