On 2014-01-17, Ignoramus29535 wrote:
(not completely idle interest)

I was just wondering, how much mercury is in mercury relays such as 80
amp three pole relays. A website claims that even a 35 amp single pole
relay contains 251 grams of mercury, which is hard to believe.

http://goo.gl/ESvOvC

As a kid, I used to reclaim Mercury from such relays, and
remember that Mercury is quite dense compared to a similar volume of
steel. Have you ever lifted a bottle filled with it?

This site:

http://www.enotes.com/homework-help/what-density-mercury-kg-m-3-what-its-density-g-cm-124349

says: "The density of mercury = 13534 kg/m^3 or 13.534 gram/cubic
centimeter"

so that would be 18.546 cc, or a cube (if you froze it) 2.64 cm on a
side (a bit over an inch). That is fairly close to my experience. So
an 80 Amp three pole would probably be on the order of 1,694 gm (making
perhaps unjustified assumptions on the needed increase in size), or
about 3/4 of a pound.

The relay needs quite a bit of mercury to handle the current,
since *it* is the conductor, and if you have too little of it, it will
vaporize (and possibly shatter the container if it is an all-glass one
(some are, some are not.) Mercury wetted reed relays, in contrast, use
a tiny drop, which simply wets both contacts, and make a clean
connection quickly when it closes, and does not suffer from contact
bounce.

Sort of somewhere between those two are the old home
thermostats, which used a small ball of it rolling around in a glass
tube to short the contacts when it tilts one direction, and open them in
the other direction. (The rolling mass of mercury also adds a bit of
hysteresis to the operation of the switch -- something which I had not
realized until I started typing this. :-)

Enjoy,
DoN.

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