Vibrational resonances of metal reeds are excited by a spinning gyro as it slows down.
How it works
The Frahm resonance gyroscope is a standard piece of equipment that can be purchased from science supply houses. 1 It consists of a heavy wheel slightly unbalanced, held in a frame to which seven metal reeds are attached, each having a different vibrational frequency. The wheel is set in motion by unwinding a string that has been wrapped around the axle. As the wheel runs down, it sets each reed successively into vibration as its rotational frequency passes through the resonant frequency of the reed.
Setting it up
This is a very small demo (designed for small classroom use) and will need a video camera/monitor to be seen by the audience in a lecture hall. A larger version using hacksaw-blade oscillators and a motor with an unbalanced wheel, all mounted on a board, can be assembled if video viewing is unpalatable.
It works well and makes the point. A good story to accompany the demo is the one about the old car that only shimmies and rattles at certain speeds while cruising down the highway.
1 Welch No. 3330, $75 (1971)