Further, it can be shown that two freely moving parallel wires conveying currents attract or repel one another according to the direction of the currents. If both currents are flowing in the same direction the wires attract one another, but if the currents flow in opposite directions the wires repel each other. Fig. 16 shows the direction of the lines of force of a wire conveying a current and passed through a horizontal piece of cardboard covered with a thin layer of iron filings; and from this figure it is evident that the passage of the current produces what we may call magnetic whirls round the wire. A spiral of insulated wire through which a current is flowing shows all the properties of a magnet, and if free to move it comes to rest pointing north and south. It is attracted or repelled by an ordinary magnet according to the pole presented to it and the direction of the current, and two such spirals show mutual attraction and repulsion. A spiral of this kind is called a _solenoid_, and in addition to the properties already mentioned it has the peculiar power of drawing or sucking into its interior a rod of iron. Solenoids have various practical applications, and in later chapters we shall refer to them again. If several turns of cotton-covered wire are wound round an iron rod, the passing of a current through the wire makes the rod into a magnet (Plate II. _b_), but the magnetism disappears as soon as the current ceases to flow. A magnet made by the passage of an electric current is called an _electro-magnet_, and it has all the properties of the magnets mentioned in the previous chapter. A bar of steel may be magnetized in the same way, but unlike the iron rod it retains its magnetism after the current is interrupted. This provides us with a means of magnetizing a piece of steel much more strongly than is possible by rubbing with another magnet. Steel magnets, which retain their magnetism, are called _permanent_ magnets, as distinguished from electro-magnets in which soft iron is used, so that their magnetism lasts only as long as the current flows.