Polarity, in physical science, a word of various application; but in all its uses there is present the idea of a directed quantity or Vector (q.v.). A sphere, situated in space, is a perfect type of all-sidedness, presenting the same aspect in every direction. Let this sphere, however, begin to rotate about some diameter, and at once it becomes a polar body; it becomes possessed of polarity (see POLES). Looked at from one end, it appears rotating clockwise; looked at from the other, it appears rotating counter-clockwise. A similar polarity is acquired by a body of any shape when it is set spinning about some axis. Hence we may take rotation as a very perfect illustration of kinetic polarity.
Perhaps the most familiar example in physics of a polar body is the magnet. Its polarity is a force-polarity, the ends or poles of one magnet having a selective action upon the ends or poles of another. This particular action is, however, only one of a host of manifestations of what is known as Magnetism (q.v.); and the general tendency in modern theory is to explain all magnetic phenomena as being essentially rotational. Thus, again, from a physical or dynamic point of view, we conceive of rotation as a true type of polarity. The phenomena of statical electricity have also been discussed as analogous to certain phenomena in vortex motion.
In electrolytic polarisation, however, it is difficult to see any rotational analogy. Here the electrodes which bring and carry away again the electric current flowing through the decomposing liquid acquire new properties and functions which have distinct directive relations to the current that produced them. See ELECTRICITY, INDUCTION, MAGNETISM.
In all the cases so far mentioned the polarity or polarisation involved is of such a nature as that originally typified by the sphere's rotation; there are two ends which in some respects have opposite characteristics. In polarisation of Light (q.v.), however, this condition is no longer always fulfilled. For instance, a plane polarised ray of light which is stopped by a Nicol prism passes more or less completely as soon as the prism is rotated round an axis codirectional with the ray. The ray has, in fact, peculiarities as regards its sides—its 'polarity' is strictly speaking lateral, not polar. On the other hand, in a circularly polarised ray we have, according to the ordinary theory, a true kinematic polarity of a rotational kind, so that, looking along the ray, we are able to distinguish right-handed and left-handed circular polarisations. It may be mentioned as a final illustration that the rotation of the plane of polarisation by means of quartz or a saccharine solution is not a real polar phenomenon, the rotation being for any one substance always in the same sense relatively to the travelling ray; but that the rotation of the plane of polarisation in a magnetic field is a true polar phenomenon, changing sign with the direction of the field.