Corona discharge is flow of current from the electrode to the outer air. Corona discharge happens (among other) in high-voltage power lines.
One way to reduce corona discharge in this case is using so-called corona rings – that's just a ring made of thick metal rod that is positioned orthogonal to the wire so that the wire passes through the ring. The ring is mounted to the power line post and earthed.
How does this help? I'd expect that instead of "wire-to-air" current flow a "wire-to-ring" current flow would start and it would be just the same. Why does using the corona ring suppress corona discharge?
Best Answer
Russell's answer is good. To clarify the way corona works:
When the electric field at a point in air is strong enough, some electrons are stripped from the atoms and the air becomes a conductive plasma instead of an insulator. When an object is at a high voltage, and has a small curvature (wire, sharp points), the field in the region around it can have a high enough field strength for this to happen, forming a conductive layer of air, which is then surrounded by normal insulating air. If the strength around this conductive layer is also high enough (if the conductive region is pointy), the conductive layer can increase and increase in size (usually in one direction) until it forms a complete conductive path to another object, and then a huge current flows in the form of an arc.
Even if the shape of the field is such that it can't form an arc, it can still form a corona discharge. This is when a conductive layer of plasma stops expanding and just sits there around the wire, and ions in the plasma layer are repelled into neutral air, where they drift slowly until they discharge on another object. This is still a flow of current, just much smaller than an arc.
The corona ring changes the shape of the electric field, spreading it out in a sense, so that it is no longer strong enough at any point to ionize the air.