Atmospheric pressure

1. Measurement of atmospheric pressure
Measured by,
Mercurial barometer
1. fortin’s barometer
2. kew pattern barometer
Aneroid barometer
Barograph
altimeter

2. Mercurial barometers
 There are two types of mercurial barometers.
A) Fortin’s barometer
B) Kew pattern barometer
Fortin’s barometer
 Principle: Balancing of column of air against a column of
mercury in a sealed glass tube. The height of the mercury
column is proportional to the pressure.

3. Construction and
maeasurement
1) It consists of a glass tube of
uniform cross section and
length, which is closed at one
end.
2) It is about one metre in length,
filled with mercury and then
inverted with its lower end open
into a movable cistern of
mercury.
3) The cistern vessel contains
mercury with a flexible leather
bag and screw at its bottom.
4) There are two scales on two
sides of the tube, one in
centimetres and the other in
inches

4.  5) For accurate readings Vernier calipers is also attached
 6) The mercury column in the tube is supported by the
pressure of the air on the surface of the mercury in the
cistern.
 7) To take the pressure reading, the height of mercury
column is measured on main scale and then Vernier scale is
read.

5. Kew pattern barometer
This is also similar to Fortin’s
barometer were the cistern vessel is
fixed and has no adjusting screw.
The divisions are made unequal
in order to allow rise or fall of
mercury column in the cistern.
In this barometer initial
adjustment of cistern is not required.

6. Aneroid barometer
Principle:
Aneroid means without liquid. Use of
symphon cell which is a partially evacuated metal
diaphragm, expands or collapses depending upon
the outside pressure.

7. Construction and
measurement  1. This is constructed with one or
more (up to 14) bellows that have
been partially evacuated.
 2. Each bellow may contain an
internal spring or may be
constructed from tempered steel
which acts as a spring.
 3. The spring forces the bellows
apart against force exerted by the
atmospheric pressure

8.  4. If the atmospheric pressure decreases the springs expand
and vice-versa if the pressure increases.
 5. The variation of the bellows with pressure changes is
mechanically linked to an indicator on a calibrated dial.
advantage:
 Aneroid barometers are suitable for outdoor measurements
and are also used in ships

9. barograph
 Principle:
 The sensitive element in this device is an aneroid
capsule which consists of a closed circular vacuum box or
boxes placed one above the other. The box is made of an
alloy of silver plated beryllium copper. As the atmospheric
pressure rises or falls the walls of the box collapse or distend
proportional to the impressed pressure changes. The motion
is communicated to a lever system connected to a rotating
drum on which recording is made

10. Purpose:
This is an instrument used to record the atmospheric
pressure continuously

11. altimeter
 Altimeter works on the principle of Aneroid barometer.
Here the principle that the pressure decreases with altitude is
used. In Altimeter the dial is calibrated in terms of Altitude. It
is used to know the elevation of a particular place