Savart's wheel
 Demonstration of Savart wheels with different numbers of teeth
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The Savart wheel is an acoustical device named after the French physicist Félix Savart (1791–1841), which was originally conceived and developed by the English scientist Robert Hooke (1635–1703).
A card held to the edge of a spinning toothed wheel will produce a tone whose pitch varies with the speed of the wheel. A mechanism of this sort, made using brass wheels, allowed Hooke to produce sound waves of a known frequency, and to demonstrate to the Royal Society in 1681 how pitch relates to frequency. For practical purposes Hooke's device was soon supplanted by the invention of the tuning fork.
About a century and a half after Hooke's work, the mechanism was taken up again by Savart for his investigations into the range of human hearing. In the 1830s Savart was able to construct large, finely-toothed brass wheels producing frequencies of up to 24 kHz that seem to have been the world's first artificial generators. In the later 19th century, Savart's wheels were also used in physiological and psychological investigations of time perception.
Nowadays, Savart wheels are commonly demonstrated in physics lectures, sometimes driven and sounded by an air hose (in place of the card mechanism).
The basic device consists of a ratchet-wheel with a large number of uniformly spaced teeth. When the wheel is turned slowly while the edge of a card is held against the teeth a succession of distinct clicks can be heard. When the wheel is spun rapidly it produces a shrill tone, whereas if the wheel is allowed to turn more slowly the tone progressively decreases in pitch. Since the frequency of the tone is directly proportional to the rate at which the teeth strike the card, a Savart wheel can be calibrated to provide an absolute measure of pitch. Multiple wheels of different sizes, carrying different numbers of teeth, can also be attached so as to allow several pitches (or chords) to be produced while the axle is being turned at a constant rate.
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