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My student Narciso Guzman built this version of the
Cavendish experiment in his garage, from a descrip-
tion on the Web at www.fourmilab.to. Two steel balls
sit near the ends of a piece of styrofoam, which is sus-
pending from a ladder by fishing line (not visible in this
photo). To make vibrations die out more quickly, a small
piece of metal from a soda can is attached underneath
the styrofoam arm, sticking down into a bowl of water.
(The arm is not resting on the bowl.)
The sequence of four video frames on the right shows
the apparatus in action. Initially (top), lead bricks are
inserted near the steel balls. They attract the balls, and
the arm begins to rotate counterclockwise.
The main difficulties in this experiment are isolating
the apparatus from vibrations and air currents. Narciso
had to leave the room while the camcorder ran. Also, it
is helpful if the apparatus can be far from walls or furni-
ture that would create gravitational forces on it.