Operation
A set of levers called keys is positioned just above the surface of the barrel. Each key corresponds to one note. A rod is connected to the rear of each key. The other end of the rod is a metal pin which operates a valve within the wind chest. When the instrument is played (by turning the crank), offsets on the crank shaft cause bellows to open and close to produce pressurized air. A reservoir/regulator maintains a constant pressure. A worm gear on the crank shaft causes the barrel to rotate slowly and its pins and staples lift the fronts of the keys. This causes the other end of the key to press down on the end of the rod which, in turn, activates the valve and allows air from the bellows to pass into the corresponding pipe.
To allow different tunes to be played from one barrel, the barrel can be moved laterally to bring a different set of pins and staples under the keys. Street barrel organs usually play 7 to 9 tunes, although small organs (usually the older ones) can play up to 15 tunes. Less commonly (and usually for large orchestrions) the pinning will form one continuous spiral and the barrel will be gradually moved as it rotates so that the pins remain lined up with the keys. In this case, each barrel only plays one long tune.
Read more about this topic: Barrel Organ
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