“Slowly the hour-hand of the clock moves round; So slowly that no human eye hath power To see it move …!”1

1.
Henry Wadsworth Longfellow, “The Two Rivers.”
2.
A pendulum clock will also advance stepwise, although not necessarily with this same increment. A clock driven by a synchronous electric motor will advance continuously.
3.
The effect can be made even larger by having the beam strike the wall obliquely. If, for example, the beam makes an angle of 60° with a line perpendicular to the wall, the spot will hop an average of 6 mm every second.
4.
In a similar application, an optical lever has been used to demonstrate the motion of a pocket watch reacting to its oscillating balance-wheel. See R.M. Sutton, Demonstration Experiments in Physics (McGraw Hill, 1938), p. 73, or “Tail Wags Dog,” Demo 07-03 in The Video Encyclopedia of Physics Demonstrations, or D. R. Carpenter and R. B. Minnix, “Watch on Watch Glass and Laser,” The Dick and Rae Demo Notebook, Demo M-772.
5.
See, for example, http://www.sterlingtimes.org/memorable_images27.htm.
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© 2005 American Association of Physics Teachers.
2005
American Association of Physics Teachers
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