Harmonium, a musical instrument, for the invention of which many claims have been advanced. The arrangement by which the sounds of the harmonium are produced is called the free vibrating reed, supposed to have been a modern discovery, but now ascertained to have been known in China long before it was ever heard of in Europe. Its construction is as follows: A narrow rectangular slit being made in a piece of brass-plate of a quarter of an inch in thickness, a thin elastic spring of the same metal, and of nearly the exact breadth of the slit, is fixed at one end by two small rivets to the surface of the plate, close to one end of the slit, and is so adjusted that it fills the area of the slit, and, when pressed into it at the free end, can pass inwards without touching the end or the sides of the slit, and when left to itself it can return back to its position of covering the slit. The spring at the free end is permanently bent a very little outwards. When a current of air is forced through the slit, the spring is put into vibration, and produces a continuous musical sound, acute or grave, according to the rapidity or slowness of the vibrations. This kind of reed is termed 'free,' in contradistinction to the reed of the organ-pipe, the spring or tongue of which entirely covers an oblong slit, in the side of a brass tube closed at one end, and vibrates against the cheeks or outside of the slit, instead of within it.
After many attempts, in various countries, to construct a keyed instrument of really a useful kind with the free reed, Debain of Paris produced his invention (1840) of the harmonium, which became more or less the model of all the others that have followed. The harmonium occupies comparatively little space, being only about 3 feet 3 inches high and 4 feet broad, the depth being according to the number of the stops, usually from 20 to 23 inches. It has a compass of five octaves of keys from C to C, the key-board being placed on the top, immediately below the lid. Under the key-board is the bellows-board, in which are valves for each key; while above the valves are the different rows of reeds. The sizes of the reeds differ, according to pitch, from about inches long to inch; and the quality of sound is affected and modified by the breadth of the vibrating part of the reed, and the shape of the aperture in the bellows-board covered by the valve. The pressure of wind is from a bellows with two feeders, which the player moves alternately with his feet, filling a reservoir, similar to the bellows of a small organ. When a key is pressed down, the valve opens, and the wind, which has access from the bellows to the wind-chest, rushes through the slit of the reed, and produces a sound which continues as long as the valve is kept open. It is a peculiarity of the free reed that an increase or a diminution of the pressure of wind does not alter the pitch of the sound, but merely increases or diminishes its volume. Advantage may be taken of this peculiarity by the harmonium-player to effect a crescendo or diminuendo by gradually augmenting or decreasing the pressure of the wind. The vibrations of the spring being like those of a pendulum, isochronous, remain fixed in rapidity or slowness, according to the length and elasticity of the vibrating slip of metal, and thus regulate the pitch of the sound without reference to the pressure of wind. For the deep bass-notes the springs are heavily loaded at the loose end, to make them vibrate slowly; while in the higher notes they are made thinner at that end.
Harmoniums are made of various sizes, and from one row of reeds (or vibrators, as they are now called) to four or more rows. Each row is divided near the middle, between an E and F; and each half has its separate draw-stop. Knee-pedals are sometimes added for producing the same modifications of tone as the swell on the organ. Some harmoniums are made with two rows of keys, thus affording a greater variety in playing solo with an accompaniment; and for more skilful performers, pedals for the feet, similar to organ-pedals, are attached. The manufacture of the harmonium in Paris has, of late years, increased almost incredibly. The various parts of the harmonium can be obtained there ready-made, from a single reed to a complete set. The best-known makers are the Alexandres and Mustel in France, and Bauer in England. The Scraphine was a similar but much inferior instrument. The American Organ, introduced in 1861 by Messrs Mason & Hamlin, is a kind of harmonium which acts by wind exhaustion or suction, and instead of force bellows, works by exhaustion bellows. Its tone is softer, and its timbre less reedy; it is also easier to play. But the true harmonium is capable of higher treatment. The percussion action for the harmonium is due to a small hammer like that of a pianoforte, which strikes a blow on the vibrator the moment the key is pressed down, and sets it instantly into vibration, thus assisting the action of the wind. The expression stop—an invention of the Alexandres, father and son—is used almost continuously by the best players on the instrument, but is very difficult to manage. By the action of this stop, the air-reservoir is cut off, and the pressure becomes entirely dependent on the management of the bellows. The latest invention of importance is the melody-attachment of Dawes—the date of the patent was 1864—which gives predominance to any special note or notes in the upper part of the harmony, by a contrivance which shuts off all notes except the highest, in certain registers of a combination. Harmoniums may now be had of various sizes and qualities, at prices from £5 to £120. Valuable for accompanying psalmody, they suitably take the place of organs in temporary places of public worship, or among the less opulent class of congregations; but of late years the French school of players, headed by M. Lémens, have treated the harmonium with success as a brilliant solo instrument. For domestic use, harmoniums are not likely to supersede the pianoforte; but possessing the important advantage of not going out of tune through humidity of atmosphere, they will be found available in climates where pianos cannot properly be kept.