Question

True atmospheric pressure is 74 cm of mercury at a given place. When a simple barometer is set up at that place, it records 76 cm. It is because:

• A. Tube may be tilting
• B. Area of the cross-section of the tube is too small
• C. Mercury contains some dissolved metals
• D. None of the above

Answer 1

The correct option is C

Mercury contains some dissolved metals

Explanation for the correct option

1. Atmospheric pressure head measured with the help of a barometer is inversely proportional to the density of liquid used in the barometer.
2. Due to the presence of dissolved metals, the density of mercury in the barometer decreases which causes an increase in the atmospheric pressure head.

Explanation for the incorrect options

1. Atmospheric pressure head measured with the help of a barometer is independent of the angle of inclination of the barometer.
2. Atmospheric pressure head measured with the help of a barometer is independent of the area of cross-section of the tube used in the barometer.

Hence, option (c) will be correct.