How to play Torricelli's Tube
- Choose a tube and read the liquid's density, the air pressure, or the column height on the card.
- The column settles where its pressure matches the air's: ρ·g·h = p, with p in pascals (1 kPa = 1000 Pa).
- Type your prediction and press Start. The column falls, or the pump empties its pipe, steadily.
- Watch the orange line: it shows the column height your prediction gives, next to the real one.
How can air hold up a column of liquid?
Air has weight, and the whole atmosphere above you presses on everything: about 101 kPa at sea level, the weight of 10 t on every square metre. You don't feel it because the air inside and around you pushes back equally. In 1643 Torricelli made it visible. He filled a glass tube with mercury, turned it upside down in a dish, and the mercury fell to about 760 mm, leaving nothing at all above it. The column stops where its own pressure at the foot, ρ·g·h, matches the air pushing down on the dish: h = p ÷ (ρ·g). That is a barometer: when the air presses less, on a mountain or before a storm, the column is shorter. Water is 13.6 times less dense than mercury, so a water barometer would be over 10 m tall. The same limit holds for a suction pump. It doesn't pull water up; it only removes air, and the outside air pushes the water up the pipe, never more than about 10 m at sea level. That is why deep wells need a pump at the bottom.
A focused learning model
The tube has the same width all the way up, and the space above the column is a perfect vacuum (the liquid's vapour is ignored). The column falls steadily for 2 s from the full tube to its balance height, h = p ÷ (ρ·g), without sloshing; the dish is wide, so its level doesn't change. The pump removes the air from its pipe steadily over 2 s, so the water rises steadily until it reaches the pump or the outside air can push it no higher; within 0.05% of the limit still reaches. A pump height is right if the water reaches the pump with at most 2% to spare; every other prediction within 2%. g = 9.8 m/s²; sea-level air presses 101.3 kPa. Plays in real time.