MAGNETISM. 



57 





the very same ratio ; no advantage, 

 therefore, as to directive power can be 

 obtained by any increase of length. 

 Beyond the limit just mentioned, there- 

 fore, all needles having the same trans- 

 verse dimensions should, according to 

 theory, be equally sensible, whatever be 

 their lengths. But it is found in prac- 

 tice, that needles which exceed a very 

 moderate length are liable to have seve- 

 ral consecutive poles, attended, as we 

 have seen, with a great diminution of 

 directive force. On this account, short 

 needles, made exceedingly hard, are 

 generally preferable. 



(226.) The next object of atten- 

 tion in the construction of a compass 

 needle is the shape which is most fa- 

 vourable to the acquisition of the 

 greatest directive power. Various 

 have been the forms given to com- 

 pass needles ; the choice having been 

 regulated more by the whim and fancy 

 of the maker, than by any reference to 

 scientific principles. The forms most 

 frequently met with are the cylindric, 

 the prismatic, that of a rhombus or 

 parallelogram, and that of the flat Iv* 

 tapering like an arrow at the extremi- 

 ties. Coulomb, who made many expe- 

 riments on the subject, gave a decided 

 preference to the last mentioned of these, 

 as being that which, with a given weight 

 of needle, retains the strongest directive 

 force. On the other hand, he found, 

 that any expansion of the needle at its 

 extremities, a form which has sometimes 

 been recommended, is attended with a 

 sensible diminution of power. From the 

 whole of his experiments, he was led to 

 the general conclusion, that in needles 

 of the same form, their directive forces 

 are to each other as their masses. 



(227.) This inquiry has been still fur- 

 ther pursued by Captain Kater, whose 

 paper in the Philosophical Transactions, 

 already alluded to ($ 201), contains an 

 account of a series of experiments for 

 determining the best kind of steel for 

 a compass needle, and the best form 

 that can be given to it. He found, 

 on comparative trial, that the directive 

 force is little, if at all, influenced by 

 extent of surface, but depends almost 

 entirely on the mass of the needle, 

 when magnetized to saturation. Two 

 needles were prepared of that kind 

 of steel which is called blistered steel, 

 and two of spur steel, each weighing 

 66 grains. They were of the form of a 

 long ellipse, five inches in length and 

 half an inch in width. One of each 



kind was pierced, as shown in fig. 61 ; 

 the weight so lost being made up by 



Fig. 61. 



additional thickness. It is evident that 

 these pierced needles had, though of 

 equal mass, much less extent of surface 

 than those which remained solid. Hav- 

 ing formerly had in his possession a 

 compass of extraordinary power, the 

 needle of which was composed of pieces 

 of steel wire put together in the shape 

 of a rhombus, he procured two needles 

 of this form (Jig. 62), made from a 

 Fig. 62. 



piece of clock spring, which J.s of that 

 kind of steel called-sheav steel. In oner 

 thf .os "piece was of brass; in the 

 urner, formed of part of a clock spring. 

 They weighed only 45 grains. 



(228.) The results of the inquiry 

 were, that shear steel is capable of re- 

 ceiving" the greater magnetic force ; and 

 that the pierced rhombus is the best 

 form for a compass needle. Needles of 

 cast steel were also tried, but were 

 found so very inferior, as at once to be 

 rejected. In the same plate of steel, of 

 the size of a few square inches only, 

 portions are found, varying considera- 

 bly in their capability of receiving mag- 

 netism, though not apparently differing 

 in any other respect. 



(229.) Captain Kater next endea- 

 voured to determine the effects of va- 

 rious modes of hardening and tem- 

 pering the needles. He found that 

 hardening a needle throughout consi- 

 derably diminishes its capacity for mag- 

 netism. The greatest directive force 

 was obtained by a needle which was 

 soft in the middle, and its extremi- 

 ties hardened at a red heat. He at 

 first thought that the most effectual 

 means of increasing its retentive power, 

 would be first to soften it throughout, 

 and then harden it at the extremities, 

 instead of first entirely hardening it, and 

 afterwards softening it in the middle. 

 But subsequent experience induced him 

 to attribute the difference of effect to a 

 difference in the degree of heat to which 

 the needle is exposed in softening it in 



