A mercury arc valve is an old piece of equipment that was used to convert AC electricity to DC. They were used at the start of the 20th century in places like factories, electric railways and streetcars. That's all well and good, but here's the real reason you should care: they look like something out of a spaceship.
You could even make one yourself! I mean, you'll need to know what you're doing, but it's possible. And if there's anything cooler than this to have humming in your living room, I don't know what it is.