Atwood's Machine
   The Rev. George Atwood (1746-1807) was a tutor at Trinity College, Cambridge when he published A Treatise on the Rectilinear Motion and Rotation of Bodies, with a Description of Original Experiments Relative to the Subject in 1784. According to Thomas Young, he "materially contributed to the progress of science, by multiplying the modes of illustration, which experimental exhibitions afford for the assistance of the instructor."

   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, 

          a = {(M - m)/(M + m)}g 

   Atwood's original illustration for the machine showed the axle of the light pulley carrying the string supported on the rims of four other wheels with similarly small moments of inertia, and every early machine I have ever examined follows this pattern.

   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.
     University of Mississippi       Benedictine College, Kansas          Marietta College         Smithsonian Institution

   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.
   This beautiful cast-brass set of pulleys from the top of an Atwood's machine is in the Garland Collection of Classical Physics Apparatus at Vanderbilt University. Unfortunately, it is unmarked, and is not even listed in the collection's catalogue.

   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.
 Variations of the Basic Atwood's machine design were made well into the twentieth century. At the left is one of several surviving at the United States Military Academy at West Point that were made by G. Cussons of Manchester, England, and imported by Knott of Boston. 

   The acceleration due to gravity could be measured more directly with other pieces of apparatus, described elsewhere on this site as free fall apparatus.

REFERENCE: Thomas B. Greenslade, Jr., "Atwood's Machine", Phys. Teach., 23, 24-28 (1985)

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