Corona ring

It is sometimes visible as a dim blue glow in the air next to sharp points on high voltage equipment.

Coronas also produce noxious and corrosive ozone gas, which can cause aging and brittleness of nearby structures such as insulators.

The electric field at the surface of a conductor is greatest where the curvature is sharpest, so corona discharge occurs first at sharp points, corners and edges.

Although they may also serve to suppress corona, their main purpose is to reduce the potential gradient along the insulator, preventing premature electrical breakdown.

The potential gradient (electric field) across an insulator is not uniform but is highest at the end next to the high voltage electrode.

In very high voltage apparatus like Marx generators and particle accelerator tubes, insulating columns often have many metal grading rings spaced evenly along their length.

This divides the potential difference evenly along the length of the column so there are no high field spots, resulting in the least stress on the insulators.