Art. 90. Terrestrial Magnetism.--We have already seen that the earth is a magnet, and like any other magnet will therefore possess its magnetic field with its magnetic lines of force. The earth's magnetic field is co-existent and co-equal with its electric field ([Art. 80]), and that is co-existent with the earth's aetherial atmosphere which is held bound to the planet by the force of gravity.
How far the earth's magnetic field reaches, is impossible to say, but we know that it extends at least as far as 260,000 miles, the distance of the moon; as we find that this satellite of the earth is affected very considerably by the electro-magnetic attractive power of the earth. Any body which is placed in the earth's magnetic field is affected by the lines of force which exist in the magnetic field; for wherever the field exists, there the lines of force exist also.
These lines of force, which are associated with the earth, extend therefore into space, and any body such as the moon would become a magnet, if not already one by the process known as magnetic induction, which physical process is well illustrated in the action of a magnet upon iron filings strewed over it as in the illustration ([Art. 88]).
An experiment which well illustrates the inductive power of the earth's magnetism, may be made by placing a poker in one of these lines of force, whose direction can be found at any part of the earth's surface by means of proper instruments. When the poker is so placed, it will be seen that it has actually become magnetized by the magnetism of the earth, and it is itself able to attract iron filings or small needles. These lines of force of the earth are closer together nearest to the earth's surface than further away in space, and congregate around the North and South magnetic poles, where they are greatest in number in a given area, and there the magnetic intensity is the greatest.
Faraday, writing on the terrestrial lines of force, says: “The lines of force issue from the earth in the northern and southern parts with different but corresponding degrees of inclination, and incline to, and coalesce with each other over the equatorial parts. There seems reason to believe that the lines of magnetic force which proceed from the earth return to it, but in their circuitous course they may extend through space to a distance of many diameters of the earth, to tens of thousands of miles.“[36]
From this extract it will be seen that Faraday was of the opinion that the lines of force extended beyond the atmosphere of the earth into the Aether, which statement is confirmed by other parts of his writings; though he was not able to give any physical explanation of how these lines extended beyond the atmosphere on account of the doubtful constitution and character of the Aether, although in another part of his work he definitively refers to the magnetic character of space.
In writing on the magnetic character of space he says:[37] “From such experiments, and also from general observations and knowledge, it seems manifest that the lines of magnetic force can traverse pure space, just as gravitating force does, and as static electric forces do (1616), and therefore space has a magnetic character of its own, and one that we shall probably find hereafter to be of the utmost importance in natural phenomena.“ With the view of the Aether presented in this work, viz. that Aether is matter, though in an infinitely more rarefied and elastic form, we can now see the physical cause of the lines of force with which by his imagination he filled all space.
Again, from the conception of the Aether presented to the reader in [Art. 45], we learn that around any body in space there are existing aetherial concentric spheres or shells which are equipotential surfaces, or surfaces of equal pressure, and that these surfaces coincide with the electric equipotential surfaces, as shown in [Art. 80].
Not only so, but they coincide with the magnetic shells which the lines of force actually form around a circular and globular magnet, as the earth. For it must not be forgotten that these lines of force exist equally on all sides of the earth, and therefore really form a spherical shell, or to speak more correctly an aetherial electro-magnetic shell, which is an oblate spheroid in shape, partaking of the shape of the earth or other planet which the lines of force surround.
If these shells were divided into two equal halves, the line so dividing them would be called the magnetic equator, and on that line any magnet would set itself in a horizontal position, so that all round the earth on the magnetic equator would correspond to a line of no dip. At the magnetic poles, a magnet would set itself vertically, or at an angle of 90°, and between these two parts, the place of no dip, and that of 90°, the dip gradually changes as illustrated in the figure. Again, in relation to the magnetism of the earth we find that there are certain variations in the magnetic force, which not only influence the dip at any place, but also the intensity at that place. The variations in Magnetic Force are chiefly three--