Potential Stresses in Dielectrics

The property of a dielectric in which one is particularly in terested in the design of high voltage insulation is its ability to resist the stresses exerted upon it by the voltage, tending to disrupt it, and to force a large current through the puncture.The fundamental assumption which is made with regard to this property is that the force tending to disrupt the material at any point is measured by the electric intensity at that point, that is, by the force which would act upon a unit charge placed at that point.Thus Maxwell makes this statement: 1 " If the electromotive intensity at any point in a dielectric is gradually increased, a limit is at length reached at which there is a sudden electrical discharge through the dielectric.The electromotive intensity when this takes place is a measure of what we may call the electric strength of the dielectric."This critical value of the electric intensity in a dielectric is itself called, in common parlance, the electric strength of the dielectric.Granting the existence of this definite physical constant, the electric strength of a dielectric, that is, a definite value of electric intensity which cannot be continuously or repeatedly exceeded without disrupting the dielectric, the voltage required to break down a given design of insulation depends evidently upon two factors :1.The electric strength of the dielectric.2. The distribution of the electric intensity, or, as one may now call it, the electric stress, in the dielectric.1.