224 Experi- juents illustrat- Magnetic properties, not depending on the circumstances in which indue- they are placed, we shall henceforth call " permanent tlon> magnets." The law of the action of one permanent magnet upon another, as we have seen, is that like poles repel and unlike poles attract each other. The action of a permanent magnet on pieces of soft iron is, at first sight, different, for either pole attracts them alike. To fix our ideas let us take a small thin bar of soft iron or of steel, and test it with a delicate magnetic needle. It w jj^ usua iiy be found, more particularly if a steel bar is auction. ta ken, that one end of the bar will repel one or other of the poles of the needle. This is a sure sign of permanent magnetism. If, however, we heat the bar to whiteness and allow it to cool in a position perpendicular to the earth s magnetic force, all permanent magnet ism will be found to have disappeared. If we now place the bar in a horizontal plane (fig. 8) with its axis perpendicular to the axis of the needle, and one of its ends A, B near either pole of the needle, that pole will be attracted, no matter whether it be the north pole or the south pole of the needle, or which end of the bar be used. Care must be taken in this experiment to avoid using a too strongly magnetized needle, and to keep the needle from touching the bar, otherwise the bar may receive traces of permanent magnetism which will disturb the result. It is very easy, by repeating the above experiment with an un- magnetized needle, to show that the power that the bar acquires of attracting the poles of the needle is temporary and depends on the presence of a magnetized body. Keeping to our principle that a magnetic cause is to be sought for every magnetic action, we are led to explain the above experiment by saying that in the magnetic field a bar of soft iron or of unmagnetized steel becomes magnetic in such a way that its north pole points as nearly as may be in the positive direction of the lines of force passing in its neighbourhood (or, in other words, in the direction, as nearly as may be, in which a magnetic needle would point if placed in its neighbourhood). A body which becomes magnetic in this way by the magnetic action of another body is said to be " magnetized " by " induction." We shall suppose, in the meantime, that it loses all the magnetism thus acquired when the inducing action is withdrawn : although this is not necessarily,- and in fact not generally, the case, as we shall see by and by. The reason why soft iron is attracted by a permanent magnet is therefore now said to be that the iron becomes magnetic by induction, and is then acted upon by the magnet like any other magnet similarly placed. The accuracy of this analysis of the phenomenon may be confirmed by many simple but striking experiments, such as the following. In the experiment above described, instead of placing the non-magnetic bar in a horizontal plane, place it in the plane of the magnetic meridian with its axis in the direc tion of the earth s force (i.e., parallel to the line of dip). The lower end of the bar will then be found to repel and the upper end to attract the north pole of the needle (figs. 9, 10). This is at once explained on the above hypothesis ; for the bar will be magnetized inductively by the earth s force, so that its lower end becomes a north pole, and its upper end a south pole. Let NS (fig. 11) be a bar magnet placed horizontally so that its axis produced passes through. O, the centre of suspension of the needle sn, then the needle will be deflected A in the direction of the arrow. If now we place between S and O a small sphere of soft iron, this deflexion will be increased, the reason being that the sphere is magnetized by induction, having a south pole towards and a north pole N N towards S, and the action of these is added to that of NS. Let NS (fig. 12) be a magnet placed in the magnetic meridian, n s a small , , magnetic needle in the ..--" "^ -, same horizontal plane, ..-- * s with its centre in the line bisecting NS at right angles. When acted on by NS alone, j ; n s will place itself j parallel to NS, with -.f its north pole pointing 5 N in the direction NS * ig- 12- Let the dotted line represent a line of force. If we move a small piece of soft iron ns along this line in the direc
tion from N towards S, it will first deflect the needle as