The viola organista is an experimental musical instrument designed by Leonardo da Vinci. It uses rotating wheels to vibrate individual strings (similar to how a violin produces sounds), with the strings selected by pressing keys on a keyboard (similar to an organ). The first known implementations made from Leonardo's incomplete designs were in modern times.
Leonardo's original idea, as preserved in his notebooks of 1488–1489 and in the drawings in the Codex Atlanticus, was to use one or more wheels, continuously rotating, each of which pulled a looping bow, rather like a fanbelt in an automobile engine, and perpendicular to the instrument's strings. The strings would be pushed downward into the bow by the action of the keys, causing the moving bow to sound the pitch of the string. In one design, the strings were fretted with tangents, so that there were more keys than strings (multiple notes, for example C and C#, would be played on one string). In another design, each note had its own string.
Leonardo's design is similar to that of the earlier hurdy gurdy, which also uses a rotating wheel to play strings. It differs in that a hurdy gurdy has a small number of strings that are constantly in contact with the wheel, rather than a larger number of strings that can be lowered onto the wheel. A hurdy gurdy has a keybox with tangents that change the pitches of the strings, rather like placing fingers on violin strings. Leonardo's innovation of a keyboard with a lowering mechanism allowed individual notes to be played, alone or in specific desired chords over a large range of pitches.
Carolyn W. Simons, "Sostenente piano", and Emanuel Winternitz and Laurence Libin, "Leonardo da Vinci," Grove Music Online ed. L. Macy (Accessed April 2, 2005 at www.grovemusic.com), (subscription access)
"Sostenente piano", The New Harvard Dictionary of Music, ed. Don Randel. Cambridge, Massachusetts, Harvard University Press, 1986. ISBN 0-674-61525-5