Magnetic pole

“ The foregoing diagram shows the situation of our magnetic pole and line of no variation, the dip of the needle by the arrows, and the magnetic equator.
Recent discoveries have shown that the magnetic force is exerted in lines and currents; that such currents, as physical lines of force, surround magnets, and currents of electricity. Doubtless such lines of force exist around the earth and the magnetic poles. There are also longitudinal lines of force existing and active, between the poles, and extending from one side of the center to the other, occupying nearly one third of the magnet. ”

“ This is in accordance with one of the fundamental laws of magnetism which states that magnetic poles of unlike polarity attract each other and those of like polarity repel each other. The amount the armature B is rotated will depend upon the relative effects of the pole of the solenoid and the permanent magnet G. The strength of the pole of the solenoid will depend upon the current in its winding and the number of times the current passes around the core, or the number of turns in the winding. ”

“ When the magnetic pole moves in a curve parallel to the sheet, the calculation becomes more complicated, but it is easy to see that the effect of the nearest portion of the trail of images is to produce a force acting on the pole in the direction opposite to that of its motion. The effect of the portion of the trail immediately behind this is of the same kind as that of a magnet with its axis parallel to the direction of motion of the pole at some time before. ”

“ The British Association Committee defined the strength of a magnetic pole by reference to the mechanical stress between it and another equal pole: hence the British Association unit magnetic pole is a pole which at a distance of one centimetre attracts or repels another equal pole with a force of one dyne. This, we have seen, is an imperfect definition, because it omits all reference to the permeability of the medium in which the experiment takes place ”

“ Sir James Ross claimed to have discovered the magnetic pole at about seventy-four degrees latitude. This is wrong—the magnetic pole is exactly one-half the distance through the earth's crust. Thus, if the earth's crust is three hundred miles in thickness, which is the distance I estimate it to be, then the magnetic pole is undoubtedly one hundred and fifty miles below the surface of the earth, it matters not where the test is made. And at this particular point one hundred and fifty miles below the surface, gravity ceases, becomes neutralized ”

“ A magnetic pole is no longer acted on by a unique force applied to that pole. We have seen, in fact, the action of an element of current on a pole is not applied to the pole but to the element; it may, moreover, be replaced by a force applied to the pole and by a couple.
(2) The couple which acts on the magnetic needle is no longer a simple director couple, for its moment with respect to the axis of the needle is not zero. It decomposes into a director couple, properly so called, and a supplementary couple which tends to produce the continuous rotation of which we have spoken above. ”

“ The difficulty was overcome by C. A. Coulomb, who by using very long and thin magnets, so arranged that the action of their distant poles was negligible, succeeded in establishing the law, which has since been confirmed by more accurate methods, that the force of attraction or repulsion exerted between two magnetic poles varies inversely as the square of the distance between them. Since the poles of different magnets differ in strength, it is important to agree upon a definite unit or standard of reference in terms of which the strength of a pole may be numerically specified. ”

“ The action of a magnetic pole diminishes so rapidly with the increase of distance that it may suffice to remove the armature to a distance relatively small compared with its own dimensions, or with those of the magnet, in order to reduce the action to a negligible value. But if the magnet, N S, and the armature, A, being at a certain distance, we bring between them a piece of iron or nickel, d, then the magnetic force upon A is immediately and very considerably increased. ”

“ The Earth's magnetic force, which is the active agent in maintaining the compass-needle in the magnetic meridian** at any particular spot, acts, not as is popularly supposed, in a horizontal plane, but at a certain angle of inclination with the Earth's surface. The nearer the magnetic poles the more nearly vertical does the freely suspended needle become. At the South Magnetic Pole it assumes a vertical position with the south end downwards ”

“ Not only has the north magnetic pole gradually been changing its position, as shown by the records of three centuries, but, according to Barlow, every place has a magnetic pole and equator of its own; and according to Faraday the earth is a great magnet, whose power, as estimated by Gauss, is equal to that which would be conferred if every cubic yard of it contained six one-pound magnets ”

“ A freely suspended magnetic needle will not maintain a horizontal position except at the magnetic equator. Over the N. magnetic pole the north-seeking end of the needle points directly downwards and dips at an intermediate angle at intermediate distances between the magnetic poles and equator. ”

“ Gary has made a discovery of a neutral line or surface, at which the polarity of an induced magnet, while moving in the field of the inducing pole, is changed. The alleged discovery appears to be an exaggerated statement of some curious facts, which, although not new, are not commonly recognized. If a bar of iron be brought up, end on, near a magnetic pole, the bar becomes an induced magnet, but an induced magnet quite different from what our elementary treatises seem to predict. ”

“ 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. ”

“ Let the coloring-matter represent the magnetism in the body charged, and we have the magnet illustrated in its polarization: the deep-red end is the positive pole, and the white or faintly-colored end is the negative pole.
It is a law of polarization that the positive poles of different magnets repel each other, and the negative poles repel each other; while positive and negative poles attract each other. The same law of polarization rules in electric or magnetic currents as in magnets at rest. ”

“ Accordingly, when a cube was thus used with the two poles, the effect was repulsion or recession from either pole, and also recession from the magnetic axis on either side.
So the indicating particle would move either along the magnetic curves or across them, and it would do this either in one direction or the other, the only constant point being that its tendency was to move from stronger to weaker places of magnetic force. ”

“ 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. ”

“ This last method of arrangement is called by Ewing the “one-pole” method, because the magnetometer deflection is mainly caused by the upper pole of the rod (Magnetic Induction, p. 40) . For experiments with long thin rods or wires it has an advantage over the other arrangements in that the position of the poles need not be known with great accuracy, a small upward or downward displacement having little effect upon the magnetometer deflection. ”

“ The N-seeking pole is sometimes called the marked pole. For convenience, we shall hereafter speak of the N-seeking pole as the N pole, and of the S-seeking pole as the S pole. We shall hereafter speak of the tendency which a bar magnet has to point N and S, as its pointing power. An unmagnetized needle has no pointing power.
26. The Magnetic Needle; The Compass. A small bar magnet, supported upon a pivot, or suspended so that it may freely turn, is called a magnetic needle. When balanced upon a pivot having under it a graduated circle marked N, E, S, W, etc., it is called a compass. ”

“ But the most serious difficulty of all is that introduced by the edge of the attracting electro-magnet. It is known that the magnetic intensity of a pole is strongest at its edges. Should the writing-index by chance be placed exactly symmetrically, as regards the right and left edges of the pole, then the two lateral pulls, being equal, will neutralise each other, and the index will vibrate to and fro perpendicularly to the recording-surface. ”

“ Her influence on terrestrial magnetism is now ascertained; the magnetism of the hemisphere that is turned toward the earth attracts the pole of our needles that is turned toward the south and increases the magnetism of our hemisphere; and as the magnetic, like the gravitating force, extends through space, the induction of the sun, moon and planets must occasion perpetual variations in the intensity of terrestrial magnetism, by the continual changes in their relative positions. ”

“ But the separate balls or poles, though no longer influencing each other, are not isolated, for each draws by induction an electrical charge opposite to its own to the extremity of the nearest conductor, and thus creates for itself a new or second pole. Polarity, in fact, involves opposition of relations, or two poles, and electrical only differs from magnetic polarity in the fact that in the latter the two poles are in the same body, while in the former they are in separate bodies. ”

“ In every magnet the strength of the south pole is exactly equal to that of the north pole, the action of the same magnetic force upon the two poles being equal and oppositely directed. This may be shown by means of the uniform field of force due to the earth’s magnetism. A magnet attached to a cork and 324 floated upon water will set itself with its axis in the magnetic meridian, but it will be drawn neither northward nor southward; the forces acting upon the two poles have therefore no horizontal resultant. ”

“ Magnetism acts through intervening non-magnetic matter with undiminished energy. Thus, the attractions and repulsions of magnetic poles manifest themselves just as strongly when the poles are separated by a stratum of wood or stone as when merely air intervenes, and the attraction of small pieces of iron by a magnet takes place through the interposed palm of one’s hand without diminution. A delicately suspended needle in even a remote apartment of a large building moves whenever a cart passes in the street. ”

“ 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 repulsion between two poles of strength and is
In air or any medium in which this is simply , but in other media the force acting between two given magnetic poles is inversely proportional to the coefficient of magnetic induction for the medium. This may be explained by the magnetization of the medium induced by the action of the poles. ”