Page 60 - Physics Form 5 KSSM_Neat

The Value of Atmospheric Pressure
The value of atmospheric pressure can be measured by using a mercury barometer. A mercury
barometer consists of a 1 metre long glass tube containing mercury. Initially, the glass tube
filled with mercury and is covered with a lid as shown in Figure 2.10(a). The glass tube is then
inverted and the covered end is immersed into a bowl of mercury as shown in Figure 2.10(b).

KEMENTERIAN PENDIDIKAN MALAYSIA
Lid

Glass tube

Mercury

Bowl

Mercury
(a) (b)
Figure 2.10 Preparation of a mercury barometer
When the lid is removed, the mercury column in the glass tube will fall until a certain height
and remain at that height as shown in Figure 2.11.

Vacuum

History
Evangelista Torricelli
h = 760 mm conducted an experiment
P P with a mercury barometer in
atm atm
the year 1643. He concluded
that the mercury column in
the glass tube is supported by
atmospheric pressure.

Figure 2.11 Mercury barometer

The atmospheric pressure acting on the surface of the mercury in the bowl supports
the mercury column. The height of the mercury column depends on the magnitude of the
atmospheric pressure. The value of the atmospheric pressure is stated in terms of the height of
the mercury column. If the height of the mercury column, h = 760 mm, then the atmospheric
pressure, P = 760 mm Hg.
atm

50 LS 2.2.2