They Didn't Ask Me (dr_phil_physics) wrote,
They Didn't Ask Me

3 Last Thoughts on the 4th of July

Someone, I think it was on NPR, spoke of some wags in England trying to make the Third of July a holiday... The last day Americans were Englishmen...

Odd thought while watching 1776... Congress was in session on a holiday? (grin)

Which brings me to a story that I heard, but didn't see, since I wasn't in the experimental physics groups during grad school at Michigan Tech. This had to be 1986.

A 3-inch bore, 4 T superconducting magnet was being installed by Oxford Magnetics for MRI work. It was late at night. The double Dewar design has an outer jacket of liquid nitrogen, the magnet immersed in liquid helium. Alas, the gas company from Chicago didn't deliver enough liquid helium. So after cooling the unit down, they started bringing up the supercurrent as far as they could safely, according to the charts.

Click. Whoosh. FOG.

The superconducting magnet has quenched. The current exceeded the critical current and the wire suddenly gained resistance, generated heat and vaporized the liquid helium. Everyone out of the room -- there is no breathable air in here. When liquid gasses because gasses, there is like a 600:1 volume change. 40 L of liquid helium at 4K, then warmed by the Ideal gas law up towards room temperature. Displaced a cube of air over 10x10x10. It was midnight.

Not to worry too much. Helium doesn't want to do chemical reactions and each atom is so small, within minutes all would be gone from the lab -- straight through the walls and ceiling, too. (Hell, for that matter, in six weeks or so of rising free in the atmosphere, all that helium will have reached escape velocity and left the Earth.)

Turns out the tables they had were for the wrong model. The man from Oxford Magnetics said that in a couple of hours he'd call the office and get the right pages faxed. They still had some liquid helium left.

But will anyone be at the office? It's Friday, why not? Because it's a holiday.

Not in England.

Dr. Phil

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