(left) Grading rings on 1.4 MV X-ray generator at the US Bureau of Standards in 1947. (right) Corona caps and rings on the Cockcroft–Walton particle accelerator from 1937, now in the Science Museum, London.
In electrical engineering, a corona ring, more correctly referred to as an anti-corona ring, is a toroid of conductive material, usually metal, which is attached to a terminal or other irregular hardware piece of high voltage equipment. The purpose of the corona ring is to distribute the electric field gradient and lower its maximum values below the corona threshold, preventing corona discharge. Corona rings are used on very high voltage power transmission insulators and switchgear, and on scientific research apparatus that generates high voltages. A very similar related device, the grading ring, is used around insulators.