Magnetism. A phenomenon of the action and reaction of the gases which till space, and to which the surface of iron presents a peculiar reaction. The plate represents the curved forms in which particles of steel-dust range themselves when gifted on paper laid over a magnet. There is no mystery in it, nor any fluid circulating, because every particle of the steel is to be regarded as a magnet with two poles ; and it is found that if a considerable number of magnetized needles were placed round the same magnet, they would be directed into curves just like those of the steel-dust. The whole is a mere mechanical affection.
Magnetism, etc. The production of magnetism by electricity is another of the important results which have arisen from an increased knowledge of Galvanism.
Almost everyone knows that property of the magnet which causes it to attract iron and a few other substances. This attractive power may be communicated temporarily to soft iron, and permanently to steel, either by the natural magnet (the loadstone) or the artificial (a magnetized bar of steel). If a magnet be suspended freely it will arrange itself north and south ; that is. one pole or extremity will point almost north, and the other in the opposite direction. This directive power, as it is called, is what makes the mariner's compass so useful to the navigator. By its aid he may traverse the path'ess ocean during the darkest night in the utmost security; and yet it consists merely of a needle (a small bar of steel magnetized) balanced on a tine point, so that it can move in every direction over a circular 'card, marked with 32 divisions (called points) in its circumference.
If a bar of steel is carefully balanced on a point, and then magnetized by rubbing it to a magnet, or by any other means - except in a part of the earth just midway between the magnetic poles - it will no longer remain in equilibrio, but will form an angle with the horizon, which is called the angle of dip. Hence, to make the needle of the mariner's compass assume and preserve a horizontal position, we are obliged to render one end of it heavier than the other. We have already said that the needle does not, when left to itself, point due north and south ; the angle it makes with a horizontal line lying in the meridian of the place is called the angle of variation of that place. It is to be remarked that this angle is not always the same even at the same plate. Both " dip " and "variation" arise from that cause which makes the needle point to the magnetic poles, namely, the earth being a great magnet and acting as such on the needle. We may illustrate both dip and variation by placing a magnetized bar of steel under the needle in such a way as that it will occupy the same positions with reference to it as the magic axis (a line passing through the magnetic poles) occupies.
It only remains to show why the earth acts towards the needle as if it were a great magnet. The earth is what is called an electro-magnet; that is, one formed by the circulation of electrical currents around it. The connexion between electricity and magnetism was long known; but that electricity circulating around the needle will cause it to be deflected from its ordinary position, and that the same current passing round a bar of iron would magnetize it, are facts which constitute a recent and very important discovery.
The currents which produce the magnetism of the earth are due to the enormous evaporation from its surface, and to the constant change of temperature caused by revolution on its axis, which exposes different parts of it in succession to the sun's rays. - Electricity developed during change of temperature has been designated thermo-electricity.
We are not to suppose that only ferruginous substances (although the best for the purpose) or even metals alone are capable of being magnetized by means of electricity. We may illustrate the most interesting facts in electro-magnetism by covering copper wire with worsted, cotton, or some other bad conductor of electricity, and then coiling it round a bar of iron. On connecting the extremities of the wire-coil or helix, as it is termed, respectively, with the plates of a Galvanic Circle, the iron bar will be found to be highly magnetic. It is necessary to cover the wire with some non-conducting substance, or the electricity, instead of traversing the length of it, and so passing round the iron, would pass directly from one part of the wire to another, selecting, as electricity always does, the shortest path.
The helve is capable not only of producing magnetism in iron, but also electricity in another helix placed around or intertwined with it; and it is found, whether used by itself, or in combination with another, to give to the electricity derived from a single Galvanic Circle an intensity which could scarcely be obtained from the combination of a very great number of circles - the zinc of one being connected with the copper of platina of the next; which is the mode ci arrangement required when we desire to give to Galvanic electricity a greater or less degree of intensity; that is, a capability of producing mechanical or physiological effects, and the power of traversing bad or imperfect conductors.