Question

The diagram shows a simple mercury barometer. Which of the following does not cause the height of the mercury column to vary?

A. Changes in the atmospheric pressure
B. Changes in the value of g
C. Evaporation of mercury from the barometer reservoir
D. Leakage of air into the tube

Answer 1

Hint: \[P=h\rho g\] can tell about the physical quantities that disturb the height of the mercury in the barometer. The elevation of the mercury depends on the pressure, density and gravity. A slight change in these quantities will disturb the system.

Complete step by step answer:
Pressure inside the mercury column can be written as,
\[P=h\rho g\], \[h\] is the height of the mercury column, \[\rho \] is the density of mercury and g is the acceleration due to gravity.
From this equation, we can say that atmospheric pressure is directly proportional to the height of mercury. Height will vary according if there is any change in gravity.
Leakage of air inside the tube will cause the vacuum. Due to this, the atmospheric pressure and pressure inside the tube will be the same. Hence the elevation of mercury with respect to the pressure is possible.
Evaporation of mercury will not make any change in this mercury column, since the column is enclosed by a glass tube. So not only air but also mercury can’t escape from the system. Thus, it won’t make any change in the height of mercury. So, the correct option is C.

Additional information:
Barometer is an instrument used for the measurement of atmospheric pressure. It works by balancing the weight of mercury in the tube against the atmospheric pressure. Torricelli invented the first barometer in 1643. According to the principle, if the weight of the mercury is less than the pressure, then the mercury will rise. If the pressure is more, then the mercury level falls.

Note: All other options are wrong since they are directly or indirectly depending on the height of mercury. Candidates will get confusion regarding the evaporation of mercury. Even if evaporation happens, mercury vapor can’t go out since they are contained in a glass tube. So the evaporation can’t make any change in height.