The
Kew Pattern Barometer is the standard type of barometer used
to measure atmospheric pressure.
The
barometer consists of a column of mercury enclosed in a vertical
glass tube. The lower end of the column ends in a cistern containing
a reservoir of mercury. The space above the mercury in the tube
is evacuated and sealed, while the surface of the mercury in
the cistern is exposed to atmospheric pressure. An increase
in atmospheric pressure causes the mercury level in the cistern
to fall and that in the tube to rise, while falling
pressure
has the opposite effect. The mercury level in the tube can then
be read off against a fixed scale on the transparent body of
the instrument.
Aspirated
Hygrometer
The
aspirated hygrometer is used to measure wet and ambient (dry)
temperatures, which in turn are used to determine humidity.
As
air flows though the instrument, a fan draws air past two thermometer
bulbs. One bulb is kept moist by a close fitting sleeve of water-absorbent
material while the other is left uncovered and dry. As the water
evaporates, the wet bulb temperature is lowered to an equilibrium
point.
The
wet and ambient temperature readings are then taken and used
to calculate (using a slide rule) the humidity.
