Describe the mercury barometer for the measurement of atmospheric pressure.

(N/A) The Italian scientist Evangelista Torricelli devised the first method for measuring atmospheric pressure.
This device is shown in the figure.
$A$ long glass tube of $1 \text{ m}$,closed at one end and filled with mercury,is inverted into a trough of mercury. The thumb is kept on the open part of the tube.
If the thumb is removed from the tube,the level of the mercury column decreases slightly.
The space above the mercury column in the tube contains only mercury vapour,whose pressure $P$ is so small that it may be neglected. Thus,$P \approx 0$.
The pressure inside the column at point $A$ is equal to the pressure at point $B$,which is at the same horizontal level. The atmospheric pressure at point $A$ is given by:
$P_{a} = P + h \rho g$
Since $P \approx 0$,we have:
$P_{a} = 0 + h \rho g$
$\therefore P_{a} = h \rho g$
Where $\rho$ is the density of mercury and $h$ is the height of the mercury column.
In this device,the height of the mercury column at sea level is $76 \text{ cm}$,which is equivalent to one atmosphere.