This article is from the Grateful Dead FAQ, by John J. Wood, Eric Nay and Ihor Slabicky email@example.com with numerous contributions by others.
It is called "The Beam", a long aluminum I-beam with twelve piano
strings streched along its length; the vibration of the strings is
sensed by a very large magnetic pickup. The sound is amplified
through Myers speakers and sub-woofers, whose loudness can be varied
with a foot-pedal allowing a controlled degree of acoustic feedback.
The best example of the Beam is on the Rythm Devil's "The Apocalypse
Now Sessions". The Beam is used heavily, but most notably on the
track, "Napalm for Breakfast". Pittsburgh 06/26/88, Shoreline
06/16/90, and Buffalo 06/06/92 are good examples of the Beam in concert.