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Thermal Physics

Measuring Atmospheric Pressure

Something similar to the experiment described above can be done using mercury.

This time the inverted test tube is placed in a reservoir of mercury. The pressure of the atmosphere (represented by the red arrows) keeps the mercury in the tube. However, this is only possible if the pressure due to the atmosphere is strong enough to support the weight of the mercury. If we repeat the experiment with a much longer tube, the situation is as shown in the next diagram.

In this case, the mercury falls until the pressure due to the weight of the mercury column (acting at the level of the surface of the mercury in the reservoir) is just equal to the pressure exerted by the atmosphere. In other words, the height, H, gives us a measure of the strength of the atmospheric pressure. If the pressure of the air increases, a little more mercury will be pushed into the tube and H will increase. The instrument shown in the diagram is called a mercury barometer.

Normal (or standard) atmospheric pressure is strong enough to support a column of mercury 760mm high. This explains why pressures are often stated in mmHg. We say that normal atmospheric pressure is equivalent to 760mmHg. Alternatively, a pressure of 760mmHg is called 1atmosphere.

If a gas is said to be at standard temperature and pressure, s.t.p. (or normal temperature and pressure, n.t.p.), this means it is at 760mmHg and 0C.

(It is left to the reader to use the equation in the section "Calculation of the Pressure Exerted by a Fluid" to find a value for normal atmospheric pressure in Nm-2.)

A barometer could be made using, for example, water or oil. What are the advantages of using mercury?

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