The idea was to cause the strings to vibrate under the influence of magnetic fields, like an e-bow does for the guitar. A synthesizer was used as the signal source. It's output was amplified by an old PA amplifier. Instead of being connected to a speaker, the output of the PA amplifier was run to an array of electromagnetic coils that were placed about a quarter of an inch away from an octave of piano strings. The cores of the coils were machined from iron rod stock about an inch in diameter. The wire gauge, number of windings, and parallel-series wiring of the coils were selected to match the 4 ohm impedance for which the PA amplifier was designed.
The resulting sounds were interesting. The strings acted as an ethereal reverb and only resonated when the magnetic fields alternated at harmonics of the strings. Slowfreqchng.mp3 is a recording of a single chord with the pitch bend wheel on the synthesizer moved very slowly. I've also got a couple of other clips below of me playing with the setup. Note that all of the recorded sounds are being produced by the piano strings and body. No effects were used. Sadly, my recording equipment and skills were pretty limited at the time, so the recordings were made with a single Sure SM-48 microphone and there's significant clipping. The magnets were close enough to the strings that occasionally they made physical contact and rattling resulted. I think I still have those coils in a box here somewhere...
