AN article by E. Rosenthal in the Electrical Review of November 27 discusses the influence of insulator glazes on porcelain, having particular reference to high-voltage insulators. Under a strong microscope the surfaces of unglazed ceramic materials show minute projecting peaks which make it evident that the quality of the surface has a great bearing on the surface resistivity. This, in turn, is largely dependent on the humidity skin which, in a moist atmosphere, forms on any surface to a thickness that increases with the surface roughness and with atmospheric humidity. With 30 per cent humidity, the surface resistance of unglazed porcelain ranges between 20 and 40 X 1012 ohms / sq. cm., but with 98 percent humidity it is only about 0.001 x 1012. This decrease in surface resistivity is smaller the smoother the surface. In a dry atmosphere glazed porcelain has approximately the same surface resistance as unglazed porcelain, but in a humid atmosphere of, for example, 98 per cent humidity the figure becomes 0-1 x 1012 ohms/sq. cm. This dependence of surface resistivity on the quality of the surface and on the humidity content of the atmosphere is a characteristic not only of porcelain but also of all insulating materials.