| At the beginning of a course of lectures in introductory
physics, I like to show how the physicist must control the environment
in which experimental data are taken. First I drop a flat sheet of paper,
then I wad it up into a ball and drop it again from the same height. The
experience of the students immediately leads them to the correct conclusion
that the very different behaviors of the flat sheet and the paper ball
are due to the effects of the air. I then turn to the tall, evacuated tube
at the right to show two different objects fall in air and in a vacuum.
This contains a slip of paper and a small, flat piece of metal. At full
atmospheric pressure, inverting the tube causes the metal to come down
with a rush, while the paper floats down. Reducing the air pressure in
the tube barely affects the metal object, but the paper now comes down
with a time which decreases with the pressure. This leads to a discussion
of extrapolation from known experimental results.
At the left is Prof. Jamie Day of Transylvania University with a similar piece of apparatus.
Traditionally the apparatus is called a Guinea and Feather Tube, and was invented by Isaac Newton. The guinea used in the early tubes was the small English coin, minted until 1813, and named after the section of West Africa from which gold was mined. In the United States, this was changed in the second half of the nineteenth century to a cent and feather apparatus. Eighteenth century books usually show the tube with a mechanism at the top for releasing the two objects simultaneously. The Kenyon tube lacks this feature, and must be turned over quickly to start the objects down. Since the tube is 5 ft long, this can be taxing to the demonstrator! Although the apparatus is unmarked, it was certainly made by Ritchie and is listed in the 1860 catalogue at $8.50. The identification is made definite by the shape of the turning on the base, so typical of Ritchie apparatus.
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