POLARITY. The n. and a. poles of the earth's axis are terms familiar to all, and ,to arc the derived terms of the n. and s. poles of a magnet (q.v.). A right-handed and a left-handed corkscrew, or helix, are also perfectly well known. The distinction between the members of. any of these pairs leads us to the consideration of polarity, which it is difficult to define except by illustrations. In the case of the helix, it is the difference between right-handed and left-handed; not as in a magnet, the difference between the two ends. If we look closely into the question, we find that it is impossible to define the term "right-handed rotation " in the abstract. We may define it as being the same as that of the hands of a wat•elm, or that of the apparent motion of the celestial bodies about us in this northern hemisphere; but to a person at the equator, or to one who had never reen a watch, such comparisons would be without meaning. In fact, it is impossiide to give a definition of even such a simple term as right, Cown, coat, etc., independent of reference to the motion or position of sonic external object. But thereis, in many cases, an-important scientific reality underlyinT, and perhaps causing these dif ficulties. To a spectator looking down upon the n. pole of the earth, the axial rotation would appear to be left-handed, or opposite to that of the hands of a watch; while at the s. Pole the appearance is time reverse. In fact, as motion in a horizontal fIrnight line appears to be from right to left, or from left to right, according to the side on which the spectator stands; so motion in a curve appears to be right-handed or left-handed, according to the side of its plane from which it is looked at.- And this is now known to be the cause of the difference of poles in a magnet; the hypothesis of two magnetic fluids is dismissed, and Amp&e's explanation, that in a magnet currents of electricity revolve round each particle in planes perpendicular to the direction of magnetization, at once accounts for dissimilarity of the poles. Such a figure as this gives a clear idea
of the subject. A little electric current, such as that in the figure, in which positive electricity passes in the direction indicated by the acts upon external bodies exactly as a small magnet would whose axis is. as in the cut, perpendicular to its Diane. the arrow-head representing the north pole; that is, the pole which turns toward the south. Again, an electric current passing in a straight wire would at first sight appear to be altogether independent of polarity; yet :t is found that such an current moving in the shwight line in time cut, in the direction Qf the arrow-head, tends to make the north pole of a magnet rotate round it in the direction indicated by the arrow-head in the circle.
Again, there are certain crystals, which, when heated, become electric.
One end of a prism of tourmaline, for instance, takes positive, the other negative electricity. Also certain crystals of quartz cause a ray of polarized light, which passes along their axis, to rotate right-handedly; others left-handedly. The difference In these cases is due to molecular arrangement, other effects of which are easily seen in -2.he tourmaline, in the dissymmetry,of the two terminals of the prism, and, in quartz, in the position of certain small faces of the crystal, so that a preliminary inspection enables us to predict the direction of the effect to be obtained front any particular specimen. The term has various other applications, amongst the least defensible of which is that to light. Sec POLARIZATION OF LIGHT.