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.