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The mercury barometer

We can measure the pressure of the atmosphere by 'balancing' the air pressure by liquid pressure. An instrument for measuring the pressure of the atmosphere is called a BAROMETER, and a barometer where the pressure of a column of mercury is used to balance the pressure of the atmosphere is shown in the diagram. Mercury is used in the barometer because it is opaque and has a high density.

(Mercury is a dangerous liquid and these experiments are described for information only).

One method of making a mercury barometer is shown in the diagram. All the air is sucked from the top of a long glass tube that is dipping into a dish of mercury. Since the dish is open the mercury will be forced up the tube by the air pushing down on the surface of the mercury in the dish. The pressure at A in the diagram is zero and, since the pressure in a liquid is the same at all points at the same level, the pressure at B equals that at C. This means that the height of the column of mercury produces a pressure that is just equal to that of the atmosphere. In fact atmospheric pressure is sometimes given as so many cm of mercury.

As the atmospheric pressure rises or falls so the mercury level in the tube rises or falls.

Normal atmospheric pressure is 76 cm of mercury or about 10 m of water. The water column is higher because of the much lower density of water. You couldn't use a straw to suck a drink up more than 10 m. You should think about why this is and try and explain it.

If the atmospheric pressure drops it will not be able to hold up so much mercury and so the level in the barometer tube will fall. Up a mountain the air pressure is low and so the barometer reading will go down.

Any air above the mercury in the tube will actually push the mercury down so giving a low reading for the atmospheric pressure.

It is the vertical height of the mercury column that should be measured and the next diagram shows the effect of tilting the barometer tube.

© Keith Gibbs 2011