MAGNETISM. 



51 



The magnets were then slid along the 

 needle backwards and forwards from 

 end to end, and this was repeated on 

 both sides, till it was conceived that the 

 full effect had been produced ; and the 

 directive force of the magnet thus ob- 

 tained was noted. The process was 

 repeated with the same apparatus, ex- 

 cepting that the magnets were separated 

 at the top by a piece of wood of the 

 same thickness as that at the bottom. 

 The effect was considerably diminished 

 by this change. When the lower ex- 

 tremities of the needle were separated 

 by a piece of wood to the distance of half 

 the length of tho needle, the upper 

 extremities remaining in contact, the 

 effect was greater than in the first expe- 

 riment. A further augmentation of 

 power was obtained when the magnets 

 were joined and placed perpendicularly, 

 as before, on the centre of the needle, 

 and then moved in opposite directions 

 from the centre to the extremities, keep- 

 ing each magnet perpendicular to the 

 needle ; afterwards rejoining them at a 

 distance from the needle, replacing them 

 on its centre, and thus continuing the 

 operation. The needles were then mag- 

 netized according to the method of 

 Duhamel, the magnets being inclined at 

 an angle of about forty-five degrees, and 

 carried as before from the centre to the 

 ends of the needle. This was attended 

 with a still greater increase of effect ; 

 but it was increased still further when 

 the magnets formed with the needle an 

 angle of about twenty degrees. The 

 maximum of effect took place when this 

 angle was reduced to about two or three 

 degrees ; for when the magnets were 

 laid flat on the surface of the needles, 

 and drawn from the centre to the ends, 

 the effect was not so great as in the last 

 case. 



(202.) On the whole, Captain Kater 

 concludes that the best mode of commu- 

 nicating magnetism to a needle, is. by 

 placing it in the magnetic meridian, join- 

 ing the opposite poles of a pair of bar 

 magnets (the magnets being in the same 

 line), and laying the magnets so joined, 

 flat upon the needle with their poles 

 upon its centre; then having elevated 

 the distant extremities of the magnets, 

 so that they may form an angle of about 

 two or three degrees with the needle, 

 they are to be drawn from the centre of 

 the needle to the extremities, carefully 

 preserving the same inclination ; and 

 having joined the poles of the magnets 

 at a distance from the needle, the ope- 



ration is to be repeated ten or twelve 

 times on each surface. 



(203.) We have seen ( 171), that 

 terrestrial magnetism may be made 

 available for procuring artificial magnets 

 by the help of percussion ; and it now 

 remains that we point out the methods 

 of employing it, in the absence of all 

 other magnetized bodies, in exciting 

 magnetism by friction, either by the 

 single touch or by a method analogous 

 to that of Dr. Knight. 



(204.) Let the needle to be magnetized 

 be placed horizontally in the magnetic 

 meridian and fixed in that situation, and 

 apply to the middle of it the lower end 

 of a long iron bar, a poker, for instance, 



Fig. 52. Fig. 53. 



held vertically ; and immediately oppo- 

 site, at the lower side of the needle, ap- 

 ply the upper end of a second bar of a 

 similar description to the first, as seen 

 in fig. 52. Then draw each bar, still 

 kept in a vertical position, towards the 

 opposite ends of the needle (Jig. 53), tak- 

 ing care that the upper bar be drawn 

 towards the side of the needle intended 

 to be its south pole, and the lower 

 bar towards the intended north pole ; 

 then, separating the bars, remove them 

 to a distance, and bring them again 

 perpendicularly to the middle of the 

 needle; and repeat this operation a 

 sufficient number of times on each side. 

 This simple process will often, if the 

 needle be small, be sufficient to magne- 

 tize it to saturation. The principle on 

 which it is founded is sufficiently obvious. 



11. Cantons Process. 



(205.) The process of Canton, already 

 alluded to, proceeds upon the same 

 principle, and does not require the pre- 

 E 2 



