This thing is a mercury arc rectifier, a giant bottle of hot, ionized mercury that glows and sparks and buzzes and looks absolutely amazing! It was invented in the early 1900s by a fellow named Peter Cooper Hewitt to
tear open a hole in the fabric of our reality convert high-voltage AC into DC. After most countries standardized on an AC electrical grid, mercury arc rectifiers became ubiquitous in industrial applications where lots of DC power was needed, like trolleys, subways, broadcast stations, foundries, and even battery chargers for early electric vehicles. Despite being essentially superfund sites in-a-bottle if they were ever broken, they remained in widespread use from their invention until the early 70s when less incredibly toxic, semiconductor-based high power rectifiers were invented. A few of them are still installed to this day though, and thanks to the magic of the internet, we can watch them in action without getting electrocuted or huffing mercury fumes:
Here’s another video that goes into a bit more depth on how they work:
Happy rectifying, ‘cados!