58 



MAGNETISM. 



the middle. Repeated exposure to heat 

 was found considerably to impair the 

 susceptibility of the needle to retain the 

 magnetic power communicated to it ; 

 an effect which does not appear to be 

 owing to any decarbonization of the 

 steel. Captain Kater suggests that this 

 deterioration may arise from a perma- 

 nent expansion produced in the texture 

 of the steel 'by the repeated application 

 of heat; for the springs of clocks, 

 which was the material used in his ex- 

 periments, being made by passing the 

 steel through rollers, when it undergoes 

 great compression, it is probable that 

 the state of condensation thus induced 

 is exceedingly favourable to the reten- 

 tion of magnetism. 



(230.) The process which, on the 

 whole, he recommends as the most ef- 

 fectual for giving to a needle the great- 

 est susceptibility of directive power, is 

 first to harden it throughout at a red 

 heat, and then to soften it from the 

 middle to within an inch of each extre- 

 liiity, by expcsi"^ it to a heat sufficient 

 to cause the blue colour which arises 

 again to disappear. 



(231.) The effect of previously po- 

 lishing the needle to be magnetized was 

 not found by Captain Kater to have any 

 sensible influence on its capacity for re- 

 ceiving directive power. Neither did 

 any advantage result from the employ- 

 ment of increased pressure in applying 

 the magnets over the surfaces of the 

 needle during the process for magnet- 

 izing them ; but, on the contrary, in 

 one instance it seemed to be attended 

 with a diminution of effect. 



(232.) It is an important requisite in 

 a compass needle that its polarities 

 should be concentrated as much as 

 possible in its two extremities, and un- 

 disturbed by the action of any conse- 

 cutive poles existing at intermediate 

 points. We have already had occasion 

 to remark ( 193) how much the direc- 

 tive force of a needle is impaired by 

 irregularities in the distribution of its 

 magnetism, attended either by a multi- 

 plicity of poles, or by an inequality in 

 the strength of the two principal poles. 

 It is on this account that Duhamel's 

 process of magnetizing is so much pre- 

 ferable to that of /Epinus for imparting 

 magnetism to compass needles ; being 

 more conducive to uniformity of effect 

 in every portion of the needle. But 

 even with all the care that can be 

 bestowed, we cannot always be certain 

 of obtaining perfect regularity in the 



disposition of the magnetic power of a 

 steel bar, whatever shape we may give 

 to it, or whatever process we may em- 

 ploy for its magnetization. 



(233.) The consequence of the un- 

 equal distribution of magnetism on the 

 two sides of the needle, is evidently to 

 produce a deviation of its axis from the 

 true magnetic meridian ; and the instru- 

 ment will therefore fail to point out the 

 real direction of this meridian. There 

 is only one way of discovering the ex- 

 istence and the amount of the deviation 

 proceeding from this cause; it is to 

 reverse the needle, that is, to turn up- 

 wards that surface which was before the 

 under surface ; and when thus reversed 

 to balance it as nearly as possible in the 

 same point in its axis as that on which 

 it was before supported. If the needle, 

 in this new state of suspension, finally 

 settles in a position somewhat different 

 from that it before assumed, we may 

 conclude that the axis indicated by its 

 figure is not its true magnetic axis ; 

 and that the latter, which alone tends to 

 arrange itself in the magnetic meridian, 

 lies, in a situation exactly bisecting the 

 i,,o positions assumed by the needle 

 in these two different modes of suspen- 

 sion. 



(234.) ; When compasses are con- 

 structed of two separate pieces of steel 

 bars, slightly bent at an obtuse angle in 

 the middle, so as to allow a space for 

 the placing of the brass cap on which 

 it is to be suspended at the centre, and 

 the two pieces joined by their extremi- 

 ties so as to compose a lozenge-shaped 

 combination, they are exceedingly liable 

 to the imperfection just noticed. For, 

 unless the ends of the separate pieces 

 which compose such a needle have been 

 brought, by tempering, to an exactly 

 equal degree of hardness, that side 

 which is the hardest will retain more 

 magnetic power than the other side ; 

 and will, consequently, have a stronger 

 tendency to place itself in the magnetic 

 meridian. The needle will, accordingly, 

 incline on the side which favours 

 this tendency, and the line joining its 

 extremities, and which must be regarded 

 as the axis of its figure, will deviate 

 from the magnetic meridian. This evil 

 will have a tendency to increase by 

 time: for the stronger magnetism of one 

 side, will tend first to impair, and at 

 length destroy, or even finally to re- 

 verse, the polarities of the parts on the 

 other side to which they are adjacent. 



(235.) The mode in which compass 



