Page 161 - Physics

Pressure

(b) What happens when a bubble of mercury is Before taking the readings, the
trapped in the simple barometer? mercury level is adjusted by the

(c) Why can’t water be used as a barometric screw until it touches the ivory
fl uid? point.

Fortin barometre However, the use of the mercury

A Fortin barometre is a modifi ed simple barometer. in a Fortin barometre has the
It consists of an inverted tube closed at its upper end following disadvantages:
with the lower open end immersed in a reservoir 1. Mercury is very expensive
of mercury, as shown in Figure 7.30. There is a and toxic to use.
vacuum at the upper end. As the atmosphere pushes 2. Mercury is in the form of
down on the mercury in the reservoir, the pressure liquid and the column is
is transmitted throughout the mercury, specifi cally approximately 1 m tall,
to the open end of the tube. This pressure forces so it is not portable.
some of the mercury up into the tube until the 3. The Fortin barometre must
height of the column of mercury exactly balances be mounted in a vertical
the atmospheric pressure. The atmospheric pressure position.
is measured in terms of the height of the column
of mercury. Aneroid barometre

Glass tube An aneroid barometer, shown
Slit in Figure 7.31, is a type of
Vernier scale barometre for measuring
Screw atmospheric pressure without the
use of a liquid, hence the word
Mercury ‘aneroid’ means without fl uid. It
consists of a partially evacuated
Thermometer
Brass tube metal chamber and a thin
corrugated lid which is displaced
by variations in external

Ivory pointer pressure. A lever connected to
the diaphragm of the chamber
moves a pointer. Any change in
Glass vessel
pressure would cause the box to
Leather cup be squashed or expanded. The
small deformation of the box
will trigger a larger motion in
Screw
the scale pointer using the lever.
Figure 7.30: Fortin barometer

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Physics Form 1 Final.indd 155 16/10/2024 20:57