Postprint version. Published in American Journal of Physics, Volume 59, Issue 9, September 1, 1991, pages 775-776.
Copyright © 1991 American Association of Physics Teachers. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Association of Physics Teachers. The following article appeared in American Journal of Physics.
NOTE: At the time of publication, the author Matthew Moelter was not yet affiliated with Cal Poly.
The definitive version is available at http://dx.doi.org/10.1119/1.16724.
Letter to the Editor.