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          Mercury Switches

     Mercury switches are a switch that uses the fact that mercury metal
conducts electricity, as do all metals, but mercury metal is a liquid at
room temperatures. A typical mercury switch is a sealed glass tube with
two electrodes and a bead of mercury metal. It is sealed because of mercury's
nasty habit of giving off brain-damaging vapors. The diagram below may help
to explain a mercury switch.

                         ______________
                     A  /              \   B
      _____wire +______/___________     \
                       \   ( Hg )  |    /
                        \ _(_Hg_)__|___/
                                   |
                                   |
                            wire - |
                                   |
                                   |

     When the drop of mercury ("Hg" is mercury's atomic symbol) touches both
contacts, current flows through the switch.  If this particular switch was in
its present position, A---B, current would be flowing, since the mercury can
touch both contacts in the horizontal position.
     If, however, it was in the | position, the drop of mercury would only
touch the + contact on the A side. Current, then couldn't flow, since mercury
does not reach both contacts when the switch is in the vertical position.
     This type of switch is ideal to place by a door. If it were placed in
the path of a swinging door in the verticle position, the motion of the door
would knock the switch down, if it was held to the ground by a piece if tape.
This would tilt the switch into the verticle position, causing the mercury to
touch both contacts, allowing current to flow through the mercury, and to the
igniter or squib in an explosive device. Imagine opening a door and having it
slammed in your face by an explosion.

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