A measure of Boyle’s law

Yasmin Jayathirtha

experiment ‘The volume of a gas is inversely proportional to the pressure’ – this is Boyle’s law. One studies this law in both physics and chemistry classes, but there is no simple setting up that seems possible. This relationship was experimentally determined by Robert Boyle and a description of his procedure shows why it is very difficult to replicate.

He prepared a glass tube in the form of the letter J, with the shorter end closed. The long side was more than 10 feet tall and didn’t fit into any room, so it was put in the stairwell. He then carefully poured mercury into the tube till the level was the same on both sides. He carefully pasted strips of paper, marking inches in eights of inches. He poured in more mercury and measured the levels again. This time, the levels were uneven, since the air in the closed tube got compressed by the mercury and resisted it more (Fig1b). The levels came to a balance depending on the pressures on both sides. Boyle knew that the atmosphere exerted pressure and so did mercury. He observed that when the level on the open side was 29 inches more than on the shorter side, the volume of the gas was half its original value. He knew that the atmosphere could support a column of mercury 29 inches high, so when the pressure was doubled, the volume was halved! He made hundreds of measurements and found that a column of mercury – 10 feet high – was needed to compress the air into a quarter of its volume. The glass broke many times, and the bottom of the J tube rested in a box so that the mercury could be caught.

The author works with Centre for Learning, Bangalore. She can be reached at yasmin.cfl@gmail.com.

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