Concept:Atmospheric pressure is measured by the vertical height of the mercury column above the free mercury surface in the trough of a simple barometer.
Explanation:In a simple barometer, the mercury column in the tube is supported by the atmospheric pressure acting on the mercury surface in the reservoir.
The pressure of the atmosphere is equal to the hydrostatic pressure exerted by the mercury column.
This pressure is determined by the vertical height of the mercury column measured from the free surface of mercury in the trough to the top of the mercury column in the tube.
In the given diagram,
Q is the mercury surface in the reservoir, and
R is the top of the mercury column.
Therefore, the distance
QR gives the height of the mercury column, which measures the atmospheric pressure.
Answer:The correct distance is
QR, so the answer is
B. QR.
