Evangelista Torricelli was born on October 15, 1608, in Faenza, Italy and died quite young on October 22, 1647 in Florence, towards the end of the great period of enlightenment known as the Renaissance. In 1641, Torricelli who was interested in physics and maths, moved to Florence to work with and learn from the renegade astronomer Galileo. who got into dreadful trouble with the Catholic Church by daring to suggest that “the sun was the centre of the world, and immovable, and that the earth moves”
Simply put, a barometer measures the pressure of the air above us. The pressure of the air on a dish of mercury can support a column of mercury over three quarters of a metre high. Here’s how to make one. Fill a tall glass tube with mercury right to the top. Hold your finger over the end and invert the tube underneath a dish of mercury. Take your finger away and the mercury level falls a little, creating a near-vacuum with a little mercury vapour. The atmosphere pushes down on the mercury in the dish and supports the weight of mercury in the inverted tube. For a long time, atmospheric pressure, just over 100kPa, was quoted as ‘760 millimetres of mercury’. You can work the calculation yourself. Mercury has a density of 13.6 x that of water, 13,600 kg/m³. h = 0.76m and g = 10m/s². P = hdg. What do we get? Just over 100,000Pa or 103kPa.
Torricelli’s ideas that the weight of the air from the atmosphere caused the liquid to stop falling seemed to be confirmed.
Torricelli also noticed that the level of the fluid in the tube changed slightly each day and concluded that this was due to the changing pressure in the atmosphere. He wrote: “We live submerged at the bottom of an ocean of elementary air, which is known by incontestable experiments to have weight”.
And, of course, if air has weight, it must also have pressure…..
On a practical note: teachers were allowed to make these in front of their students, then somebody realised that mercury vapour, a heavy metal, was incredibly poisonous so we weren’t allowed to do it any more. Sorry, all.