Mercury Arc Rectifier

The common type 83 was a non-arc mercury rectifier used in old TVs and power supplies.

When warmed up under operating conditions the gaseous metal concentration from such a volatile element conducts quite a bit more current between the electrodes than can be beamed across similar sized structures through plain vacuum.

Arcs are to be avoided on type 83 since it can fry them and risk other components but here is a video, note it is not really arcing just the blue-violet glowing:

https://www.youtube.com/watch?v=tBKbp7oxG-Q

This was a consumer tube so a hobbyist could use one to build a battery charger for something like an escooter. If you did it right you would be able to tell the amount of current draw from the glow.

Too bad it would take more energy to run the tube than it would to charge the battery.

See one running here https://youtu.be/latzy2NDcYg

These eerie ghosts used to roam the London underground up until not so many years ago, well into this century.