Pressure

-is the pressure exerted by the weight of air in the atmosphere of Earth. It is measured in millibars. It is measured using an instrument known as a barometer. There are two different types of barometers: the mercury and aneroid barometers.

Mercury barometer

A diagram showing how a Mercury barometer works.

• It is very large, the glass tube is typically almost 1m in height.
• It gives very accurate readings.
• It has a bulb that is dipped in a container of mercury.
• When the atmospheric pressure rises, it exerts force its force on the mercury which becomes compressed.
• The mercury is pushed up the bulb to a height of 760 mm.
• When pressure decreases the mercury contracts and moves down the thermometer.

Advantages

• It gives accurate readings.
• It is simple to construct and maintain.

Disadvantages

• It is very large and cumbersome.
• Mercury must be at least a meter long.
• They break easily.
• They cannot produce a continuous reading, so readings have to be taken regularly.
• Mercury is a dangerous poison.

Aneroid Barometer

A diagram showing an Aneroid barometer.

• These are portable and can be safely used at homes and schools as they do not contain mercury.
• They are made up of an air-tight metal box.
• If pressure increases the box is squashed inwards.
• Conversely if pressure decreases the box expands outwards.
• A complex series of springs is attached to the box.
• The other end of the spring is connected to a point which has a calibrated dial on the other end to record the various pressure readings.
• The spring amplifies the expansions and contractions of the box in accordance to atmospheric pressure.

Advantages

• It is portable.
• It can be safely used in the home or at school since it does not rely on mercury which is poisonous.
• It is easier to read since it comes with a calibrated dial.
• Can make continuous readings.
• Can be attached to a computer to make automated continuous readings.

Disadvantages

• It is less accurate.
• Requires great skill and expertise to make.

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