How is the vacuum measured ..... ???
Why Mercury?, you askas did I. This results from measurement made by an Italian mathematician named Evangelista Torricelli (1608-1647), who Galileo asked to work on the problem of water pump capabilities.
Torricelli devised an experiment in which he filled a tube sealed on one end with mercury. Holding his finger over the open end of the tube, he inverted it, lowered the open end into a bowl full of mercury, removed his finger, and observed how far the mercury inside the tube fell. The portion of the tube that was vacated held a vacuum, and the distance from the top of the column of mercury to the top of the mercury in the bowl was the differential pressure between the vacuum in the tube and the ambient air (what we call barometric pressure today). This was recorded in millimeters of mercury, resulting in standard ...more