Measurement of Atmospheric Pressure: Barometer
The atmospheric pressure was first measured by Torricelli in about 1644. To repeat Torricelli’s experiment, take a thick glass tube about one meter long and close it at one end. Fill it completely with mercury and invert the tube vertically over a basin containing mercury so that the open end closed with the thumb, is under the mercury. On removing the thumb, the mercury level drops down but remains steady at a certain height. The height of the mercury column is found to be nearly 76 cm.
The question that naturally arises is, why does the mercury level in the tube remain 76 cm higher than the level in the basin? The explanation provided by Torricelli is that the atmospheric pressure supports the pressure of the mercury column. In other words, the atmospheric pressure is equal to the pressure exerted by the column of mercury contained in the tube. The height of the mercury column supported by the atmosphere may, therefore, be taken as a measure of the atmospheric pressure. The arrangement shown above is thus capable of measuring the atmospheric pressure.
An instrument that is used for the measurement of atmospheric pressure is known as the barometer.
The barometer shown above is known as the simple barometer. For accurate work, Fortin’s barometer is used.
Effect of tilting the barometer tube- Figure (a) shows the effect of slanting the barometer tube. The vertical height of the column of mercury always remains the same and is a measure of the atmospheric pressure.
Effect of pushing the barometer tube- Figure (b) shows the effect of pushing the barometer tube in the mercury cup. The barometric height remains unchanged in doing so.
Meaning of 76 cm of mercury- The atmospheric pressure is usually expressed in units of cm of mercury. When we say that the atmospheric pressure is 76 cm of mercury, we simply mean that the atmospheric pressure is equal to the pressure exerted by 76 cm of mercury column. Since the density of mercury, d = 13.6 x 103 kg/m3, and acceleration due to gravity, g = 9.8 m/s2, therefore, putting h = 0.76 m in the expression
| ρ = h dg |
| We have, 1 atm = (0.76) (13.6 x 103) (9.8) ⟹ 1 atm = 1.013 x 105 Pa A pressure of 76 cm of mercury at 0°C at sea level is known as the standard atmospheric pressure. Thus 1 atm = 1.013 x 1015 Pa In meteorology, the pressure is usually expressed in the bar, where 1 bar = 105 Pa therefore, 1 atm = 1.013 bar Another unit used for expressing the atmospheric pressure is known as torr, where 1 torr = 1 mm of Hg = 133.3 Pa In terms of this unit 1 atm = 760 torr |
Water Barometer- Suppose we use water instead of mercury in a barometer, then what height of water would be able to balance the atmospheric pressure? Since mercury is 13.6 times heavier than water, the
Height of water barometer = 76 x 13.6 = 1033.6 cm
Pascal while repeating Torricelli’s experiment using water showed that the atmospheric pressure could be balanced by a water column of about 10 m in height.
Why mercury is used in barometers- There are several advantages of using mercury as a barometric substance:
- The height is manageable- it is about 76 cm with mercury and 1034 cm with water.
- The space above the liquid column should not contain any vapor. With mercury, practically vapors are produced.
- Mercury does not wet or stick with glass.
- It is shining and opaque, so reading can be easily taken.
Pressure on the Human Body- Taking the total area of an average human body to be about 1.5 m2, force on a human body due to atmospheric pressure is
| F = pA = (1.013 x 105) (1.5) ≃ 1.5 x 105 N |
Which is equivalent to 1.5 x 104 kgf or 15 tons (nearly). The question which now arises is, how are we able to support such an enormous force on our body?
This can be explained with reference to the tin-can experiment. When the tin-can was full of air, it was able to support the atmospheric pressure because the force exerted by the external air was balanced by the force exerted by the inside air. However, when the inside air was extracted, the can was crushed under the influence of external force. The same happens with our body. Our body had innumerable pores on the skin. There is free communication between the inside and outside air. So inside air also exerts equal pressure. The force due to the pressure of the inside air balances the force due to the pressure of the atmosphere, hence we do not feel discomfort.
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