252 MAGNETISM Deter mination of devia tions by swing ing." Thom son s compass, Experi mental difficul ties. The chief part of the heeling deviation is the term Jicos , depending on the coefficient J = (e- &-R/Z)tan0/A. This coeffi cient may be reduced to zero by increasing or diminishing the earth s vertical force by means of a vertical magnet under the compass. The usual way of ascertaining the deviations of a ship s compass is to " swing " the ship gently round so that her head comes into various positions, and to observe with the compass the magnetic bearing of some well-defined distant point (compass mark) on shore. The true magnetic bearing of this point is then ascertained, which may be done by taking the compass ashore, carefully placing it in a line join ing the compass mark with the point on board at which the compass was formerly placed, and then taking the mag netic bearing of the mark once more. Care must of course be taken that there is no local magnetic disturbance at the shore station. The differences between the bearings on board and the bearing on shore give of course the devia tions for the various positions of the ship s head. When the deviations have thus been ascertained they may be either corrected by means of tables, by graphical methods, such as the steering diagram of Napier or the dygograms of Smith, or mechanically as we have partially explained. For full details on the subject the reader should consult the Admiralty Manual on the Deviation of the Compass. Of late years Sir W. Thomson has devoted his great scientific knowledge and well-known practical sagacity and inventive skill to the improvement of the compass. By reducing the size of the magnets and increasing their number he has succeeded in reducing Airy s apparatus for the mechanical correction of the quaclrantal deviation within convenient bulk, and by lightening the card and suspension of the magnets in a very ingenious manner (at the same time throwing all the remaining weight as much as possible to the circumference) he has reduced the friction on the pivot to a minimum while retaining a sufficiently long period of vibration to secure perfect steadiness. He has also contrived apparatus for facilitat ing the determinations of the deviation on different courses and of the heeling error. 1 The experimental investigation of induced magnetism reduces itself mainly to the investigation of the dependence of the magnetic susceptibility K 2 (or the magnetic per meability -or) upon the magnetizing force |p. Confining ourselves to the strongly magnetic metals, iron, nickel, and cobalt, it will be seen presently that K depends, not only upon p, but also upon the magnetic condition of the body at the actual moment when H is in action, and upon its previous magnetic history. K also depends greatly on the temperature, on the state of the body as to purity (notably in the case of iron and steel on the percentage of carbon present), and on the temper. Thus, if we make one experiment on a body by magnetizing it in any way, we permanently alter its magnetic properties, and can restore it to the magnetically virgin condition only by heating it to a high temperature ; but in this process we are very apt to permanently alter its molecular condition, so that, although magnetically indifferent, it is physically changed. Owing to the fact, already insisted upon, that we cannot infer the magnetic distribution inside a heterogeneously magnetized body from its external magnetic action, and to the fact, presently to be established, that K varies with |p, 1 For a description of liis compass see art. COMPASS, vol. vi. p. 228. Detailed descriptions of the compass with instructions for its adjust ment are issued in the form of a small pamphlet (Maclehose, Glasgow 1879). 2 K is sometimes called by Continental writers the magnetization function. They have also a habit of speaking of the ratio of whole magnetic moment of a body of any form divided by its volume to the strength of the field in which it is placed as the magnetization function for that particular form. This is a most inconvenient practice, and has led to considerable confusion. it is of the last importance to choose the experimental Import- circumstances so that both the magnetic field and the in- ance ot duced magnetization shall be uniform, or very approximately um 013 j Q so. A further necessity for the fulfilment of these con- i za tion. ditions arises from the fact that we must in all cases be able to render an account of the effect of the form of the magnetized body, because the true argument of K is not the strength of the original field but the whole force |) due to the original field and the induced magnetism together. The simplest method for securing a uniform field whose Unifor.!i strength can be controlled is to place the body inside a held. hollow cylindrical coil (usually called the magnetizing spiral), whose length so far exceeds that of the body that the disturbance arising from the ends of the coil may be neglected in the neighbourhood of the body. The results in all cases where the length of the body or core is nearly equal to or exceeds that of the coil are impure, and cau only be ussd with the greatest caution in drawing general conclusions as to the value of K. The core should always Best be either exactly or approximately one of the calculable form of forms, but preferably such that the dimension parallel to core> the axis of the spiral very much exceeds the others, because in this case the effect of the form is of secondary importance compared with the effect of the susceptibility (see above, p. 245). Thus a very thin cylindrical core is convenient, because the force inside it differs very little from that of the undisturbed field, and any small difference can be easily calculated by supposing the cylinder replaced by a very elongated ellipsoid. On the other hand, a thick -cylindrical bar is a bad form of core for the determination of K, both because the magnetizing force inside it is less than the intensity of the undisturbed field by a large quantity, which it is impossible to calculate, and because the magnetization at the end is not uniform, and the disturbance thereby arising is so great that it may mask the general character of the function K altogether. A further question arises as to how far the time during which a magnetizing force acts affects the resulting magnetization, whether temporary or permanent. It is also Disturb- important to consider the disturbances arising during the ances at make and break of the current in the magnetizing spiral. ^ As the resistance in the circuit is usually small, and the break. self-induction and capacity sensible, oscillatory currents rnay arise ; to these will correspond oscillatory magnetizing forces, which may even vary in sign. When we consider that the permanent magnetization produced by any force may be very much weakened or even altogether destroyed by a smaller force in the opposite direction, it is evident that we have no right to conclude that these disturbances, especially at break, will be without effect upon the per manent magnetization. In order to elude these difficulties, some experimenters have followed the practice of first establishing the current, then gently 3 introducing the core into its place, and finally removing it before breaking the circuit. In this way the disturbances just alluded to are avoided ; but another difficulty is raised, for it is clear that in this operation the core passes through a heterogeneous field before it reaches the final position where the magnetiz ing force is uniform ; different parts of it have therefore been subjected successively to different influences, and we are not at liberty a priori to conclude that this fact will not influence the results. Perhaps the best plan would be to place the core in its position, and allow the current to rise very slowly to the maximum value required, and then to fall slowly to zero. This, however, is not the place to dogmatize concerning the best method of experimenting ; all that is necessary is to furnish the reader with points of 3 Carefully avoiding all shocks or tremors which exercise a very
important influence on the induced magnetism, see below, p. 268.