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Polarity

POLARITY signifies, in general, a disposition in a body or in an elementary molecule of a body to place its mathematical axis in some particular direction; frequently also it denotes in a body the existence.either naturally or induced, of two points possessing contrary properties.

If iron-filings be strewed over a mass of natural loadstone, it will be found that there are two points on its surface at which the filings are most abundantly attracted, and whero they dispose themselves nearly in the direction of a line imagined to be drawn through the mass. [magnetism.] Then, if the loadstone bo cut in the form of a Sphere, having this line for a diameter, the symmetrical arrangerncnt of the iron-filings with respect to this line affords an indication that the particles of the loadstone may he symmetrically disposed ahout the line; and, from an analogy with the axis and poles of the earth, this line is called the axis of the loadstone, and its extremities are called the poles. If the mass of loadstone be cut in the form of a prism, the length of the latter being in the direction of the axis, and if the prism be suspended by its centre of gravity, it will be found to take one particular direction with respect to the horizon and the meridian of the observer. The two extremities of the prism so formed have received the denomination of poles, and the term is now applied to the opposilo extremities of any body or molecule, when it assumes or can be brought into a particular direction.

What has been said respecting the properties of a prism formed of the natural loadstone, is true of a magnetised bar of steel [magnet], and the poles or opposite extremities of either material are found to possess a contrariety of character. One extremity always tends towards the northern part of the horizon only, and the other towards the southern part; and if two such prisms or bars are formed, and suspended by their centres of gravity, on bringing the northern or southern pole of one near the like polo of the other, they exorcise upon each other a mutual repulsion; but if either polo of one be brought near the opposite pole of the other, they mutually attract each other.

A piece of natural loadstone, if it could be removed beyond the influence of the magnetic power in the earth, would probably exhibit no signs of that attractive and directive power which we observe in it, the maguetic lluid, or whatever be the cause of the former, being then in equtlibiio in the mass; and it may be conceived that the magnetic power in the earth by somo means disturbs that equilibrium, forcing the fluid molecules which possess opposite properties, or are in contrary states, towards the opposite extremities of the mass. In magnetising a steel bar it is probable that the natural magnetism is decomposed in a similar manner. Since in magnetisud bars the poles of contrary names attract each other, and that the earth may be considered as a body possessing boreal magnetism towards the north, and austral magnetism towards the south, it is evident that the magnetism which exists in the northern extremity or pole of a suspended bar (as a compass needle) must be austral, and that which exists in the southern extremity must be boreal.

If a cylinder of wood or metal be insulated on a glass stand, and it be then brought near a body which has been electrified by the usual machine, it will be rendered polar; that is, one end will possess the vitreous or positive electricity, and the other the resinous or negative electricity, and near the middlo the cylinder will bo in a neutral state. These conditions may be rendered evident on electrifying a pith ball, insulated by means of a silk thread, and presenting it to the cylinder, when it will be attracted towards one end and repelled from the other. It appears, from the effect of the cylinder on the electrified ball, that the particles of fluid of the same kind repel each other, and those of unlike kinds attract each other.

Polarity is also obtained by what is called galvanism, which indeed differs from electricity only in the manner in which a change in the electrical condition of a body is produced; in the latter case friction is employed for this purpose, but in the former it results from the contact of motals susceptible of different degrees of oxidability. In an ordinary battery, the fluid, by chemical action on the zinc, produces a separation of the two kinds of electricity; that which is called positive is carried to the copper plate, and the latter communicates it to the zinc plato with which it is connected. This action is repeated at every pair of plates in the battery; and from the last zinc plate the electricity enters the conducting wire, or that which is employed to connect the opposite extremities of the battery: thus the zinc extremity constitutes the positive pole of the battery. At the same time an opposing current of negative electricity passes from the copper, through the fluid, to the zinc, from thence to the next eopper plate, and so on to the last, which is in connection with the conducting wire at that extremity of the battery: this copper end is called the negative pole of the battery.

The attractive power in a magnetised steel bar increases from each extremity to about one-quarter of an inch from thence, where it is the greatest, and it then diminishes gradually towards the centre; this distribution is similar to that of the induced electricity in the insulated cylinder. But experiments show that if a prism of loadstone or a magnetised bar be divided into several parts perpendicularly to its length, each part is a complete magnet, having poles of contrary denominations at its extremities; and this condition is accounted for by Coulomb ift the following manner: —He supposes that every molecule of loadstone is a small magnet possessing opposite polarities at its extremities, the axes of all being parallel to or coincident with tho magnetic axis of the mass; and that a similar disposition of the molecule is induced in a bar of steel when the magnetic power is communicated to it. The austral polarity of each molecule, w hile in the mass, is destroyed by the boreal polarity of that which is contiguous to it in the common directions of their axes; but on separating, as above, the parts of the loadstone or magnetised bar, that extremity of each which is farthest from the northern pole of the bar exhibits the austral magnetism appertaining to the like extremities of the moleculos ; and that extremity which is nearest exhibits boreal magnetism. >: rln v batnv: • 1 A magnetised steel bar, when tried by means of a small compass-needle, is often found to exhibit, at different places in the direction of its length, a change from boreal to austral magnetism, and the contrary. These places are called consecutive poles.

Effects precisely similar to those which have been mentioned are also found to exist in the crystals of tourmaline and Siberian topaz when their electrical properties are developed by heat. The crystals of tourmaline are of a orismatical form with an uneven number of faces, and they aro terminated at each extremity by a small pyramid ; the polar axes being coincident with what may be called the geometrical axes of tho prisms. The crystal being heated to any degree between about 100" and 212° (Fahrenheit), it is found that its extremities possess opposite kinds (positive and negative) of electricity; about the middle there is a space In which the electricity is insensible; and when the crystal is broken across, each fragment exhibits the like phenomena. (Hauy, Mincralogie, torn, i.) Boracite, which crystallises in cubes, is found to have four polar axes, the extremities of which possess opposite kinds of electricity when heated. These are nearly in the directions of the four diagonals of the cube.

The intensity of the force, either of attraction or repulsion, exercised by one of the pules of a magnet on any body is inversely proportional to the square of tho distance of such body from that pole; and if a very small compassneedle, supported or suspended in the usual way.be brought near a magnetised bar, it must settle, between those opposing forces, in the direction of a tangent to some curve lino passing through the two poles of the magnet. This is called the magnetic curve, and tho direction of the tangent at any given point may bo thus investigated :—

Let N be the north and S the south pole of a magnet; let P be any given point at which the centre of gravity of a small suspended needle may be placed, and j0'n FN, PS.

Let the attraction of N on P be expressed by p^, and be represented by PB; also let the repulsion exercised by S on P be expressed by pgj and bo represented by PC, in the direction of SP produced. Imagine the parallelogram CB to be formed; then, by mechanics, PQ, its diagonal, -will represent the resultant of the forces acting on a particle at P; it will therefore be the direction of the needle and of a tangent to the curve at that point. Let ZQPC be represented by 6; ZQPB by and let fall CD pcrpen cos. 6 dicularly on PQ: then by trigonometry, "pgr «* tne cos.s 6' sin. 0 _ sin. 6' pjsj j whence "pgV = pjvja PN1.

Now, produce QP till it meets NS, produced, if necessary, in T; and draw SA parallel to NP: then, by trigonometry, SA : PS :: sin. APS : sin. PAS :: sin. QPS : sin. QPN; that is, SA": PS :: sin. 6 : sin. 6'; hence SA : PS :: PS« : PNS, which being compounded with the identical proportion PS : PN :: PS : PN gives SA : PN :: PSS : PN3. But SA being parallel to PN, SA : PN :: ST : NT; therefore ST : NT :: PS3 : PN3.

Thus the ratio of ST to NT is known; and, consequently the position of the tangent PT, from the given position of P. If the poles N and S are unlike, as above supposed, the curves are of the kind called convergent, as NPS; but if the poles are similar, the curves will be divergent, asNP'S'.

An analogy has long been known to subsist between electricity, galvanism, and magnetism, in respect of the power of imparting polarity to bodies and of modifying each other's effects. Coulomb found, in 1802, that needles of gold, silver, copper, lead, and even wood, when about one-third of an inch long and one-fiftieth of an inch thick, if suspended between the opposite poles of two strong magnets, presently arranged themselves in lines joining those poles. (Biot, Traitc de Physique, torn, iii.) Sir Humphry Davy, Mr. Faraday, and other philosophers, both in this country and abroad, succeeded in exciting magnetism in steel bars by the galvanic battery; and gold needles galvanised have been found to acquire polarity, and to be subject, with respect to the horizon and the meridian, to a certain variation and dip, different however from those of a steel bar when magnetised.

A small piece of steel-wire may be magnetised, or made to acquire polarity in a short time, by merely placing it perpendicularly across the conducting wire of a galvanic battery in action; and it will be found that one end of the steel attracts either the north or south pole of a compass-needle according as it is placed above or below the conducthigwire. But the method employed by M. Ampere to magnetise needles was to twist a wire spirally about a cylinder, and to place the needle in a glass tube within the spiral; then, connecting the two extremities of the spiral with the poles of a galvanic battery, the needle was found, after a few minutes, to be strongly magnetised. When the spiral was formed from the right hand downwards, to the left, above the axis, that extremity of the needle which was nearest to the negative end of the battery pointed towards the north, and the extremity nearest to the positive end towards the south; and when the spiral was formed in the contrary direction, the extremity of the needle which was nearest to the positive end of the battery pointed towards the north. By this method a great intensity of magnetic power is produced; the action of the wire upon the needle being repeated as many times as there are revolutions of the spiral about the latter.

In 1820, M. Oersted observed that if a magnetic needle, suspended as usual, be placed under and near the wire connecting the opposite poles of a galvanic battery, the pole of the needle which, is nearest to the negative end of the battery declines westward; and if the needle be placed above the wire, that pole declines eastward. The amount of the deviation depends on the distance of the wire from the needle; and when the distance is about three-quarters of an inch, it amounts to about 45°. When the wire is in the same horizontal plane as the needle, the latter suffers no deviation; but, if on the western side of the needle, the pole of the latter, which is nearest to the negative end of the battery, becomes depressed; and if on the eastern side, that pole becomes elevated. The hypothesis proposed by Oersted and Ampere to account for the action of the wire on the needle is, that a current, passes along the conducting wire of a galvanic battery from each pole towards the middle; and that these currents, on meeting, turn each other from their rectilinear directions, so that both are compelled to move spirally in opposite directions round the wire. In thus revolving, they act upon the magnetic particles in the needle, or upon the electric currents supposed to circulate about them, producing changes in the positions of those particles, and thus turning the needle from its place. It was a discovery of Biot, that if a perpendicular line were let fall from any point in a magnetised needle to the conducting wire of a battery, the electric force acting on that point is perpendicular to the line and to the axis of the wire; and if the magnetic particles in the needle were free to move under the impulses communicated to them by the electrical currents about the wire, it is supposed that those particles would arrange themselves so as to place their axes in planes coincident with those of the electric currents, and at right angles to the lines let fall from them perpendicularly on that axis. Such were fouYid by Mr. Barlow to be the dispositions assumed by very small magnetised needles when placed in any positions near the conducting wire; the effects of terrestrial magnetism on them being neutralised by means of magnets properly placed for the purpose.

Note - this article incorporates content from The Penny Cyclopaedia of the Society for the Diffusion of Useful Knowledge (1840)

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