Without this ability the electric motor would not revolve, the electric light would not burn, the click of the telegraph would not be heard, the telephone would not talk, nor would the telautograph write.
I have said that the permanent magnet would hold its charge after once having been magnetized. This is true only in a sense and under favorable conditions. If made of the best of steel for the purpose and hardened and tempered in just the right way, it will hold its charge if it is given something to do. If a piece of iron is placed across its poles it also becomes a magnet and its molecules turn and work in harmony with those of the mother magnet. These magnetic lines of force reach around in a circuit. Even before the iron, or "keeper," as it is called, is put across its poles there are lines of force reaching around through the air or ether from one pole to another. (For a description of Ether see Chap. V.) This is called the "field" of the magnet, and when the iron is placed in this field the lines of force pass through it in a closed circuit, and if the "keeper" is large enough to take care of all the lines of force in the field the magnet will not attract other bodies, because its attraction is satisfied, like its prototype in the molecular ring described above.
We speak of lines of force, not that force is necessarily exerted in a bundle of lines but as a convenient way of telling the strength of a magnetic field. The practical limit of the magnetization of soft iron (called saturation) is 18,000 lines to the square centimeter. As long as we give our magnet something to do, up to the measure of its capacity, it will keep up its power. We may make other magnets with it, thousands, yea, millions of them, and it not only does not lose its power but may be even stronger for having done this work. If, however, we hang it up without its "keeper," and give it nothing to do, it gradually returns to its natural condition in the home circle of molecular rings. Little by little the coercive force is overcome by the constant tendency of the molecule to go back to its natural position among its fellows.
The magnet furnishes many beautiful lessons, as indeed do all the natural phenomena. Every man has within him a latent power that needs only to be aroused and directed in the right way to make his influence felt upon his fellows. Like the magnet, the man who uses his power to help his fellows up to the measure of his limitations not only has been a benefactor to his race, but is himself a stronger and better man for having done so. But, again, like the magnet, if he allows these God-given powers to lie still and rust for want of legitimate use he gradually loses the power he had and becomes simply a moving thing without influence or use in a world in which he vegetates. But let us leave philosophy and go back to science.
One of the striking exhibitions of magnetism is found in the earth. The earth itself is a great magnet; and there is good reason for believing that it is an electromagnet of great power. The magnetic poles of the earth are not exactly coincident with the geographical poles, and they are not constant. There is a gradual deviation going on, but as it follows a certain law mariners are able to tell just what the deviation should be at a certain time. The magnetic pole revolves around the polar axis of the earth once in about 320 years. A thermal current (one produced by heat) of electricity seems to flow around the earth caused by the irregularities of temperature at the earth's surface, as the sun makes his daily round. These earth currents vary at times, and other phenomena are the occasion. This will be discussed when we come to electric storms.
The value of the earth's magnetism is seen most in the science of navigation. A magnetic needle is only a slender permanent magnet suspended very delicately, and when not under local influence it points north and south on the magnetic axis. The law of its action may be explained as follows: Take a straight bar magnet of fairly good power and suspend a magnetic needle over it. The needle will arrange itself parallel to the bar magnet. The north pole of the needle will point toward the south pole of the bar magnet. In the presence of the magnet the needle is not affected by the earth, but yields to a superior force. If, however, the bar magnet is taken out of the way of the needle it will immediately arrange itself north and south. Of course if the earth's magnetic axis changes the needle will vary with it. This variation is uniform and in navigation is reduced to a science, so that the mariner knows how much to allow for the variation. Columbus, as heretofore mentioned, was supposed to have first noticed this variation and it made him trouble. He did not know how to account for it, and as his crew thought the laws of nature were changing because they were so far from home he saw the necessity for some sort of explanation. So, like the brave man that he was, he hatched up a theory that satisfied the crew, and although in the light of the closing years of the nineteenth century it was a questionable one, it worked well enough in practice to serve his purpose.
We have already stated that the earth was a great magnet, and that probably it was an electromagnet, caused by earth currents circulating around the globe. You want to know how the earth can be a magnet unless it has an iron core like an electromagnet. Magnetism or magnetic lines of force may be developed without the presence of iron. When we pass a current of electricity through a wire, magnetic lines of force are thrown out at right angles with the direction of the current. This will be fully explained further on. If we wind the wire into a coil, or helix, these magnetic lines are concentrated. If now we suspend this helix, or, better, float it on water so that it can move freely, and pass a current of electricity through it, the helix will arrange itself north and south the same as a magnetic needle. Its attractive properties are feeble in comparison with that of the iron, but it obeys the laws of a magnet. The earth is probably a magnet of this kind, consisting mostly of lines of force.
However, the iron in the earth is affected magnetically, as we have evidence in the loadstone. The earth has the power also to magnetize iron through the medium of its magnetic field, that reaches out in lines of force from pole to pole like those of the artificial magnet. If we hold a bar of iron in line with the magnetic axis of the earth and dip it in line with the dipping needle and then strike it a few blows on the end, it will be found to be feebly magnetic. The blows have partly loosened the molecules and during the moment that they unclasped themselves the earth's magnetism has through its lines of force caught them for a time and held them a little out of their natural position—as they are in a state of rest. The peculiar changing light that we sometimes see in the northern sky, that is called the Aurora Borealis (Northern Light), is indirectly due to intense magnetic lines of force that radiate from the north magnetic pole of the earth. Those lines of force are able to cause the rarified air molecules to become feebly incandescent, giving them the appearance that we see in a tube that is a partial vacuum when electricity is passed through it. While these auroral displays may be seen almost any night in the far north, they vary greatly in their intensity, so it is only once in a while that they are visible in the temperate latitudes.
What are called magnetic storms occur occasionally, and at such times the telegraph service will sometimes be paralyzed on all the east and west lines for many hours. Strong earth-currents will flow east and west, and be so powerful and so erratic that it is sometimes impossible to use the telegraph. It sometimes happens that the operators can throw off their batteries and work on the earth-current alone. Sometimes it is necessary to make a complete metallic circuit to get away from the influence of the earth in order to use the telegraph. Currents equal to the force of 2,000 cells of ordinary battery have been developed sometimes in telegraph wires. This of course is a mere fraction of what is passing through the earth under the wire through which the current flowed. On the 17th and 18th of November, 1882, a magnetic storm occurred that extended around the globe, as it was felt wherever there were telegraph wires. These magnetic storms are attended by brilliant displays of the aurora, and this fact strengthens the theory that the earth is a great electromagnet; for the stronger the electrical current the more powerful we should expect the magnetism to be, and this is shown by the action of the magnetic needle at such times. The stronger the magnet the more intense will be the lines of force, and naturally the more intense the light, if indeed these lines of force are the cause of the light. There is evidently some close relation between the two.
Another coincidence is that at the times of these storms there is an unusual display of sun-spots. These sun-spots seem to be great holes that have been blown through the photosphere of the sun. The photosphere is a great luminous body of gaseous matter that is believed to envelop the sun,
