Question Video: Analyzing the Structure of the Barometer

The apparatus shown in the diagram is used to measure atmospheric pressure. Which of the following occupies region A of the test tube? [A] Air [B] Mercury Vapor [C] Glass [D] Vacuum

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Video Transcript

The apparatus shown in the diagram is used to measure atmospheric pressure. Which of the following occupies region A of the test tube? (a) Air, (b) mercury vapor, (c) glass, (d) vacuum.

The apparatus in the diagram is from the Torricelli experiment, what we would call today a barometer. It has a dish on the bottom that is filled with mercury that is open to the atmospheric pressure and a test tube in the middle that is also filled with mercury. When the test tube is first placed into the dish, it is filled with mercury all the way to the top. The mercury then flows out of the test tube and into the dish below, leaving the space that we see in our diagram between the top of the mercury and the test tube. Because the test tube was full before and now it’s not, that means that there is a vacuum left in this space, leading us to answer choice (d). Vacuum is what occupies region A of the test tube.

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