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 con trary properties.
If iron-filings be strewed over a mass of natural lbadstone, it will be found that there are two points on its surface at which the filings are most abundantly attracted, and where they dispose themselves nearly in the direction of a line imagined to be drawn through tho mass. [31.sexErisu.) Then, if the loadstone be cut in the forts of a sphere, having this hoe for a diameter, the symmetrical arrangement of the iron-filings with respect to this line affords an indication that the particles of the loadstone may be symmetrically disposed about the line ; and, from an analogy with the axis and poles of the earth, this line is called the axis of the loadstono, 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 opposite extremities of any body or molecule, when it assumes or can be brought into a particular direction.
Wbat has been said respecting the properties of a prism formed of the natural loadstono, is true, of a magnetised bar of steel [111soNzr], and the poles or opposite extremities of either material are found to possesa a contrariety of character. One extremity always tends toward+, 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 exercise upon each other a mutual repulsion ; but if either pole of one be brought near the opposite pole of the other, they mutually attract each other.
Since in magnetised bars the poles of contrary names thus 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 suspendiel bar (as a compass needle) must lie (mere!, 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 middle the cylinder will be in a neutral state.
There: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 galvanic electricity. In an ordinary battery, the fluid, by chemical action on the zinc, produces a separa tion of the two kinds of electricity ; that which is called positive is carried to the copper plate, and the latter communicates it to the eine plate with which it is connected. This action is repeated at every pair of plates in the battery ; and from the last rine plate the electricity enters the conducting wire, or that which 6 employed to connect the opposite textremities of the battery : thus the seine extremity con stitutes the positive polo of the battery. At the same time an opposing ourrent of negative electricity puree from the copper, through the fluid, to the zinc, from thence to the next copper plate, and so on to the hat, 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. [GALVANISM.] 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 deno minations at its extremities; and this condition is accounted for by Coulomb in the following manner :---He supposes that every molecule of loadetono is a small magnet possessing opposite polarities at its extremities, the axes of all being parallel to or coincident with the magnetic axis of the mass; and that a similar disposition of the mole cule is induced in a bar of steel when the magnetic power is communi cated to it. The austral polarity of each molecule, while 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 molecules ; and that extremity which is nearest exhibits boreal magnetism.