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While looking at pictures from the underworld, I came across this one. Anyone want to guess what it is? This may help or it may not (depending on what you know about me) but my grandfather used to tell me stories about them when I was a little kid - at my request. He worked underground and came into contact with them from time to time. They're ludicrously dangerous, but up until now I'd never seen one. I hadn't realised how pretty they were.

Do you know the expression 'mad as a hatter'? Hatters used to dip hats in mercury and the vapour from the mercury would send some of them insane. You don't want mercury vapour around, particularly not underground in a confined space. Neither do you want hard ultraviolet, which is what that purple glow is.

In the days before transistors and the like, London Underground used the technology of the day to provide the electricity for their trains. Pour a few pints of mercury into a thin glass balloon, pump out some air, connect a gazillion volts of AC electricity from the grid and strike an arc. Out the other side comes DC electricity that tube trains can run on, and in the meantime you have a handy supply of both poison gas and dangerous radiation. And according to my granddad (who was an engineer on the Underground) sometimes they'd shatter, releasing clouds of mercury vapour and a torrent of liquid metal that presumably still had a few thousand volts connected to it. I ask you, who wouldn't want to be an engineer?

And while I'm reminiscing, I also remember my granddad explaining to me the pitfalls of working with voltages above ten thousand volts. It had, he said, a tendency to 'come out and meet you'. He always offered to read my brother and I bedtime stories from books and I'd always want to hear about all the machines in the substations, the noises they'd make (which he was excellent at mimicking) and what happened when one of them blew up. And the time when even at full output one of the trains wouldn't start, so they sent a man down there who reported that the tunnel was flooded and the water between the tracks was now boiling thanks to the power they were applying down there. Or what happened to mice who ran along the live busbars once their tails got long enough to touch the ground (complete with appropriate sound effects). Or the cat they kept at Mansion House station which was the size of a small dog and the only creature capable of fighting the giant rats down there. Rupert Bear stories never really stood a chance.

Re: H's suggestion below of what she thought this device might be. To steal a line from Leila, I'm not sure whether I believe in the existence of weather balloons.