Earth's magnetic poles

“ Terrestrial Magnetism Consider the Earth as a huge magnet surrounded by lines of magnetic flux connecting its two magnetic poles. These magnetic poles are near, but not coincidental with, the Earth’s geographic poles. Since the north seeking end of a compass needle is conventionally called the north pole, or positive pole, it must therefore be attracted to a south pole, or negative pole. ”

“ A.D. 1683.—Halley (Edmund) , LL.D., who became English astronomer royal, makes known his theory of four magnetic poles and of the periodical movement of the magnetic line without declination. He states that the earth’s magnetism is caused by four poles of attraction, two of them being in each hemisphere near each pole of the earth. By the word pole he means a point where the total magnetic force is a maximum, or, as he himself styles it, “a point of greatest attraction” (Walker, “Magnetism,” p. 317, etc.) ”

“ An entirely similar experience will result from carrying a small needle through the magnetic field of the steel bar. At the neutral ground it will be parallel to the bar, while, as we approach either end, the dip toward the pole becomes more and more until it stands vertical at the pole. And as it was stated, regarding the steel bar, that the intensity of its magnetic field varies from point to point, so with the earth, it also has a magnetic field which is powerful near its poles and steadily moderates in strength as we approach the magnetic equator. ”

“ If we look away from the actual condition of the earth and take the question in its generality, there may certainly exist more than two magnetic poles; but it does not appear to have been noticed that if, for example, two north poles exist, there must necessarily be between them yet a third point, which is likewise a magnetic pole, but is properly neither a north nor a south pole, or is both if that expression be preferred. ”

“ He remarks that the term “poles,” as used by his predecessors, involves an assumption arbitrary, and, as it is now found, false; namely, that certain definite points, two, four, or more, acting according to the laws of ordinary magnetical poles, will explain the phenomena. He starts from a more comprehensive assumption, that magnetism is distributed throughout the mass of the earth in an unknown manner. On this assumption he obtains a function V, by the differentials of which the elements of the magnetic force at any point will be expressed. ”

“ Unless the diurnal inequality vanishes in its neighbourhood, a somewhat improbable contingency considering the large range at the “Discovery’s” winter quarters, the position of the south magnetic pole has probably a diurnal oscillation, with an average amplitude of several miles, and there is not unlikely a larger annual oscillation. Thus even apart from secular change, no single spot of the earth’s surface can probably claim to be a magnetic pole in the sense popularly ascribed to the term. ”

“ But it very often happens at sea and on land that the magnetick iron does not point to the true pole, and that not only a versorium and magnetick pieces of iron, and the needle of a compass, or a mariners' compass, but also a terrella in its boat, as well as * iron ore, iron stones, and magnetick earths, properly prepared, are drawn aside and deviate towards some point of the Horizon very near to the meridian. For they with their poles frequently face termini away from the meridian. ”

“ If a magnet is dipped into a mass of iron filings and withdrawn, filings cling to certain parts of the stone in moss-like tufts, other parts remaining bare. There are generally two regions where the tufts are thickest, and the attraction therefore greatest, and between them is a zone in which no attraction is evidenced. The regions of greatest attraction have received the name of poles, and the line joining them is called the axis of the magnet; the space around a magnet in which magnetic effects are exhibited is called the field of magnetic force, or the magnetic field. ”

“ For indeed the moving magnetick virtue is stronger in a higher latitude; and the larger regions extending toward the poles dominate the more, as is easily apparent anywhere on a terrella. For as in the case of true Direction magnetick bodies tend toward the pole (namely, toward the stronger end, the whole earth causing the motion) , so also do they incline a little toward the stronger and higher parts by the action of the whole along with the conjoint action of iron bodies. ”

“ On approaching the magnetic poles, compasses become sluggish, for the horizontal deflecting force falls off rapidly. The force, acting in a vertical direction, tending to make the needle dip, correspondingly increases, but is of no value for navigation purposes. However, in the scientific discussion of terrestrial magnetism, both the horizontal and vertical components as well as the absolute value of the total force are important, and the determination of these "elements" is the work of the magnetician. ”

“ But in no turnings-about of bodies, in none of the motions of the planets are there to be recognized, beheld, or assured to us by any reasoning any sensible or natural poles in the firmament, or in any Primum {221}Mobile; but those are the conception of an unsettled imagination. Wherefore we, following an evident, sensible and tested cause, do know that the earth moves on its own poles, which are apparent to us by many magnetick demonstrations. ”

“ This theory is not yet definitely proved, but it furnishes a sufficient theoretical basis for future research. The earth itself is a gigantic magnet, a fact which makes the compass possible, and it is well known that the earth's magnetism is affected by those great outbreaks on the sun called sun-spots. Now it has been recently shown that a sun-spot is a vast whirlpool of electrons and that it exerts a strong magnetic action. ”

“ This close relationship between the periodicity of sun-spots and the daily movements of the magnetic needle is not the sole proof we possess that there is a connection of some sort between solar phenomena and terrestrial magnetism. A time of maximum sun-spots is a time of great magnetic activity, and there have even been special cases in which a peculiar outbreak on the sun has been associated with remarkable magnetic phenomena on the earth. ”

“ Whatever may be the main factors in making the Earth a great magnet, to whatever extent the rotation factor may be important, the Earth's magnetic properties point strongly to a very irregular distribution of magnetic materials in the outer strata where the temperatures are below that at which magnetic materials commonly lose their polarity. ”