Page 30 - Curriculum Visions Dynamic Book

Using liquid mercury
Mercury is the only metal that is liquid at room temperatures.
It has a high coefficient of expansion, it is very dense, and it is a good conductor of electricity. Each of these properties has been exploited
in applications from thermometers and barometers to electrical switches.
The mercury barometer
The fact that air has weight was discovered by Evangelista Torricelli in 1643. Using a tall tube that had been filled with mercury then inverted into a bath of mercury, he found that the mercury did not run out, but that a vacuum was created in the top of the tube and that the mercury was supported by air pressure.
He noticed that the height
of mercury in the tube changed with weather conditions,
thus behaving as a measure
of air pressure. This was the foundation of the barometer.
All liquids can be supported in tubes; but since other liquids are less dense than mercury, the tubes have to be much taller before a vacuum space
is created at the top. A tube using water, for example, would have to be 10 m high, whereas a tube using mercury only has to be 760 mm high. This makes a mercury-filled barometer a practical instrument to use.
A mercury barometer is also known as a Fortin barometer.
Vacuum
The upper curved surface of the mercury is called a meniscus. It is the opposite of a water meniscus, which curves down rather than up.
Mercury switches
Mercury is a good conductor of electricity and can be used as a switch.
A mercury switch is a closed tube containing two electrodes and a small amount of mercury. When the mercury runs between the contacts, an electrical connection is made; if it flows away from the contacts, the connection is broken.
This property can be used
in two ways: for detecting movement and for detecting changes in temperature. In a tilting switch, as the tube tilts, the mercury flows away from one of the contacts and the electrical connection is broken. If the switch moves upright, the mercury flows back and contact is made once more.
In a thermostat, as the temperature rises, the mercury expands until it touches
the contacts and makes a connection. When it cools, it contracts from the contacts and the connection
is broken.
Glass tube
Height of column
Air Air pressure pressure
 A diagram of a mercury barometer.
Mercury Container
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