Some Contemporary Advances in Physics -V Electricity in Solids

N considering such topics as the flow of electricity through solids and the outflow of electricity across their boundaries, we have to forego the assistance of the great system of laws, models, and wordpictures which constitutes the contemporary theory of the structure of the atom. This imposing and truly powerful theory, which nowadays seems to bulk larger than all of the rest of physics, is after all limited to certain restricted fields; it deals successfully with particular properties of isolated atoms, and also with certain qualities of atoms which seem to be localized in their inner regions; but it avails little or nothing in the study of the behavior of liquids and solids. Much of the present-day theory of electrical conduction in solids is based only on the very simplest assumptions as to the nature of the atoms of which they are built, some would even remain valid under the old-fashioned ideas of continuous electrical fluids; and profoundly as we may believe that solids are built of atoms resembling Bohr's famous model, it is highly doubtful whether t h a t model has ever helped to interpret a single one of the phenomena of conduction or clone more than to provide a new language for old ideas. We have first to make the distinction between the substances in which atoms migrate along the path of the flowing current and apparently carry the moving charge, and the substances in which the atoms stand still while the current flows past them. It is universally conceded that elements, and likewise the alloys of metals and a number of solid compounds, belong to the latter class; whatever it is that carries the current flows through and past the substance, leaving it at the end as it was at the beginning.