A barometer tube filled with water hangs from a spring scale as in the photograph. The water reservoir (a beaker) rests on a table, with the bottom of the tube below the water surface. A hole on the top of the tube is opened, allowing air to enter the tube, so the water in the tube flows into the reservoir. Q: After the water in the tube has been replaced by air, is the reading on the spring scale (a) greater than, (b) less than, or (c) the same as the reading with water filling the tube? A: The answer is that the scale reads less, as can be seen in the photograph at the right.
This is an interesting question for people who know that atmospheric air pressure is what keeps the water in the tube. They are then left with the problem of what exactly is the force which changes to reduce the spring scale reading after the water flows out of the barometer.