Obtain as many harmonics as your arm can handle.
What it shows:
Generation of a standing wave by reflection from a fixed end.
How it works:
A two person demonstration using a 2m (2cm diameter) steel spring. 1 One party acts as the fixed end, standing holding the spring rigidly at chest height. The other sends the pulses down the spring by vigorous up-and-down movements. The frequency is adjusted to set up a standing wave from the fundamental up to whatever you're capable of (see Comments). Amplitudes of up to a meter can be achieved for the low harmonics.
Setting it up:
The demonstration can be performed by lone vibrator, by fixing one end with a C-clamp to the lecture bench.
It is easier to generate a standing wave by making circular movements with the hands, but it doesn't look as good. The record so far is the seventh harmonic (this is more in the wrist than the arm!). The following page illustrates the long spring in action.
Roy Glauber and Wolfgang Rueckner at the ends of their rope. (Photo by Laura Wulf)
1 CENCO 84740