Terrestrial magnetism

“ Now the magnet when employed in the deflexion experiments is placed with its axis east and west, so that the action of terrestrial magnetism is transverse to the magnet, and does not tend to increase or diminish . When the magnet is made to vibrate, its axis is north and south, so that the action of terrestrial magnetism tends to magnetize it in the direction of the axis, and therefore to increase its magnetic moment by a quantity , where is a coefficient to be found by experiments on the magnet. ”

“ If it is true that terrestrial magnetism is a necessary consequence of the magnetism and the rotation of the earth, other bodies comparable in these qualities with the earth, and comparable also with the earth in respect to materials and temperature, such as Venus and Mars, must be magnets, comparable in strength with the earth; and they must have poles similar to the earth, North and South poles on the North and South sides of the equator. ”

“ Gilbert further pointed out that the loadstone is surrounded by a sort of atmosphere or "orbe of virtue" within which the magnetical effects can be observed. Book 3, on the directive force of the magnet, is full of most instructive experiments, in which the terrella figures largely, relating to the question how one magnet influences another and tends to make it point toward it. All this was leading up to the theory of terrestrial magnetism; for we find him naming the parts of his loadstone globes with poles, equator and meridians. ”

“ Terrestrial magnetism did not acquire a right to be regarded as a science until partial results were graphically connected in a system of lines of 'equal declination, equal inclinatiion', and 'equal intensity'. ”

“ Of greater importance for ascertaining the true nature of northern lights has been the demonstration of the relation these phenomena hold to terrestrial magnetism, a mysterious power of which our globe is the conductor. Powerful magnetic currents appear suddenly and disappear as quickly, and we are not able to name the cause of their existence.
Like a vast ocean the magnetic force is constantly surging through our globe, here appearing, there receding, but forever seeking to attain its equilibrium. ”

“ 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. ”

“ The association between the sun-spot period and terrestrial magnetism is of a far more marked character, though I must premise that the Astronomer Royal, after careful analysis of the Greenwich magnetic records, denies the existence of any such association whatever. There is, however, a balance of evidence in its favor. ”

“ No doubt the magnetic lines of force in the earth are closed curves, as in artificial magnets; but in their circuitous courses they may extend to any distance in space, or rather in the ethereal medium, even to thousands or tens of thousands of miles; for the ethereal medium is permeable to lines of magnetic force, or rather transmits them, otherwise the solar spots could not affect the variations of terrestrial magnetism; besides, they pass through the Torricellian vacuum, which is nearly a void with respect to air, but not to the ethereal medium. ”

“ In connection with this, I may tell you that Lamont of Munich, after patient inquiry, has come to the conclusion, that there is a decennial period in the variations of the magnetic declination; it increases regularly for five years, and decreases as regularly through another five. If it can be discovered that the horizontal intensity is similarly affected in a similar period, another of the laws of terrestrial magnetism will be added to the sum of our knowledge. ”

“ From an analysis of seventeen years data at St Petersburg and Pavlovsk, Leyst [56] concluded that all the principal planets sensibly influence the earth’s magnetism. According to his figures, all the planets except Mercury—whose influence he found opposite to that of the others—when Planetary Influence. nearest the earth tended to deflect the declination magnet at St Petersburg to the west, and also increased the range of the diurnal inequality of declination, the latter effect being the more conspicuous. ”

“ When the field is removed, many of the newly formed combinations are but slightly disturbed, and the group may consequently retain a considerable resultant moment. This corresponds to the second stage of magnetization, in which the susceptibility is large and permanent magnetization is set up. A still stronger magnetizing force has little effect except in causing the direction of the needles to approach still more nearly to that of the field; if the force were infinite, every member of the group would have exactly the same direction and the greatest possible resultant moment would be reached ”

“ Unless the path of the induction is entirely inside the metal, free magnetic poles are developed at those parts of the metal where induction enters and leaves, the polarity being south at the entry and north at the exit of the flux. These free poles produce a magnetic field which is superposed upon that arising from other sources. The resultant magnetic field, therefore, is compounded of two fields, the one being due to the poles, and the other to the external causes which would be operative in the absence of the magnetized metal. ”

“ When therefore sensible uniformity is desired, the radius of the ring should be large in relation to that of the convolutions, or the ring should have the form of a short cylinder with thin walls. The strongest magnetic fields employed for experimental purposes are obtained by the use of electromagnets. For many experiments the field due to the earth’s magnetism is sufficient; this is practically quite uniform throughout considerable spaces, but its total intensity is less than half a unit. ”

“ This period covered five complete sun-spot periods, and the approximate synchronism of the maxima and minima, and the general parallelism of the magnetic and sun-spot changes is patent to the eye. Ellis [23] has also applied an analogous method to investigate the relationship between sun-spot frequency and the number of days of magnetic disturbance at Greenwich. A decline in the number of the larger magnetic storms near sun-spot minimum is recognizable, but the application of the method is less successful than in the case of the inequality range. ”

“ All magnetism is certainly not lost until we arrive at a white heat,* and it is manifested when iron is at a dark red heat, however different, therefore, the modifications may be which are excited in substances in their molecular state, and in the coercive force depending upon that condition in experiments of this nature, there will still remain a considerable thickness of the terrestrial stratum, which might be assumed to be the seat of magnetic currents. ”

“ According to Balfour Stewart, auroræ and earth currents may be regarded as secondary currents resulting from changes in the earth’s magnetism. Magnetic phenomena are included under the general term terrestrial magnetic elements, and consist of magnetic declination, inclination, and intensity.
These are for convenience determined separately; the first by an instrument called a Declinometer, and the second by an Inclinometer or Dipping Needle. ”

“ In heat the predominant principle is light; whereas the predominant principle in magnetism is gravitation. To these we find new powers still added, among which the mechanical and the chemical appear to be allied to gravitation and magnetism, (for the laws of mechanics are essentially connected with those of gravitation, as the chemical laws include the compositions and decompositions, the attractions and repulsions of ponderable bodies,) while electricity and galvanism, on the contrary, being more connected with light and heat, are found less inherent in terrestrial substances. ”

“ The magnetism acquired by soft iron while it is in a magnetic field. Soft iron will lose its induced magnetism when it is removed from a magnetic field. The strength and polarity of the induced magnetism will alter immediately as its magnetic latitude, or its orientation in a magnetic field, is changed. The induced magnetism has an immediate effect upon the magnetic compass as the magnetic latitude or heading of a craft changes. ”

“ 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. There is doubtless a connection between these outbreaks of electronic activity and the consequent changes in the earth's magnetism. ”

“ The great magnet or terrestrial globe directs iron (as I have said) toward the north and south; and excited iron quickly settles itself toward those termini. Since, however, the globe of the earth is defective and uneven on its surface and marred by its diverse composition, and since it has parts very high and convex (to the height of some miles) , and those uniform neither in composition nor body, but opposite and dissimilar: it comes to pass that the whole of that force of the earth diverts magnetical bodies in its periphery toward the stronger and more prominent connected magnetick parts. ”

“ That half of the globe turned away from the sun will partake of the density of the ether at that distance, which is greater than on the side next the sun; the magnetic intensity ought, therefore, to be greater in the night than in the day. The poles of the great terrestrial magnets, or even the position of a magnetic needle on the surface, are continually placed by the earth’s rotation in a different relation to the axes of the terral vortex, and the tangential current, which is continually circulating around the globe, has its inclination to a given meridian in a perpetual state of change. ”

“ 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. ”

“ Only, to find the phenomena from which to read the absolute characteristics of the two sides of the magnetic polarity, we must not turn to the body of man but to that of the earth, one of whose characteristics it is to be as much the bearer of a magnetic field as of gravitational and levitational fields. There is significance in the fact that even to-day, when the tendency prevails to look for causes of natural phenomena not in the macrocosmic expanse, but in the microscopic confines of space, the two poles of magnetism are named after the magnetic poles of the earth. ”

“ For if a revolvable needle is shut up in a rather thick gold box (this metal indeed excels all others in density) or a glass or stone box, nevertheless that magnetick needle has its forces connected and united with the influences of the earth, and the iron will turn freely and readily (unhindered by its prison) to its desired points, North and South. * It even does this when shut up in iron caverns, if they are sufficiently spacious. Whatever bodies are produced among us, or are artificially forged from things which are produced, consist of matter of the terrestrial globe ”

“ It does not generate the ordinary magnetism which attracts iron and steel, nor the special type of magnetism which we call gravity, but something between the two. It attracts the sun enough to disturb the tilt of the earth's axis, but not enough to pull the earth out of its orbit. ”

“ Therein he shows, as the result of his customary careful experimental explorations that the magnetism of every known substance (even tissues of the human frame) is manifested in one of two ways. Either the body is, like iron, attracted by the magnet, taking a position coincident with the magnetic forces which he calls paramagnetic (para beside or near, magnetes, magnes, magnet) or bodies—like bismuth, for instance—are repelled by the poles and should therefore be called diamagnetic (dia, across) for they set themselves across, equatorially, or at right angles to the magnetic lines. ”

“ The simplest hypothesis of this kind is that which supposes a very small magnet in the centre of the earth; or rather (as it is not likely that any one ever believed in the actual existence of such a magnet) supposes magnetism to be so distributed in the earth, that its collective action at and beyond the surface is equivalent to the action of an imaginary infinitely small magnet; much as gravitation towards a homogeneous sphere is equivalent to the attraction of a sphere of equal mass condensed in its central point. ”