The water barometer
A barometer over 10 m tall is very memorable.
Apparatus and materials
Length of clear tubing (PVC or other plastic), 12 m
Trap (to prevent the water being 'sucked' into the vacuum pump)
Food colouring (OPTIONAL)
Health & Safety and Technical notes
A trap should be used to prevent water being 'sucked' into the vacuum pump.
The water needs to be coloured, otherwise it is very difficult to see where the water column ends.
a Find a tall (10 m or more) teaching block.
b Fix a suitable length (12 m) of clear plastic tubing to a vertical wall. Mark the tube with coloured tapes at 0.5 m intervals.
c Dip the lower end of the tube into a bucket of water. Connect the upper end to the vacuum pump (preferably with the gas-ballast valve open), with a trap.
d Pump the air out gently.
e An alternative procedure is to fill the polythene tube with water and seal its ends. One end of the tube is then hauled up to a height of over 10 m and secured while the lower end is fixed below the water surface in the bucket. The bottom end of the tube is then opened. The water level will settle at a height equal to the atmospheric pressure measured in metres of water.
1 Introducing the idea of a water barometer:
Suppose we didn’t have air all around us but had mercury instead, how high up would that mercury atmosphere extend from the ground to the top of the atmosphere if it were to make the same pressure that we feel here? What depth of mercury would make the same pressure? 76 cm, the same height as the mercury barometer.
How high would a water atmosphere be? Mercury is 13.5 times as dense as water so the water atmosphere would have to be 13.5 x 76 cm, about 10 m.
At this point, show the water barometer.
2 Extending the idea to determine the height of the atmosphere:
Now what about an atmosphere of air, assuming that the air does not vary its density at greater altitudes and there was nothing more above it?
Mercury has a density of 13 600 g /litre and air has a density of 1.2 g /litre. This means that mercury is about 11 300 times denser than air and so the height of an air atmosphere would be 76 x 11 300 cm or about 8 500 m.
Howeve,r the real atmosphere gets thinner and thinner and so it will be greater than this. Mount Everest is more than 8 300 m high and the atmosphere is thin up there – that is why mountaineers need to carry oxygen.
3 The water will evaporate/boil at the top of the water column because there is a vacuum above it. This water vapour will depress the level of water in the water barometer. That is why mercury is used in a barometer: its vapour pressure is low and little evaporates into the vacuum. (The water barometer is also very cumbersome due to its height.)
This experiment was safety-checked in July 2007