A Dictionary of Music and Musicians/Piston
PISTON. A name given to one form of valve used in brass instruments for altering the course of the vibrating column of air, and thus producing alteration of pitch. The other form is termed a rotatory valve. The piston consists of a vertical tube inserted in the main air-way; usually, but not necessarily, at right angles to it. Four orifices communicate with it laterally; two belonging to the original bore; two connected with a bye-path or channel of greater length termed the 'valve slide.' In the vertical tube itself slides an air-tight cylinder or piston, pressed upwards by means of a spiral spring beneath it, and prolonged above into a circular button or finger-piece which can be depressed at pleasure. Across the cylinder are two oblique perforations occupying its central portion. In a state of rest, one of these is continuous on either side with the bore of the instrument, and the bye-path is obstructed. But when the fingerpiece is depressed in opposition to the action of the spiral spring, the former is closed, and communication is established by the other between the main bore and the valve slide or channel. The ordinary cornet à pistons, so named from this ingenious contrivance, usually possesses three of these pistons worked by the first three fingers of the right hand, the musical effect of which has been described under that title. [Cornet, vol. i. p. 403.] The Euphonium or bass saxhorn is generally furnished with a fourth valve for the left hand. The series may, however, be extended to six or more, though it is rare to see the above numbers exceeded. The French horn, from the closeness of its harmonic sounds, hardly needs more than two, respectively depressing the open note a tone and a semitone: these are usually attached to a removable slide, and can be replaced by a plain metal tube. [See the woodcut under Horn, vol. i. p. 747.] The early pistons were of complicated plan, causing several abrupt angles in the air-way, which to a certain extent interfered with the purity and freedom of the tone. Modern improvements have to a great degree removed this defect; though there still exists a prejudice against their use, especially among players of the French horn.
In the rotatory valve the vertical piston is replaced by a horizontal fourway cock, also kept in position by a spring, moved by a lever like that of a clarinet or flute, but possessing on its circumference the same pair of orifices, and establishing exactly the same connexions between tube and slide as does the piston. The rotatory valve, when really well made, is perhaps the more perfect of the two as a mechanical contrivance; but it is somewhat more liable to stick fast, and less easily accessible for cleaning than the piston-valve. The device is quite of recent invention, due in great measure to M. Adolphe Sax, and has completely superseded the older contrivance of keys, as in the key-bugle, ophicleide, and the ancient serpent. It is liable to considerable imperfections of intonation from the fact that it does not distinguish between major and minor tones and semitones; also from the different theoretical length of the valve-slides due to alterations of key or of crook. Mr. Bassett has ingeniously added to the trumpet an extra valve, which he terms the 'comma valve' or piston, and which corrects the former error; the latter must be left to the ear of the performer, and is often sadly neglected.