MAGNETISM 223 purely magnetic action on a magnet has its source in some other magnetic body, we are naturally led to the con clusion that the reason why at every point of the earth s surface the axis of a freely suspended magnet assumes a definite position is simply that the earth itself is a great magnet, and that in observing the declination and dip we are simply exploring the magnetic field of the earth. It is true that, according to the experiment above described, the declination would every where be zero, and the magnetic equator would coincide with the geographical, but that arises merely because we assumed our earthkin, for simplicity of explanation, to be symmetrically magnetized, so that its lines of force ran in planes passing through its axis. It remains to be discussed whether the most general assumption, viz., that the earth is a magnetic body, will not account for the facts of terrestrial magnetism. The answer to this question has been given, as we shall see, by Gauss. This idea, whose simplicity is the truest measure of its greatness, is due to Gilbert, and "was by him made the foundation of his work on magnetism. The boldness of his theory will be appreciated when we remind the reader that in his day the dip was but newly discovered, and had been measured only at London, so that Gilbert s very full and clear exposition of this phenomenon, which we have given above, was in fact a scientific prediction, which was not fully verified till long afterwards. 1 Before Gilbert a variety of wild conjectures had been made as to the cause of the directive property of the magnet. 2 Many, like Columbus, Cardan, and Paracelsus, believed that the magnet was attracted by a point in the heavens, possibly some magnetic star. Others supposed that the attracting point was situated in the earth ; Fracastorius imagined hyperborean mountains of loadstone situated near but not quite at the north pole ; and to this theory others con tributed the detail that the magnetism of these mountains was so powerful that ships in these regions have to be built with wooden nails instead of iron ones, which would be instantly drawn out by the magnetic attraction. It is clear that, if we call that magnetic pole of the earth which lies in the northern hemisphere its north pole, we ought, in accord ance with our fundamental law of magnetic action, to call the north- seeking pole of an ordinary magnet a south pole. When it is necessary to speak of magnets from this point of view, the diffi culty is got over by calling the north-seeking pole the austral pole, and the south-seeking pole the boreal pole. In reality the danger of confusion is more imaginary than real. The reader should be warned, however, that in some French works the ordinary nomenclature is reversed, and that Faraday uses "marked" and "unmarked," and Airy "red " and "blue," in the sense in which north and south are commonly used. vrth s The Earth s Action on a Magnet is a Couple. Norman rce of i n hi s JYeive Attractive (chapters v. and vi.) discusses very s a j acutely the question whether there is any force of transla tion exerted upon a magnet. He advances three conclusive experiments to prove the negative. First, he weighed several small pieces of steel in a delicate gold balance, and then magnetized them, but could not detect the slightest alteration in their weight, " though every one of them had received vertue sufficient to lift up his fellow." Secondly, he pushed a steel wire through a spherical piece of cork, and carefully pared the latter so that the whole sank to a certain depth in a vessel of water and remained there, taking up any position about the centre indifferently. After the wire was magnetized very carefully, without disturbing its position in the cork, it sank to the same 1 The first verification was by Hudson, who, in 1608, found the dip in 75 22 N. lat. to be 89 3.0 . Gilbert found 72 at London in 1600. The place of vertical dip in the northern hemisphere was first reached by Sir James Ross in 1831. It was found about 70 5 17" N. lat. and 96 45 48" W. long. 2 See Gilbert, De Magnete, lib. i. cap. i. depth as before, neither more nor less, the only difference being that now the wire set itself persistently in a definite fixed direction parallel to the magnetic meridian, the north end dipping about 71 or 72 below the horizon. Thirdly, he arranged a magnetized needle on a cork so as to float on the surface of water, and found that, although it set in the magnetic meridian, there was not the slightest tendency to translation in any direction. 3 He concludes that there is no force of translation on the magnet, either vertical or horizontal. He was evidently somewhat puzzled how to put this result into a positive form, and his "point respective," as he calls it, is not a very clear explanation of the earth s action. What he wanted was the modern idea of a "couple," i.e., a pair of equal but oppositely directed parallel forces acting on the two ends of the needle ; but such an idea was not conceived in Norman s day. Gilbert adopts Norman s result in this matter, adding nothing essential, reproducing even Norman s diagram of the spherical cork with the wire through it. It is clear therefore that Gilbert had a forerunner in the practice, as Bacon had in the theory, of inductive science ; for Norman says, speaking of the mass of fables that had passed for truth in geography, hydrography, and navigation before his time, " I wish experience to bee the leader of Writers in those Artes, and reason their rule in setting it downe, that the followers bee not Jed by them into errors, as often times have beene seene." The Magnetic Property is Molecular. Apart altogether Magnet- from the question as to how we are to represent the action ism a of a magnet upon other magnets, there arises another quite m distinct question, as to where the cause of this action resides. That these two questions are really distinct, although there has always been a tendency in the more superficial treatises on the subject to confuse them, will be obvious from the fact that we shall afterwards obtain more than one perfectly general way of representing the action of a magnet at external points, whereas there must be one and only one cause of this action. A very old experiment 4 at once throws considerable light on this point. If we break a bar mag- ,., property. net into two pieces, it will be found that each of these is itself a magnet, its axis being in much the same direc- N S N" S" N 3, Fig. 7. tion as that of the original magnet, and its poles in cor responding positions, see fig. 7. The same holds if we break the bar into any number of pieces ; and, quite generally, if we remove any piece, however small, from a magnet, this piece will be found to be magnetic, the direc tion of its axis usually bearing a distinct and easily recognizable relation to the direction of the axis of the whole magnet. We are therefore driven to the conclusion that the magnetic quality of a body is related to its ultimate structure, and not simply to its mass as a whole, or to its surface alone ; and this conclusion is not to be invalidated by the fact that we can in general, as will afterwards appear, represent the action of the magnet at external points by means of a proper distribution of centres of attractive and repulsive forces upon its surface merely. Temporary Magnetism of Soft Iron and Steel in the Magnetic Field. Bodies which possess permanent magnetic 3 Hartmann (see his letter above cited) was in error on this subject. He describes a somewhat similar experiment, and distinctly states that the needle has a motion of translation. " Schwimmt mit dem Ort welcher ist mitternachtlich am Stein, bis er kam an den Port der Schiissel, da das Wasser in war. "
4 Of. Gilbert, De Magnete, lib. i. cap. v.