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George Huxtable wrote,

>Later, at university, we found a similar environment. We would use mercury
>in pint quantities for diffusion-pumps in high-vacuum systems.
>
>I suspect many physics students from my generation, the world over, could
>tell a similar story.

Quite so. When they pulled down the old Royal College of Science in the
early '60's (to build what is now Imperial College, London), the mercury
vapour in the Spectroscopy lab had reached the level where absorption lines
of mercury were always present in any spectra taken in the lab. A lake of
mercury was found under the floor boards when they pulled them up! My ten
years in the Spectroscopy group was spent when it had moved to the new
physics department of Imperial College, but the researchers who worked in
old Spectroscopy lab were still alive and working when I was there - and it
goes without saying that they were hale and hearty and lived to a ripe old
age...

>I'm not convinced about the virtues of floating a solid mirror on a
>disc-raft on liquid. The liquid and the solid would need to have a
>repulsive surface tension between them to ensure blobs wouldn't gather up
>the sides of the raft. That surface tension would require to be exactly
>even around the edges of the disc or the raft would be unbalanced. How
>would one prevent the raft from nearing the edges of the container, which
>would unbalance the surface-tension forces or give rise to friction which
>would constrain the self-levelling? There are serious problems here which
>would need resolving.

I am not so sure that this is as much of a problem as you paint it George.
It is quite easy to work a glass disc so that it is flat and the two sides
parallel to a micron or so, and with a sharp uniform edge. In my
experience, the main problem was making sure the surface of the mercury was
absolutely clean, or the glass would sit on top of a spec of dust and the
glass would not be level.

The surface tension forces between mercury and glass are repulsive, so the
problems of blobs of mercury adhering to or sitting on the glass raft
disappear.

Geoffrey Kolbe.

   

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