Completely and totally off-topic and without any merit

From: William Donzelli <>
Date: Tue Sep 7 20:40:52 2004

> Telescope makers use it to make small "pin holes" in photo graphic
> plates and film. You smash some and it makes hundreds of tiny specs
> that are almost perfectly spherical. These cast shadows on the the
> photo material. The film that is exposed turns dark, leaving the
> tiny clear spots. One simple mask off the ones that are not used.

One interesting place mercury is used is in near-frictionless
bearings. Gyrocompasses from World War 2 used a big oscillating mercury
bearing at the base to hold the sensitive element, which can get sort of
heavy. The bearings essentially are a pool of mercury in a special bowl,
and the part to be supported (in the case of gyros, the sensitive
element) neatly just fits in, so it barely floats. In an Arma gyro, about
a pint of mercury is used, I think.

The term "oscillating" comes from a little mixer that sits in the bottom
of the bowl, always running to keep the mercury in constant motion. This
is to overcome any static vicosity in the mercury, which, like friction,
is always greater than the dynamic viscosity. The difference is tiny, but
the forces that work on a gyrocompass are tiny as well.

I have an old Mk 10 Arma gyro, and I suppose that someday I will need to
get some mercury, but to be realistic, I don't have nearly the skill to
balance the thing. An unbalanced gyrocompass quickly turns into a very
broken gyrocompass.

William Donzelli
Received on Tue Sep 07 2004 - 20:40:52 BST

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