(b) EXPERIMENT TO SHOW THE PRODUCTION OF MAGNETISM BY AN ELECTRIC CURRENT.
Any one who experiments with magnets must be struck with the distance at which one magnet can influence filings or another magnet. If a layer of iron filings is spread on a sheet of paper, and a magnet brought gradually nearer from above, the filings soon begin to move about restlessly, and when the magnet comes close enough they fly up to it as if pulled by invisible strings. A still more striking experiment consists in spreading filings thinly over a sheet of cardboard and moving a magnet to and fro underneath the sheet. The result is most amusing. The filings seem to stand up on their hind legs, and they march about like regiments of soldiers. Here again invisible strings are suggested, and we might wonder whether there really is anything of the kind. Yes, there is. To put the matter in the simplest way, the magnet acts by means of strings or lines of force, which emerge from it in definite directions, and in a most interesting way we can see some of these lines of force actually at work.
Place a magnet, or any arrangement of magnets, underneath a sheet of glass, and sprinkle iron filings from a muslin bag thinly and evenly all over the glass. Then tap the glass gently with a pencil, and the filings at once arrange themselves in a most remarkable manner. All the filings become magnetized by induction, and when the tap sets them free for an instant from the friction of the glass they take up definite positions under the influence of the force acting upon them. In this way we get a map of the general direction of the magnetic lines of force, which are our invisible strings.
Many different maps may be made in this way, but we have space for only two. [Plate III.a] shows the lines of two opposite poles. Notice how they appear to stream across from one pole to the other. It is believed that there is a tension along the lines of force not unlike that in stretched elastic bands, and if this is so it is easy to see from the figure why opposite poles attract each other.
[Plate III.b] shows the lines of force of two similar poles. In this case they do not stream from pole to pole, but turn aside as if repelling one another, and from this figure we see why there is repulsion between two similar poles. It can be shown, although in a much less simple manner, that lines of electric force proceed from electrified bodies, and in electric attraction and repulsion between two charged bodies the lines of force take paths which closely resemble those in our two figures. A space filled with lines of magnetic force is called a magnetic field, and one filled with lines of electric force is called an electric field.
A horse-shoe magnet, which is simply a bar of steel bent into the shape of a horse-shoe before being magnetized, gradually loses its magnetism if left with its poles unprotected, but this loss is prevented if the poles are connected by a piece of soft iron. The same loss occurs with a bar magnet, but as the two poles cannot be connected in this way it is customary to keep two bar magnets side by side, separated by a strip of wood; with opposite poles together and a piece of soft iron across the ends. Such pieces of iron are called keepers, and [Fig. 13] shows a horse-shoe magnet and a pair of bar magnets with their keepers. It may be remarked that a magnet never should be knocked or allowed to fall, as rough usage of this kind causes it to lose a considerable amount of its magnetism. A magnet is injured also by allowing the keeper to slam on to it; but pulling the keeper off vigorously does good instead of harm.
If a magnetized needle is suspended so that it is free to swing either horizontally or vertically, it not only comes to rest in a north and south direction, but also it tilts with its north-pointing end downwards. If the needle were taken to a place south of the equator it would still tilt, but the south-pointing end would be downwards. In both cases the angle the needle makes with the horizontal is called the magnetic dip.
PLATE III.
(a) LINES OF MAGNETIC FORCE OF TWO OPPOSITE POLES.
