Magnetic needle

“ If a piece of steel be carefully balanced upon a support, and then magnetized, it will be found that it will no longer balance. The north pole will dip or point downward. Fig. 33 shows what happens to a needle when it is held in different positions over a bar magnet. It simply takes the directions of the lines of force as they pass from the north to the south pole of the magnet. As the earth's lines of force pass in curves from the north to the south magnetic pole, you can see why the magnetic needle dips, unless its south pole is made heavier than its north. ”

“ He caused a circular copper plate to revolve immediately beneath a magnetic needle or magnet, freely suspended so that the latter might rotate in a plane parallel to that of the copper plate, and he found that the needle tends to follow the circumvolution of the plate; that it will deviate from its true direction, and that by increasing the velocity of the plate the [479] deviation will increase till the needle passes the opposite point, when it will continue to revolve, and at last with such rapidity that the eye will be unable to distinguish it. ”

“ If a needle is magnetized by being left some time in contact with a magnet, and is balanced on a point in such a way that it will move freely on the point, then no matter in which direction the magnetic needle is turned, as soon as it is set free, it will come to rest with one pole pointing to the south, the other pointing to the north. ”

“ There was no iron near, and the magnetic needle when placed on various parts of the stone, showed no magnetic influence in it. Next day the experiment was repeated with this difference, that the needles were left exposed to the sun, under the blue glass, six hours; and then the needles had not only acquired very sensible magnetism, but still retain it, at the distance of nearly six months. Pieces of clock spring, which had been heated as formerly mentioned, also became magnetic under the blue glass. ”

“ Astatic needle, a magnetic needle having another needle of the same intensity fixed parallel to it, the poles being reversed, so that the needles neutralize one another, and are unaffected by the earth's magnetism ”

“ When either the magnetization I or the induction B corresponding to a given magnetizing force H is known, the other may be found by means of the formula B = 4πI + H.
Magnetometric Methods.—Intensity of magnetization is most directly measured by observing the action which a magnetized body, generally a long straight rod, exerts upon a small magnetic needle placed near it. The magnetic needle may be cemented horizontally across the back of a little plane or concave mirror, about 1⁄4 or 3⁄8 in. in diameter, which is suspended by a single fibre of unspun silk ”

“ The center of the needle is annealed or softened by holding it in the flame of an alcohol lamp and then allowing it to cool.
Lay the needle on a piece of soft metal such as copper or brass, and dent it in the center with a punch.
Balance the needle on the end of a pin stuck through the bottom of a pill-box.
Magnetic Substances are those which are attracted by a magnet. Experiment with a number of different materials, such as paper, wood, brass, iron, copper, zinc, rubber, steel, chalk, etc. It will be found that only iron and steel are capable of being attracted by your magnet. ”

“ Amsterdam and Leipzig, 1752, he speaks (Vol. I. pp. 14–18 and Vol. II. pp. 30–31, 92–94, 113, 123, 128) of the defective magnetic needles given him as well as of the means of correcting them, and he details at great length the variations of the needle observed during the voyage. He also alludes to the variation charts of Dr. Halley and to the alterations therein made by advice of William Mountaine and Jacob Dooson—James Dodson—of London, as well as to the methods of ascertaining the variation of the magnetic needle pointed out both by Manuel de Figueyredo, at Chaps. ”

“ The direction of magnetization of the equivalent magnet may be predicted by remembering that a current travelling round the earth from east to west as the sun appears to do, would be equivalent to that magnetization which the earth actually possesses, and therefore in the reverse direction to that of a magnetic needle when pointing freely. ”

“ It is an attribute of a
paramagnetic substance only and is identical for permanent magnets or
for electro-magnets. An air space intervening between armature and
magnet poles, or a space filled with any diamagnetic substance prevents
continuity, although the lines of force to some extent still find their
way around. The leakage is increased by discontinuity.
Magnetic Control.
Control of a magnetic needle, magnet, iron index or armature, in a
galvanometer, ammeter or voltmeter by a magnetic field; the restitutive
force being derived from a permanent magnet. ”

“ And it was a passage found in the Leyden manuscript (Q 27) which led to the belief that Peregrinus had first observed the variation or declination of the magnetic needle. The [54] passage is as follows: “Take note that the magnet, as well as the needle that has been touched by it, does not point exactly to the poles, but that the part of it which is supposed to point to the South sometimes declines a little to the West, and that the part which looks towards the North sometimes inclines to the East. ”

“ The only assumption relative to induced magnetism which is involved in these equations is, that if a magnetic force X due to external magnetism produces in the iron of the ship an induced magnetization, and if this induced magnetization exerts on the compass needle a disturbing force whose components are X', Y', Z', then, if the external magnetic force is altered in a given ratio, the components of the disturbing force will be altered in the same ratio. ”

“ 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. ”

“ A balanced needle, highly magnetized, which points to the magnetic pole, when not influenced by the local attraction of neighbouring iron. The magnetism may be discharged by blows, or a fall; hence, after an action at sea, the needles are often found to be useless, until re-magnetized. ”

“ As the N pole of the magnetic lever is lowered, it produces an increasing deflection of the suspended needle which is magnified by the spot of light reflected from the attached mirror. The sensitiveness of the apparatus is very greatly enhanced by the employment of a perfect system of astatic needles. ”

“ But in recent times iron is so largely used in the construction of ships that it has become impossible to use the compass at all without taking into account the action of the ship, as a magnetic body, on the needle.
To determine the distribution of magnetism in a mass of iron of any form under the influence of the earth s magnetic force, even though not subjected to mechanical strain or other disturbances, is, as we have seen, a very difficult problem. ”

“ N and S represent the Earth’s north and south magnetic poles, and the arrow heads are the north poles of the needles.
Fig. 14.—Diagram to illustrate Magnetic Dip.
Since the Earth is a magnet, we should expect it to be able to induce magnetism in a bar of iron, just as our artificial magnets do, and we can show that this is actually the case. If a steel poker is held pointing to and dipping down towards the north, and struck sharply with a piece of wood while in this position, it acquires magnetic properties which can be tested by means of a small compass needle. ”

“ The intensity (at any point) of the field due to the magnetization may be denoted by Hi, that of the external field by H0, and that of the resultant field by H. In certain cases, as, for instance, in an iron ring wrapped uniformly round with a coil of wire through which a current is passing, the induction is entirely within the metal; there are, consequently, no free poles, and the ring, though magnetized, constitutes a poleless magnet. Magnetization is usually regarded as the direct effect of the resultant magnetic force, which is therefore often termed the magnetizing force. ”

“ When the load on a hardened wire is gradually increased, the maximum value of I is found to correspond with a greater stress than when the load is gradually diminished, this being an effect of hysteresis. Analogous changes are observed in the residual magnetization which remains after the wire has been subjected to fields of different strength. The effects of longitudinal pressure are opposite to those of traction; when the cyclic condition has been reached, pressure reduces the magnetization of iron in weak fields and increases it in strong fields (Ewing, Magnetic Induction, 1900, 223) . ”

“ Horse-shoe magnets should have a short bar of soft iron adapted to connect the two poles, and should never be laid by without such a piece of iron adhering to them. Bar magnets should be kept in pairs with their poles turned in contrary directions, and they should be kept from rust. Both kinds of magnets have their power not only preserved but increased, by keeping them surrounded with a mass of dry filings of soft iron, each particle of which will re-act by its induced magnetism upon the point of the magnet to which it adheres, and maintain in that point its primitive magnetic state. ”

“ If we place two or more needles of iron near the magnet, the action becomes more complex, for then the needles are not only acted on by the magnet, but they act upon each other. And if we pass to smaller masses of iron—to iron filings, for example—we find that they act substantially as the needles, arranging themselves in definite forms, in obedience to the magnetic action. ”

“ 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. ”

“ It is very certain that magnetism is an inherent property of the molecules of iron and steel, and, to a small degree, other forms of matter. That is to say, the molecules are little natural magnets of themselves. It is as unnecessary to inquire why they are magnets as it is to inquire why the molecules of all ordinary substances possess the attraction of cohesion. The one is as easy to explain as the other. People of all ages have insisted upon making a greater mystery of all electrical and magnetic phenomena than they do of other natural forces. ”

“ In all substances except iron, nickel, and cobalt, the coefficient of magnetization is so small that the induced magnetization of the body produces only a very slight alteration of the forces in the magnetic field. We may therefore assume, as a first approximation, that the actual magnetic force within the body is the same as if the body had not been there. The superficial magnetization of the body is therefore, as a first approximation, , where is the rate of increase of the magnetic potential due to the external magnet along a normal to the surface drawn inwards. ”

“ For a given strength, therefore, the potential depends solely upon the boundary of the shell, and the potential outside a closed shell is everywhere zero. A magnet which can be divided into simple magnetic shells, either closed or having their edges on the surface of the magnet, is called a lamellar magnet, and the magnetism is said to be distributed lamellarly. ”

“ When a piece of soft iron is held sufficiently near one of the poles of a powerful magnet, it becomes by induction endowed with magnetic poles, and will support another bit of soft iron, such as a nail, brought in contact with it. When the magnet is removed, the inductive action ceases, and the soft iron loses its magnetic power. This experiment affords another example of the connexion between the phenomena of electricity and magnetism. ”

“ The state of excitement into which iron is thrown by the influence, of a magnet, is sometimes called 'magnetisation by influence.' More commonly, however, the magnetism is said to be 'induced' in the iron, and hence this mode of magnetising is called 'magnetic induction.' Now, there is nothing theoretically perfect in Nature: there is no iron so soft as not to possess a certain amount of coercive force, and no steel so hard as not to be capable, in some degree, of magnetic induction. ”

“ Until 1820 all the artificial magnets in practical use derived their virtue, directly or indirectly, from the natural magnets found in the earth: it is now recognized that the source of all magnetism, not excepting that of the magnetic ore itself, is electricity, and it is usual to have direct recourse to electricity for producing magnetization, without the intermediary of the magnetic ore. ”

“ Brass, copper, zinc, etc., are not visibly affected by a magnet. Magnetism will act through paper, glass, copper, lead, etc.
Fig. 24.
30. Making Magnets. One of the strangest properties that a magnet has is its power to give magnetism to another piece of steel. If a sewing-needle be properly rubbed upon one of the poles of a magnet, it will become strongly magnetized and will retain its magnetism for years. Strong permanent magnets are made with the aid of electromagnets. Any number of little magnets may be made from a horseshoe magnet without injuring it. ”