Mechanics

Torricelli’s Experience

The water suction pump has a working principle based on an experiment proposed by Torricelli

When pumping water from wells with suction pumps, it is found that it rises inside the pipe to a height of approximately 10.3 meters and no more. For a long time this fact remained unexplained.

In the 17th century, however, Torricelli solved the problem through the following experiment:

He filled a tube of mercury to the top, capped it, and placed it in a vessel containing the same liquid. When he uncapped the tube, the mercury inside it descended and stopped at 76 cm (or 760 mm) from the level of the mercury in the container (equilibrium situation). Between the upper level of the mercury column and the walls of the tube, a vacuum region was formed.

In the equilibrium situation, points C and B, which are at the same level, are subjected to the same pressure: in C is the atmospheric pressure at sea level; at B is the pressure exerted by the liquid column.

How does Torricelli’s experience explain the 10.3 m limit for the rise of the water column in suction pumps?

Water is 13.6 times less dense than mercury. Thus, for a column of water to exert at its base the same pressure exerted by the atmosphere, as with the column of mercury, the relationship between the heights of the columns must be:

height of the water column=13.6 .0.76=10.3 m

Thus, we can say that 760 mmHg = 10.3 mca (mca: meters of water column). Another unit used to measure pressure is the atmosphere (atm).

atm =1 atm=760 mmHg=10.3 mca=1.013 .10 5   Pa