PURPOSE: To illustrate longitudinal standing waves in an aluminum rod.
DESCRIPTION: Apply powdered violin rosin to your fingers or wear a rosined glove and stroke the aluminum rod firmly while holding it at a nodal point. Holding it in the center produces the fundamental, holding at 1/4 of the way from one end produces the second harmonic, holding at 1/6 of the way from one end produces the third harmonic, etc. The rod is about 6 ft long, and the speed of sound in aluminum is about 16,700 ft/sec, so the frequency of the fundamental is about 1400 Hz. The sound is very loud and lasts a long time; the Q for this system is around 100,000!
SUGGESTIONS: Practice! This one is a bit weird at first, but is a very dramatic demonstration if done adequately.
EQUIPMENT: Aluminum rod with rosin.
SETUP TIME: None.
Go back to Lecture-Demonstration Home Page.