(Sent to me by Chris Suslowicz, who got it from Alister, the lowly worker
In the 60s, I was a lowly orker at a transmitter site in the frozen wastes of Scotland. To me fell all the most significant jobs; check the mast lights were on, check the dummy loads for leaks (concentrated caustic soda spoiled the shine on the linoleum), polish the VHF transmitter panels and sit through long hours of nothing very much, in case the transmitter should start playing up.
And every so often Test the Oil.
Bear in mind that this station was fed with 11.6Kv 3 phase via huge isolating/balancing transformers, OCBs (oil circuit breakers for those of low power leanings), and PFC capacitors about 4 foot high. And... there was a 25KW VHF AM transmitter, anode modulated at about 25Kv of plate voltage (1 amp plus loss currents) which means a BIG mod transformer with largish inductive spikery when someone shouted.
All of these nice things were filled with a substance called Shell Diala B - an oil with NO FREE CARBON and NO WATER. Except that time and the Scottish climate tended to upset the second of those - and if some little tiny arcing happened, it produced the former. Hence the Testing.
Imagine a transformer about two foot cubed on a trolley with a mains lead attached to a Variac, attached to the primary. The Variac was marked in tens of KV. On top of the transformer were two big ceramic pillars about 6 inches apart with brass hooks on them. Into the hooks could be placed a glass cell, open at the top, containing two brass balls about 2 inches apart, into which was poured the oil sample. Well now, that oil has a normal working proof of 50Kv and the idea was to see when it broke down - you getting the idea?
Breakdown was, er, spectacular. But, much more spectacular was what happened if you placed a pair of specially fabricated horns on the thing. They call it a Jacob's Ladder, I believe, and very pretty it was too. I wonder what they do now, in these days of health and safety - certainly the same oil is in use. OCBs are no more, having been replaced by tiny little things full of some clever gas, but there's plenty of both transformers and large voltages about still.