On Sun, 15 Feb 2009 23:56:52 +0000, Frank Erskine
wrote:

On Sun, 15 Feb 2009 20:22:09 +0000, Andy Champ
wrote:

Andrew Gabriel wrote:

Many years ago, my father had a Citroen GS which had a mercury
switch for the boot light. Being sealed, they are normally very
reliable. Sounds like Matty's one wasn't sealed.

With regards to more recent cars, I've had the both the light
switch fail high resistance due to dirt ingress, and the earthing
of the boot hatch go high resistance through its hinges (which
stopped the rear window heater and rear wiper from working).
In the latter case, I connected an explicit earth tail between
the hatch and the car bodywork, and that was still working fine
when I sold the car ~5 years later.


I'm not sure what Matty saw. Surely mercury would evaporate over enough
time? And I'm really not sure how you clean it. Kind of hard to polish
a liquid!

A mercury switch has a curved or bent glass tube with 'contacts'
typically at the middle and one end, and obviously containing a bit of
mercury. As it's tilted the mercury bridges the contacts to complete
the circuit.

The one have in the shed is a straight glass tube with contacts at one
end. The tube has to be tilted to at least 30 deg from horizontal to
let the contacts be bridged. In actual use they were set to flip up to
a vertical position.