MAGNETISM. 



45 



immediate result is a loss instead of an 

 augmentation of power ; " and the bar 

 remains only magnetized to a degree 

 corresponding with the lesser power of 

 the last magnet by which it has been 

 touched. The reason of this will easily 

 be understood, when we consider that 

 the first effect produced by the second 

 magnet is, to reverse the poles -which 

 already exist in the bar, and afterwards 

 to induce the same kind of polarity ; 

 but this last effect it can produce only 

 in a degree corresponding to its own 

 strength. It therefore destroys, during 

 the former part of the operation, more 

 than it can supply in the latter. 



The only exception which might be 

 conceived to exist to this destructive ac- 

 tion of a weaker magnet, would be in the 

 case where the latter was composed of a 

 very soft material, so that the magnetism 

 of the bar was capable of affecting its 

 polarity, so as to destroy it when of a 

 similar kind to the part of the bar with 

 which it came in contact, and convert it 

 into an opposite polarity. 



(181.) An attentive 'consideration of 

 the stages of the process we have detailed, 

 will show us that the destructive opera- 

 tion of the second magnet is produced 

 chiefly in the first half of the bar ; for 

 if the'weaker magnet were first applied 

 to the middle of the bar, and then made 

 to slide on to the end in the same direc- 

 tion as before, over the latter half, its 

 effect on the first half would only tend 

 to strengthen the polarity already im- 

 pressed upon it. Nor would there, in 

 that case, be any injurious effect pro- 

 duced if the second magnet were suffi- 

 ciently soft in its texture to admit of 

 having its polarity changed by the mag- 

 netism of the bar. This consideration 

 leads us to another important stage in 

 the progress of improvement in the art 

 we are studying. 



5. Dr. Knight's Method. 

 (182.) This improvement consists in 

 employing two magnets in the same 

 operation, applying two dissimilar poles 

 of these magnets each to a different half 

 of the bar to be impregnated, and con- 

 fining its action to that portion of the 

 bar, which of course should be much 

 smaller than the magnets. For this pur- 

 pose the two magnets are to be joined 

 lengthwise, with their dissimilar poles in 

 contact, and laid on the bar to be magne- 

 tized, in the manner represented in/g-. 

 46, where A and B are the magnets, and 

 C the bar to be magnetized ; so that the 



Fig. 46. 



point of junction of the magnets shall be 

 immediately over the middle of the bar. 

 Then separating the magnets, by draw- 

 ing them opposite ways in the direction 

 of their length as far as the extremities 

 of the small bar, they are next to be re- 

 moved to a considerable distance, and 

 again joined; and afterwards laid a se- 

 cond time on the middle of the bar, in 

 the same manner as at first. This ope- 

 ration is to be repeated several times on 

 each of the sides of the bar. By this 

 method, which was first practised by Dr. 

 Gowan Knight about the middle of the 

 last century, steel bars could be rendered 

 much more powerfully magnetic than by 

 any of the means before in use. 



(183.) The great superiority of Dr. 

 Knight's method is owing, not merely 

 to the circumstance before noticed that 

 each pole of the magnets acts only upon 

 that half of the bar which is intended to 

 receive a magnetism of an opposite kind, 

 and that its inductive effect on the other 

 half has never to be destroyed, but 

 also to the inductive influence of the two 

 poles being combined together during 

 the whole of the operation. In every por- 

 tion of the bar which lies between the 

 two poles of the magnets that are thus 

 applied, their influence conspires to in- 

 duce the kind of magnetism that it is 

 desired to produce. In those portions 

 of the bar, indeed, which lie on the other 

 sides of the poles of the magnets, they 

 oppose each other: but it will be per- 

 ceived that their effect is here only that 

 resulting from the difference of their re- 

 spective influences ; while, in the former 

 case, when they act upon the interme- 

 diate portions, it is as the sum of that 

 influence. The superiority of the com- 

 bined influence is even greater than the 

 united powers of the single magnets, as 

 we have already had occasion to point 

 out. 



6. Duhamel's Method. 



(184.) If the magnets employed be 

 large and powerful, and the bars very 

 short and slender, it is easy, by the' pre- 

 ceding method, to magnetize them to 

 saturation. Soon after the publication 

 of Dr. Knight's method, small bars thus 

 magnetized were distributed over Europe, 

 and were eagerly sought after by the cul- 

 tivators of natural philosophy. It was 



