Q.What is corona discharge?

Because a conductor's surface charge density -- and hence the electric field just outside its surface, E = sigma/epsilon0 -- is greatest at its most sharply curved regions (points, edges, corners), the field can become locally strong enough at such a point to exceed the dielectric strength of the surrounding air, ionising the air right around the point and allowing a continuous, steady leakage of charge from the point into the air. This phenomenon is called action at points, or corona discharge (sometimes visible as a faint glow or audible as a faint hiss). It is usually an unwanted charge loss in high-voltage equipment, but it is also deliberately exploited in the lightning arrester (lightning conductor): a thin, sharply pointed metal rod mounted at the top of a tall building, connected by a thick low-resistance cable to a metal plate bu …