A barometer measures air pressure.

In 1643 the Italian scientist Toricelli, took a glass tube about 80 cm long and closed at one end, and filled it with mercury. He then inverted this so that the open end was below the surface of mercury contained in a bowl. He found that the mercury in the tube dropped until it was about 76 cm above the mercury level in the bowl.

• What would happen if the tube was titled sideways at an angle?

• What would happen if the apparatus was taken to the top of a mountain?  What would this show?       
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Most modern barometers look something like the one ilustrated at left. It is called an aneroid barometer. The position of the indicating hand is determined by the effect of the air pressure on the elastic lid of a partially evacuated, sealed box.

The barometer


• If the tube is placed on an angle, the surface
of the mercury remains level, and the
vertical height remains the same.

• At the top of a mountain, the height of the mercury column is less. This is because the air pressure decreases as we climb. Thus the air pressure is inversely proportional to the height (above sea level, say).