§ 11. "If a current be caused to flow over and parallel to a freely suspended magnetic needle, previously pointing north and south, the north pole will be impelled to the LEFT of the entering current. If, on the contrary, the wire, or conductor, be placed below the needle, the deflection will, under similar circumstances, be in the opposite direction, viz.: the
north pole will be impelled to the RIGHT of the entering current." In both these cases the observer is supposed to be looking along the needle, with its N. seeking pole pointing at him.
§ 12. From a consideration of the above law, in connection with the experiments performed at [§ 9], it will be evident that inside the tumbler the zinc is positive to the copper strip; while, viewed from the outside conductor, the copper is positive to the zinc strip.[3]
§ 13. A property of current electricity, which is the fundamental basis of electric bell-ringing, is that of conferring upon iron and steel the power of attracting iron and similar bodies, or, as it is usually said, of rendering iron magnetic. If a soft iron rod, say about 4" long by ½" diameter, be wound evenly from end to end with three or four layers of cotton-covered copper wire, say No. 20 gauge, and placed in proximity to a few iron nails, etc., no attractive power will be evinced; but let the two free ends of the wire be placed in metallic contact with the wires leading from the simple battery described at [§ 9], and it will be found that the iron has become powerfully magnetic, capable of sustaining several ounces weight of iron and steel, so long as the wires from the battery are in contact with the wire encircling the iron; or, in other words, "the soft iron is a magnet, so long as an electric current flows round it." If contact between the battery wires and the coiled wires
be broken, the iron loses all magnetic power, and the nails, etc., drop off immediately. A piece of soft iron thus coiled with covered or "insulated" wire, no matter what its shape may be, is termed an "electro-magnet." Their chief peculiarities, as compared with the ordinary permanent steel magnets or lodestones, are, first, their great attractive and sustaining power; secondly, the rapidity, nay, instantaneity, with which they lose all attractive force on the cessation of the electric flow around them. It is on these two properties that their usefulness in bell-ringing depends.
§ 14. If, instead of using a soft iron bar in the above experiment, we had substituted one of hard iron, or steel, we should have found two remarkable differences in the results. In the first place, the bar would have been found to retain its magnetism instead of losing it immediately on contact with the battery being broken; and, in the second place, the attractive power elicited would have been much less than in the case of soft iron. It is therefore of the highest importance, in all cases where rapid and powerful magnetisation is desired, that the cores of the electro-magnets should be of the very softest iron. Long annealing and gradual cooling conduce greatly to the softness of iron.
Fig. 3.
Magnets, showing Lines of Force.
§ 15. There is yet another source of electricity which must be noticed here, as it has already found application in some forms of electric bells and signalling, and which promises to enter into more extended use. If we sprinkle some iron filings over a bar magnet, or a horse-shoe magnet, we shall find that the filings arrange