The Electrical Resistivity of Insulating Materials

A brief review is given of the literature on conduction through insulators. Every dielectric has a definite resistivity when the potential gradient is below a certain value, different for each substance. If the potential gradient is continually increased, a point is reached where an increase in voltage does not affect the current. This is called the saturation current. With still greater potential gradients, a point is reached where the current increases rapidly and breakdown soon results. All these phenomena of conduction are explainable as the movement of ions in an electric field. The resistivity of a dielectric depends on the number of ions in unit volume, on the charge on each ion, and on their mobility (velocity under unit potential gradient). The saturation current depends on the charge on each ion and on their rate of production. Breakdown is preceded by ionization by collision, which is determined by the ionization potential of the substance and the length of path of an ion. The number of ions normally present in a dielectric depends not only upon the rate of producing them but also upon their rate of recombination. The rate of recombination is a constant of the material, but the rate of production may depend either upon outside agencies or inside forces. In gases, ions are generally produced by outside agencies, the important ones being rays from radioactive materials, X-rays and ultra X-rays.