A barrel organ is a mechanical musical instrument made of a series of pipes, and bellows, like any other traditional organ, and of a cylinder studded with staples or bridges or pins corresponding in their placement to a particular tune. While the cylinders are called barrels, they are usually much smaller than the barrels used as storage containers for several centuries. Usually made of finely crafted metal, the organ barrels had to be extremely sturdy in order to keep their precise alignement over the years, since they played the same programming role as piano rolls and had to deal with more mechanical stress given the greater number of rods and levers involved.

The continuous rotation of the barrel causes the staples to come into contact with levers and rods which open valves to let air from the bellows into the organ pipes. The bellows is usually actuated by the same power source which, through reduction gearing or worm gearing, causes the drum to slowly turn around. Because of the very solid and sonorous presence of the organ grinder in the literature and the pictorial history of the 19th century and the early 20th century the power source is commonly assumed to be a crank, turned by this gentleman or by his monkey. With a few exceptions, the organ grinders used one of the smaller portable versions of the barrel organ. There were many larger versions located in churches, fair grounds, music halls and other large establishments such as sports arenas and theatres. The large barrel organs were often powered by very heavy counterweights and springs, like a more powerful version of a grandfather clock. They could also be hydraulically powered, with a wheel arrangement giving the mechanical force while columns of water forced greater pressure in the columns of air which sounded the pipes. The last barrel organs were electrically powered, or converted to electrical power.

The large barrel organs can easily be confused with normal organs because many of them were built as "barrel and finger" organs: They were furnished with a normal set of organ keyboards making it possible to play them without using the barrel, when a human organist was available. The barrels were all too often out of sight.

At the beginning of the 20th century large barrel organs were often converted to run on piano rolls or book music instead of their original barrels because of the much greater variety of melodies available on the rolls or on the books.

The barrel organ should not be confused with the hurdy gurdy, which, although it is also powered by a crank and often used by street performers, produces sound with a rosin-covered wheel rotated against tuned strings. Another difference is that the barrel organist is confined to pre-programmed tunes, while the hurdy gurdy player is free to play any tune he or she desires.

References

• Diagram Group. Musical Instruments of the World. New York: Facts on File, 1976.
• Ord-Hume, Arthur W.J G. Barrel Organ: The Story of the Mechanical Organ and Its Repair. London: George Allen & Unwin, 1978.
• Reblitz, Arthur A., Q. David Bowers. Treasures of Mechanical Music. New York: The Vestal Press, 1981.
• Smithsonian Institution. History of Music Machines. New York: Drake Publishers, 1975.