Atwood's name is forever attached to the pulley-wheel arrangement shown at the right (from the apparatus collection of Kenyon College). Endless generations of students have solved the problem of the massless, non-stretching string passing over the pulleys with masses M and m on the ends. The acceleration of the masses is, they show,
In his book Atwood described a series of twelve demonstration experiments to be done with his apparatus. All of these involve timing. When I tried out these experiment in 1984 with the apparatus above, I used photogates and millisecond timing, technologies not available to Atwood two centuries earlier. Instead, he used a pendulum which ticked off the seconds, and adjusted the distances of fall to make the times of fall integral numbers of seconds.
The four examples below show the clock mechanism controlled by the seconds pendulum, and the vertical scale showing the distance of fall. The only one still in operation is the example at Marietta College, made by Pixii of Paris in the eighteen thirties. It was listed at 800 francs (about $200) in the 1849 Pixii catalogue. The acorn of knowledge adorns its top. The examples from Mississippi and the National Museum of American History of the Smithsonian Institution were also made by Pixii; the Smithsonian machine was originally at the United States Military Academy at West Point.
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Portions of three Atwood machines are shown below. At the top left is the top of an Atwood's machine made by Queen of Philadelphia, and, when I took the picture in September 1979, at Wittenberg University, the apparatus was almost certainly upside down! The pulley mechanism sat on the top of the apparatus, probably supported on the points of the four brass screws. The square clamp in the center supported the wooden rod with the scale marked on it, and the electromagnet on the right released the falling weight. The fine thread ran through the two holes in the top.
At the top right is the top of an Atwood's machine in the collection of historical scientific instruments at the Smithsonian Institution.
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The Garland Collection also has an excellent complete Atwood's machine by Deleuil dating from about 1875. It's column is topped with an pineapple; the Pixii machine at Marietta is topped with an acorn. This machine is listed at 775 francs in the 1865 Deleuil catalogue.
| The picture at the right shows West Point cadets using an Atwood's machine about 1900. This instrument is the one shown above, currently on display at the National Museum of American History of the Smithsonian Institution. |  |
The acceleration due to gravity could be measured more directly with other pieces of apparatus, described elsewhere on this site as free fall apparatus.
