Atmospheric pressure shown using a mercury-filled manometer
Pumping air from one side of a manometer.
Apparatus and materials
Mounted U-tube manometer, 1.5 m tall
Bottle of mercury
Length of pressure tubing, 1 m
Mercury spill tray
Trap (to prevent the water being 'sucked' into the vacuum pump)
Health & Safety and Technical notes
When mercury is being used, the laboratory should be well ventilated, and equipment for dealing with spills ready at hand. The apparatus itself should be placed in a tray so that any beads of mercury can be collected easily.
Wash hands thoroughly after using mercury.
1 The pressure must be reduced gradually by careful operation of the tap on the pump. This is made easier if there is a needle valve on the pump, alternatively a side tube can be used with rubber and clip to provide a leak.
2 It is essential to insert a trap between the pump and the tube to ensure that mercury does not enter the pump. This has the additional advantage of making it easier to evacuate slowly.
3 The spill tray should be plastic and smooth surfaced. It should be deep enough to hold all the mercury.
4 When the manometer is not in use, use a stopper to keep the ends open.
a Connect the vacuum pump to the manometer as illustrated.
b Reduce the pressure gradually until the difference in heights stops increasing.
c Measure the difference in heights.
1 For many students, the idea of the atmosphere is half taken for granted, half a mystery. Asked if the air is here in the room a student will say, Of course it is, I breathe it in and out, I can feel it. Yet when asked if air is real stuff that you can weigh and put in a box, like water or sand, many students will show uncertainty. Air is not as real to them as water or sand, nor was it to early scientists.
The idea that we live at the bottom of an ocean of air that exerts as good a pressure as an ocean of water some 10 m deep is new and strange; essentially unthought of rather than difficult. (James Conant, in his excellent discussion of the tactics and strategy of science, quotes the idea of an 'ocean of air' as an example of a conceptual scheme that enabled science to advance.)
2 Students may have heard that the air exerts a pressure, pushes on things. Ask, Well, if the air does press on everything, could we use the U-tube and mercury to measure the pressure of the air in this room, the pressure of the atmosphere as we call it? (If someone asks, Why mercury?, reply, Let us try it with mercury first, in case the pressure is so big that the water pressure scale is not tall enough.) This is not quite the same as the discouraging reply, because mercury is the right liquid to use - it is nearer to the sensible admonition Try the 10-amp ammeter before you try the 1-amp one! That is good scientific procedure, and so worth pointing out.
3 If students do not know what to suggest, point out that if you blow into a U-tube pressure gauge, there are two pressures. The lung pressure plus atmospheric pressure on one side and atmospheric pressure only on the other side.
Now look at the U-tube. Both ends are open, not connected to anything. There is mercury at the same level on both sides. Suppose we wanted to measure the pressure of the air in this room. There it is pushing on the mercury in the left side, and there it is pushing on the mercury in the other side. What must we do?
Elicit the suggestion of pumping air away from one side. Pump a little and ask what is happening. Then pump some more. Then stop and raise the question of the pump being damaged by pumping the mercury into the pump. By this time a group of students are frantic to see that happen. Go on pumping and show there is a definite limit. Ask whether one can be sure that there is a good vacuum, 'just nothing', on that side.
Measure the levels of mercury. What would it be if we had used water?
This experiment was safety-checked in July 2007