Robert M. Keolian
Dept. of Phys., Code PH/Kn, Naval Postgraduate School, Monterey, CA 93943
A tuning fork instrumented with an electromechanical drive and pickup is used to show many of the properties of the simple harmonic oscillator as well as self-maintained oscillations (negative damping) and the active damping of vibrations (positive damping). The apparatus is used both as a teaching laboratory experiment and as a lecture demonstration. The drive and pickup consist of coils of wire taken out of relays, and small but powerful rare-earth magnets available at Radio Shack. An eraser between the tines can be used to vary the passive Q. By amplifying the signal from the pickup coil, compensating for inductively induced phase shifts, and applying this signal back to the drive coil, a force proportional to the velocity of the tines can be used to subtract from or add to the passive damping, resulting in a self-excited tuning fork, a tuning fork with infinite Q, or one with very low Q.