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**Idea: **It is possible to measure pressure by relating pressure
differences to the variation in the height of a column of liquid.
Two devices
for measuring pressure are:

**1. Open-tube manometer**

The pressure *P* of the gas in the closed cylinder on the left in fig.9.5 is
related to atmospheric pressure *P*_{0} (exerted on the top of the
column of liquid on the right) by:

P - P_{0} = gh
| (11) |

*P*_{ absolute} = *P*_{ gauge} + *P*_{0}.

**2. Mercury Barometer**

Atmospheric pressure is measured by ensuring that the pressure at the closed end of the barometer tube in Fig.9.6 is zero (i.e. it is vacuum). Thus

P_{0} = gh
| (12) |

It turns out that the pressure of 1 atmosphere (1 atm), is equivalent to a column of mercury exactly 0.76 m high
at a
temperature of 0^{ o } C with
*g* = 9.80665 *m*/*s* ^{2} and
= 13.595 `x` 10^{3} *kg*/*m* ^{3}. From this
information, we conclude that

1 atm = 1.013 x 10^{5} Pa = 101.3 kPa.
| (13) |

10/9/1997