Christian Gottlieb Kratzenstein (1723–1795), professor of physiology at Copenhagen, was credited with the first free-reed instrument made in the Western world, after winning the annual prize in 1780 from the Imperial Academy of St. Petersburg. The harmonium's design incorporates free reeds and derives from the earlier regal. A harmonium-like instrument was exhibited by
Gabriel-Joseph Grenié (1756–1837) in 1810. He called it an orgue expressif (expressive organ), because his instrument was capable of greater expression, as well as of producing a crescendo and diminuendo. Alexandre Debain improved Grenié's instrument and gave it the name harmonium when he patented his version in 1840. There was concurrent development of similar instruments. A mechanic who had worked in the factory of Alexandre in Paris emigrated to the United States and conceived the idea of a suction bellows, instead of the ordinary bellows that forced the air outward through the reeds. Beginning in 1885, the firm of Mason & Hamlin, of Boston made their instruments with the suction bellows, and this method of construction soon superseded all others in America.
Beatty's Parlor Organ, 1882
Harmoniums reached the height of their popularity in the West in the late 19th and early 20th centuries. They were especially popular in small churches and chapels where a pipe organ would be too large or too expensive; in the funeral-in-absentia scene from Mark Twain's The Adventures of Tom Sawyer, the protagonist narrates that the church procured a "melodeum" (a conflation, likely intended by Twain for satirical effect, of the names "melodeon" and "harmonium") for the occasion. Harmoniums generally weigh less than similar sized pianos and are not as easily damaged in transport, thus they were also popular throughout the colonies of the European powers in this period not only because it was easier to ship the instrument out to where it was needed, but it was also easier to transport overland in areas where good-quality roads and railways may have been non-existent. An added attraction of the harmonium in tropical regions was that the instrument held its tune regardless of heat and humidity, unlike the piano. This "export" market was sufficiently lucrative for manufacturers to produce harmoniums with cases impregnated with chemicals to prevent woodworm and other damaging organisms found in the tropics.
Modern portable harmonium with 9 air stop
At the peak of the instruments' Western popularity around 1900, a wide variety of styles of harmoniums were being produced. These ranged from simple models with plain cases and only four or five stops (if any at all), up to large instruments with ornate cases, up to a dozen stops and other mechanisms such as couplers. Expensive harmoniums were often built to resemble pipe organs, with ranks of fake pipes attached to the top of the instrument. Small numbers of harmoniums were built with two manuals (keyboards). Some were even built with pedal keyboards, which required the use of an assistant to run the bellows or, for some of the later models, an electrical pump. These larger instruments were mainly intended for home use, such as allowing organists to practise on an instrument on the scale of a pipe organ, but without the physical size or volume of such an instrument. For missionaries, chaplains in the armed forces, travelling evangelists, and the like, reed organs that folded up into a container the size of a very large suitcase or small trunk were made; these had a short keyboard and few stops, but they were more than adequate for keeping hymn singers more or less on pitch.
The invention of the electronic organ in the mid-1930s spelled the end of the harmonium's success in the West (although its popularity as a household instrument declined in the 1920s as musical tastes changed). The Hammond organ could imitate the tonal quality and range of a pipe organ whilst retaining the compact dimensions and cost-effectiveness of the harmonium as well as reducing maintenance needs and allowing a greater number of stops and other features. By this time, harmoniums had reached high levels of mechanical complexity, not only through the need to provide instruments with a greater tonal range, but also due to patent laws (especially in North America). It was common for manufacturers to patent the action mechanism used on their instruments, thus requiring any new manufacturer to develop their own version; as the number of manufacturers grew, this led to some instruments having hugely complex arrays of levers, cranks, rods and shafts, which made replacement with an electronic instrument even more attractive.
The last mass-producer of harmoniums in North America was the Estey company, which ceased manufacture in the mid-1950s; a couple of Italian companies continued into the 1970s. As the existing stock of instruments aged and spare parts became hard to find, more and more were either scrapped or sold. It was not uncommon for harmoniums to be "modernised" by having electric blowers fitted, often very unsympathetically. The majority of Western harmoniums today are in the hands of enthusiasts, though the instrument remains popular in South Asia.
Modern electronic keyboards can emulate the sound of the pump organ.