Testing Transformer: What is the Top Metal Thing?

[eric336]

Hi everyone.

What is the metal thing on the top of the testing transformer (figure below)? It looks like electrode, but what is the use? I find it ubiquitous in many HV equipments.

 

[Carl Pugh]

Corona Ring.
Usually a hollow aluminum spinning.

 

[Windadct]

At high voltages ( Starting around 1000V and becoming very pronounced > 100KV) the voltage gradient around sharp corners will cause the air to ionize, causing Corona. The rounded shapes help to reduce the effect. And to clarify CP's post, the aluminum is not spinning, that is the manufacturing process often used to make the ring. Google (as usual ) has some good info. - Also note how a lightning rod is physically the opposite.