220 MAGNETIC M coincides with its centre of figure. Suspend this spherical magnet by a fine thread of untwisted silk, attached to any point of its surface, say P. After the magnet has come to rest, mark the vertical plane through the centre which falls in the geographical meridian ; this may be done by trac ing a great circle on the surface of the magnet. Next find the point P in which the vertical through P cuts the surface again, and suspend the magnet by P , again marking the plane which falls in the meridian. Now, find the plane which bisects the acute angle between the two former planes, mark it by a great circle, and call it the axial plane of P. If we thus find the axial planes of any number of points, we shall find that they all intersect in one common line passing through the centre of the sphere. We may call this line the " axis " of the magnet. Let us mark the points where it cuts the surface ; we may Poles, call these the "poles" of the magnet. We shall then observe north t ^ a ^ h owever we suspend it, the magnet will always come south to rest so that fc h e ver fci ca l plane through the axis makes a definite angle with the meridian. This angle (8) is called the " declination" (also, by sailors, the " variation "); it varies from place to place, and from time to time, but very slowly, so that throughout a limited area of the earth s surface, and for a limited time, it may be regarded as constant. 1 One end of the axis always turns northwards, and the other always southwards ; we shall call the former the "north" and the latter the "south pole," although, for reasons to be afterwards explained, it would be more appro priate to invert the order of these names. Henceforth the vertical plane in which the axis of the magnet comes to Magnetic rest will be called the magnetic meridian, and the two meridian, horizontal directions in this plane magnetic north and magnetic south respectively. It must be carefully noticed that there is a certain amount of arbitrariness in our definition of the axis and poles of a magnet. In reality it is only the direction of the axis that is fixed in the body, and not its absolute position. This will be made plain if we repeat all our experiments with the spherical magnet after fastening to it a piece of wax or other non-magnetic body, so as to leave its magnetic properties unchanged, but to throw its centre of gravity out of the centre of figure. Everything will fall out as before, only the axial planes of the different points of suspension will now meet in a line, parallel, it is true, to the axis determined before, but passing through the new centre of gravity. In point of fact, therefore, we might choose any point in the body, draw a line through it in the proper direction, and call this the axis. Hereafter we shall, unless the contrary is stated, draw the axis through the centre of gravity of the body, or through its centre of figure if it has one ; and we define the poles, for the present, as the points in which the axis cuts the sur face of the magnet, supposing, as will be generally the case, that the line cuts the surface in two points and no more. Magnetic Having now obtained a definite idea of the axis of a needle, magnet, and seen that it has, in the first instance at least, nothing to do with the- external form of the body, let us proceed to make an artificial magnet of the particular kind usually called a " magnetic needle," and briefly examine its properties. Take a tolerably thin flat piece of pretty hard-tempered steel, of the elongated symmetrical form NS shown in fig. 1. We suppose it, in the first place, in an unmagnetic condition. Let it be pierced by a well- turned axis a&, passing accurately through its centre of gravity, and perpendicular to its plane, so that, when the axis is placed on two horizontal knife edges, the needle will rest in any position indifferently. Further, let four very small hooks, c, d, e,f, be attached, two (c, d) to the ends of the axis, and other two (e,f) to the edges of the needle in a line perpendicular to NS. Now rub the half of the needle 1 For the early observations on the declination the reader is re ferred to the treatment of the subject of terrestrial magnetism in the article METEOROLOGY. At the present time the declination at Green wich is a little over 18 ; at Edinburgh it would be about 4 more. Fig. 1. towards N with the south pole of the spherical magnet whose properties we have just discussed, beginning the stroke at the middle and ending it at the point of the needle, and for symmetry s sake let us do the same to the other side of the needle, and then repeat this process with the north pole of the sphere on the other half towards S. Let us examine the properties of the needle thus <l magnetized." If we suspend it first by the hook c and then by the hook d, we shall find that in both cases the line joining NS 2 makes very nearly the same angle with the geographical meridian. Hence the mag netic axis must lie in a plane through NS perpendi cular to the plane of the needle. A similar experiment with the two hooks e, f will show that the magnetic axis lies approximately in the plane of the strip, which we may suppose for the present to be infinitely thin. Hence the magnetic axis may be taken to be coincident with the line NS joining the points of the needle. This coincidence is, however, in general only approximate, and in delicate measurements corrections have to be made on that account, of which more hereafter. If we now mount our magnetized needle on a piece of cork or two straws, and float it in a basin of water, or replace its axle by a small cap and set it on a pivot, we have the mariner s compass in its early form. We shall call it a magnetic needle, to distinguish it from the more elaborate compass of the present day. A favourite way of showing the directive property of a magnet, described by Gilbert, is to magnetize a sewing- needle, and lay it very gently, by means of a fork of wire, on the surface of water ; it will float and turn until it takes up its position in the magnetic meridian. A needle mounted in this way, so as to have great freedom to move in a horizontal plane, is of great use in magnetic experiments. Gilbert calls it a " versorium." When very delicate applications are in view, the point of the pivot on which it is mounted must be very hard (say of hard tempered steel or iridium), and the cap should be fitted with an agate or other hard stone having a polished cavity of the form of a blunted cone to receive the pivot. A still better arrangement, also used by Gilbert, is to suspend a short and very light piece of steel wire a fine sewing needle may be used by means of a single fibre of silk. The most delicate arrangement of all is to use one of Sir W. Thomson s light galvanometer mirrors with the magnets attached, and follow its movements by means of the lamp and scale as usual. See GALVANOMETER. Such, with as much of modern accuracy imported into them as was necessary for clearness of exposition, were the facts of magnetism as known up to the beginning of the IGth century. Another experiment with our magnetized needle will Pisco> enable us to describe the next important magnetic dis covery. In its unmagnetized condition the needle rested indifferently in any position when its axis was placed on 2 Or the vertical plane through it, should it happen to be not quite
horizontal.