The semi-conducting ceramic glazes are described, and details are given of their electrical properties. The surface resistivity (i.e. the resistance between opposite edges of a unit square) of typical glazes varies from 0 2 to 50 MΩ at 20° C. For the range 0-150° C., the surface resistivity (ρ) at a temperature of T° K is given by ρ=ρ0eb/T, where ρ0 has values of from 25 to 5000 Ω, and b is approximately 3000, corresponding to an activation energy of about 0 25 eV. The contact resistance between an electrode and the semi-conducting glaze is investigated, and it is shown that this resistance decreases as the applied voltage increases. If a voltage greater than a certain limiting value is applied to an insulator glazed with semi-conducting glaze, the current will increase cumulatively until the glaze is destroyed by the excessive temperature rise. Even with small currents, disintegration of the glaze often occurs in time, especially in the vicinity of the electrodes, due to local heating effects resulting from contact resistance and non-uniformities in the structure of the glaze. Insulators glazed with semi-conducting glaze operate in humid atmospheres without the spark discharges which are characteristic of an ordinary glazed porcelain surface.