The phenomena accompanying the disruptive discharge of electricity are, in the present state of electric science, among the most interesting known, because they are the least understood, and, so far as we know, the least concordant with our preconceived ideas. The simplest way of representing the facts is to imagine with Faraday that the non-conducting medium, or dielectric, between two charged conductors is the seat of mechanical stress, consisting of tension along, and pressure perpendicular to, the lines of force. The rupture of the dielectric may then be conceived as a phenomenon precisely analogous to the rupture of an elastic body under stress. We are thus led to the conclusion that the commencement of the rupture happens at that particular point where the tension first reaches a certain value, called the breaking tension or dielectric strength, which depends merely on the material of the dielectric, and on its physical condition at the time being. The main thing in any experiment on dielectric strength is to know the tension at the point where the rupture begins.