Principle of the mercury barometer
(see page 27 and page 28 in the
ClassNotes)
Air pressure at any level in the atmosphere depends on the
weight of the air overhead. A mercury barometer is
basically just a scale that measures the weight of the
atmosphere above.
|
|
Easily the most
impressive seesaw (teeter totter) that I've ever
seen (source
of this image). If you understand
how this works you'll be able to figure out how
mercury barometers function.
|
A barometer is essentially a balance.
The weight of the atmosphere is balanced by the weight
of a much shorter mercury column.
|
You can't use an
ordinary pan balance to weight the atmosphere (because
air is pushing down on both sides). A
U-shaped tube filled with some kind of liquid that can slosh
back and forth would work. Such an instrument is
called a manometer and is often filled with mercury.
|
|
To turn the manometer into a
true barometer, we'll extend the tube on the right and close the
top so that air isn't pushing down on the mercury. We'll
also use somewhat larger cylindrical columns of air and mercury
that completely fill the insides of the tube.
The weight of a very tall cylindrical column of air is balanced
by a much shorter cylindrical column of mercury. The
height of the mercury column will change as air pressure varies.
Mercury is a liquid. You need a liquid that can slosh back
and forth in response to changes in air pressure. Mercury
is also very dense which means the barometer won't need to be as
tall as if you used something like water. A water
barometer would need to be over 30 feet tall.
With mercury you will need only a 30 inch tall column to
balance the weight of the atmosphere at sea level under normal
conditions (remember the 30 inches of mercury pressure units
mentioned earlier). Mercury also has a low rate of
evaporation so you don't have much mercury gas at the top of the
right tube (there's some gas, it doesn't produce much pressure,
but it would be hazardous you if you were to start to breath
it).
Here is a more conventional barometer design.
The bowl of mercury is usually covered in such a way that it
can sense changes in pressure but sealed to keep poisonous mercury
vapor from filling a room.