Corona Ring on Surge Arresters and other EHV equipment

Have you ever seen a ring around the Surge Arrester or other EHV equipment? What is the purpose of the Ring around the Extra High Voltage (EHV) equipment? What is the use of big rings at EHV testing centers?

Corona Ring:

This Metal Ring what we see in the substations and other transmission line network is "Corona Ring". At higher voltages such as in EHV transmission networks corona discharge effect is prominent. Corona Ring distributes the electrical potential gradient evenly and maintains the maximum value below the threshold limits of the corona effect. Corona Rings are employed at high altitude electrical equipment where density of the air is less.