Action of points or Corona discharge

Because the surface charge density on an irregularly shaped charged conductor is greatest at its most sharply curved regions (as shown in section 1.9.1), and the field just outside any charged conductor's surface has magnitude E = sigma/epsilon0, the electric field is also strongest right at those sharp points and edges. If the conductor's charge (and hence sigma at the sharp point) becomes large enough, the local field there can exceed the dielectric strength of the surrounding air (about 3x10^6 V/m, from Table 1.1), ionising the air molecules immediately around the point. This creates a steady, continuous leakage of charge from the point into the surrounding air, a phenomenon called action at points, or corona discharge (named for the faint glow, and often a barely audible hissing sound, that can accompany it). Corona discharge is generally an unwanted loss of charge in high-voltage equipment, but it is also deliberately exploited: since it lets a sharply pointed conductor bleed charge away into the air far more readily than a smooth or rounded one, sharp metal points are used, for instance, at the tip of a lightn …