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I first "learned" the singing rod demo from Fred Leitner at Rensselaer, but
it was also used as a lab exercise to measure the speed of sound in brass,
steel, and aluminum. A couple of hints:
Mark the rod using a file (whatever its length, but ~ 6' seems to work well
with all these materials) at intervals from each end 1/2, 1/3, 1/4, 1/5 its
length. Holding the rod at each of these will allow you to generate all
the harmonics as well as the fundamental. For odd harmonics, once
excited you can grab the rod at the center and the "other" node without
killing the sound. Pretty convincing standing wave demo! Beware of
neighborhood dogs responding to the call of the rod!
I've found that having rosin on thumb and forefinger works better than on
the palm, I suspect because you are trying to "excite" a smaller region of
the rod.
I haven't followed up in the literature to confirm this, but Fred Leitner
told me that the earliest reference to this demo was attributed to Benjamin
Franklin, perhaps in a letter to Lavoisier.
--
George Spagna ******************************************
Department of Physics * *
Randolph-Macon College * "Imagination is more important *
P.O. Box 5005 * than knowledge." *
Ashland, VA 23005-5505 * *
* - Albert Einstein *
phone: (804) 752-7344 * *
Internet: gspagna@rmc.edu ******************************************