Describe how a mercury barometer measure air pressure?

Answer 1

The basic principle of barometer is Torricelli's experiment. Imagine that you fill a tube of one metre length (100cm) with mercury up to the brim. Then close the mouth of the tube with your thumb and invert the tube and place it in a cistern of mercury with its end immersed in the mercury say some 4 cm. Now you remove your thumb. Mercury would come down but it stops at one level to our astonishment. This level would be some 20 cm from the closed end of the tube. So there will be almost 76cm column of mercury in the tube. Why is it so? As mercury comes down the space left vacant will not be having any air molecule and so it is almost vacuum. This is named as Torricelli's vacuum. Now the air outside in the atmosphere would exert a pressure and wants to push the mercury towards this vacuum place. But the weight of the mercury column (76cm) will balance this pressure. Hence we say the atmospheric pressure will be 76 cm of mercury. This is how we have mercury scale for measuring the pressure. Usually pressure is defined as the force per unit area. Hence the unit will be N/m2 or pascal. But we give pressure only in cm or mm of mercury. The reason is explained properly. Same technique is applied in Fortin's barometer and vernier arrangement is made to measure the pressure so accurately.