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The Denis d'or ("golden Dionysus" - spelling variants: Denisdor and Denydor) is, in the broadest sense, the first electric musical instrument in history.
It was invented and constructed by the Czech theologian Václav Prokop Diviš (1698 - 1765) — his surname is pronounced "Deevish" and often spelled "Divisch" — at his parish in the Moravian town Prímetice near Znojmo in the south-east of what is now the Czech Republic. He was also a pioneer of research into electricity, being most famous for having invented the lightning rod in Europe, contemporaneously with but independently of Benjamin Franklin.
His passionate interest in music was crowned by the construction of quite an extraordinary musical instrument he named "Denis d'or", with the French "Denis" etymologically going back to "Dionysus", whose Czech counterpart is "Diviš" — hence the name.
The earliest written mention of the Denis d'or dates from 1753, but it is likely that it already existed around 1748. Some sources even date its existence back to the year 1730, which claim is historically untenable and not supported by any known fact about Diviš's biography and work. Unfortunately, after Diviš's death in 1765 the unique instrument was sold and eventually brought to Vienna, where it soon vanished without trace. What is more, very few and short are the documents about the Denis d'or which are extant, so our image of it must remain rather fragmentary. But at least some facts are known and these can be summarized as follows:
The Denis d'or had 14 registers, most of which were twofold, and its complex mechanism fitted in a symmetrical wooden cabinet equipped with a keyboard and a pedal. It was about 150 cm long (5 feet), 90 cm wide (3 feet), and 120 cm high (4 feet). Basically, it was a chordophone not unlike a clavichord — in other words, the strings were struck, not plucked. However, the suspension and the tautening of the numerous metal strings (which, it is said, numbered the astonishing total of 790) were much more elaborate. The ingenious mechanism, which had been worked out by Diviš with painstaking mathematical accuracy, was such that the Denis d'or could imitate the sounds of a whole variety of other instruments, including chordophones such as harpsichords, harps, lutes and even wind instruments. This was mainly owing to the exceptional responsiveness and combinability of the stops, which permitted the player to vary the sound in multifarious ways, thereby generating far more than a hundred different tonal voices altogether. But the most special feature was that Diviš (temporarily) charged the iron strings with electricity in order that the sound quality might be enhanced — "purified", so to speak. This was an absolute novelty at the time. Additionally, he installed a slightly sadistic gimmick so that, any time he wanted, the player could be given an electric shock.
Diviš was the first person to foster the idea of an aesthetic connection between music and electricity. Before him nobody had sensed the aesthetic potential of electro-acoustic effects. In the face of electrical research still being in its early infancy in the middle of the 18th century, this revolutionary idea could then, of course, only be technically realized by Diviš in the most primitive way. But, nevertheless, those historical circumstances cannot belittle the fact that the Denis d'or can justifiably be regarded as the forefather of all electrophones, at least from the idealistic point of view.
This conclusion is strongly substantiated by the remarkable fact that the German theologian Johann Ludwig Fricker (1729-1766), who visited Diviš in 1753 and saw the Denis d'or with his own eyes, refers to it in a journal of the university of Tübingen as an "Electrisch-Musicalische[s] Instrument", the literal translation of which is "electric musical instrument".
The complete, up-to-date and editable article about Denis D'or can be found at Wikipedia: Denis D'or