EUPHONIUM (Fr. baryton; Ger. Tenor Tube), a modern brass wind instrument, known in military bands as euphonium and in the orchestra as tuba. The euphonium consists of a brass tube with a conical bore of wide calibre ending in a wide-mouthed bell; it is played by means of a cup-shaped mouthpiece. The sound is produced as in the bombardon, which is the bass of the euphonium, by the varied tension of the lips across the mouthpiece, whereby the natural open notes or harmonics, consisting of the series here shown, are obtained.
The intervening notes of the chromatic scale are obtained by means of valves or pistons usually four in number, which by opening a passage into additional lengths of tubing lower the pitch one, half, one-and-a-half, two-and-a-half tones (see Bombardon; Tuba; Valves). The euphonium gives out the fundamental, or first note of the harmonic series, readily, but no harmonic above the eighth. Euphoniums are made in C and in B♭, the latter being more generally used. By means of all the valves used at once, the B♭, an octave below the fundamental, can be reached, giving a compass of four octaves, with chromatic intervals. The bass clef is used in notation. The euphonium is treated by French and German composers as a transposing instrument; in England the real notes are usually written, except when the treble clef is used. The quality of tone is rich and full, harmonizing well with that of the trombone. The euphonium speaks readily in the lower register, but slowly, of course, owing to the long dip of the pistons. Messrs Rudall Carte have removed this difficulty by their patent short action pistons, which have but half the dip of the old pistons. On these instruments it is easy to execute rapid passages.
The euphonium is frequently said to be a saxhorn, corresponding to the baryton member of that family, but the statement is misleading. The bombardon and euphonium, like the saxhorns, are the outcome of the application of valves to the bugle family, but there is a radical difference in construction; the tubas (bombardon and euphonium) have a conical bore of sufficiently wide calibre to allow of the production of the fundamental harmonic, which is absent in the saxhorns. The Germans classify brass wind instruments as whole and half  according to whether, having the wide bore of the bugle, the whole length of the tube is available and gives the fundamental proper to an organ pipe of the same length or whether by reason of the narrow bore in proportion to the length, only half the length of the instrument is of practical utility, the harmonic series beginning with the second harmonic. (See Bombardon.)
 See Dr Schafhäutl's article on "Musical Instruments" in sect. iv. of Bericht der Beurtheilungs- Commission bei der Allg. deutschen Industrie Ausstellung (Munich, 1854), pp. 169-170; also Fried. Zamminer, Die Musik und die Musikinstrumente in ihrer Beziehung zu den Gesetzen der Akustik (Giessen, 1855).
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)