Shab Levy on his various Gravitrams:
The largest ball that I used was 6 inches in diameter and the smallest less than one inch. Some of the Gravitrams that were created in my studio were made from 3/16" stainless steel track, using balls that are between 2-4 inches made of hard plastic. Some Gravitrams were musical. In one Gravitram, 25 feet long and less than 2 feet deep, the tracks were made of hardwood. Another Gravitram used copper troughs in which water flowed and tripped various devices. The largest and most complex Gravitram was built for a museum in Brazil in 1995. It is approximately 15' tall and 12' in diameter and is controlled by the visitor through a computer console allowing different gates and tricks to operate according to the visitor’s input.
See many more of Shab Levy's Gravitrams on his web site.