The purpose of the paper is to show the conditions that determine the disruptive strength of an air path along the surface of a solid dielectric of higher specific inductive capacity, and what steps must be taken to insure the most efficient use of the two dielectrics in combination. Part II of the paper discusses the effect on the electric field between two given terminals of the presence of a solid dielectric. An experimental means of determining the shape of the field is given. An explanation of the conditions that exist at the surface of the dielectric when in an electric field is given. The conclusion is drawn that if the surface of the dielectric be made tangential to the lines of force at every point, the strength of the path between the two given terminals will not be weakened. Part III shows the results of applying this theory to practical purposes, such as insulating a terminal rod passing through a casing, and directing the external field on the standard form of condenser type terminals. In this portion of the paper, several curves are given, showing the distribution of potential over the surface of the dielectric. Part IV gives the summary and conclusions, which are to the effect that breakdowns of an air path over a surface have been obtained, which average as high as 9.4 kv. per cm. effective value (23,900 volts per inch) over a distance of 17.0 cm. (6.7 inches). Conditions of design are such that these same averages may be maintained for any voltage, by increasing all dimensions of the structure proportionately. The strength of the air path in this form of design is independent of the specific capacity of the dielectric, and an important thing, therefore, is to shape the terminals properly in order to obtain a high average intensity over the given path.