56 



MAGNETISM. 



in all namely, a magnetised bar of 

 steel, generally termed the needle, hav- 

 ing at its centre a cap fitted to it, which 

 is supported on a sharp-pointed pivot 

 fixed in the base of the instrument. In 

 the mariner's compass, the needle is 

 also affixed to a circular plate, or card, 

 the circumference of which is divided 

 into degrees, while an inner circle de- 

 scribed upon it is marked with the 

 thirty-two points of the compass, or 

 rhumbs, as they are called. The pivot of 

 support rises from the bottom of a circu- 

 lar box, which contains the needle and 

 its card, and is covered with a piece of 

 glass,* in order to protect them from 

 dust, and prevent their being disturbed 

 by the agitations of the external air. 

 The compass box is suspended within a 

 larger box, by means of two concentric 

 brass circles, or gimbals, as they are 

 called; the outer one being fixed by 

 horizontal pivots, both to the inner 

 circle which carries the compass box, 

 and also to the outer box ; and the two 

 sets of axes being in directions at right 

 angles to one another. By the combi- 

 nations of movements determined by 

 these axes, the inner circle, with the 

 compass box and its contents, always 

 retains a horizontal position, during the 

 rolling of the ship. 



(222.) The qualities required in the 

 needle of the compass, for the perfect 

 performance of its office, are these : 

 first, its directive force compared with 

 its weight, or with the mass which that 

 power has to set in motion, should be 

 as great as possible ; while, secondly, 

 the impediments to the exertion of that 

 force, and which consist principally in 

 the friction between the cap and pivot, 

 should be as small as possible. Hence 

 it becomes important to consider the 

 relation subsisting between these op- 

 posing forces, and to ascertain those 

 conditions which give the greatest pre- 

 ponderance to the directive force. 



(223.) The friction that takes place 

 between the pivot and the cap which 

 rests on it, will, in different compasses, 

 bear a certain proportion to the pressure 

 on the points of support, provided these 

 parts are constructed precisely in the 



* An electrical state of the glass cover, acci- 

 dentally excited by friction, has been known to 

 occasion a sensible disturbance of the needle, by 

 attracting its ends. This attraction, when it ex- 

 ists, may be at once destroyed by moistening the 

 surface of the glass. See Phil. Trans, for 1746, 

 p. 242. See also the observations on the local and 



ectrical influences on compasses by Lieutenant 

 Johnson, in the 21st volume of the Quarterly Jour- 

 nal of Science, p. 274. 



same manner in each case. This pres- 

 sure is proportional to the weight of 

 the needle and the parts which turn 

 with it. Coulomb concluded, from a 

 set of experiments he made with a view 

 to ascertain this particular point, that 

 when the pivots terminate in a sharp 

 point, and the caps are made of very 

 hard materials, the friction is very nearly 

 proportional to the square root of the 

 cube of the weights. But after long 

 use, the point of the pivot becomes 

 blunted, and the surface of contact with 

 the bottom of the cap is considerably 

 enlarged. In this state the friction is 

 found to be simply proportional to the 

 pressure. 



(224.) Assuming this, then, to be the 

 law of relation between them, let us 

 take a magnetized needle of any given 

 size and shape, and support it upon a 

 pivot in the usual manner. Let us next 

 place upon it another needle, precisely 

 similar in all its dimensions, and mag- 

 netized to the same degree. The pres- 

 sure on the pivot will now be double 

 what it was before ; and therefore the 

 friction, which is proportional to that 

 pressure, will be double also. But the 

 directive force, though increased, will 

 not be twice as great as with the single 

 needle ; because, as was formerly shown, 

 the reaction of the similar poles of the 

 two magnets tends to diminish the 

 power of each. Hence the ratio be- 

 tween the directive force and the resist- 

 ance is diminished, and the compound 

 needle is less sensible to the magnetic 

 influence of the earth, and less fitted for 

 indicating the magnetic points of the 

 compass. The same mode of reasoning 

 applies to any increase of thickness that 

 may be given to the needle. Hence it 

 appears, that when all other conditions 

 are the same, needles of very small 

 thickness possess the greatest sensibi- 

 lity to terrestrial magnetism. To this 

 general proposition there is, however, a 

 limit ; inasmuch as excessive thinness 

 in the needle would endanger its bend- 

 ing by its own weight, which would be 

 attended with a considerable loss of 

 power. 



(225.) With regard to the most ad- 

 vantageous length for a compass needle, 

 it appears that when we have passed a 

 certain limit, which is about five inches, 

 an increase of length is accompanied by 

 an increase in the directive force in the 

 same proportion ; but when the thick- 

 ness remains the same, the weight, and 

 consequently the friction, increases in 



