The vibration microscope is an electromagnetically-driven adaptation by Hermann von Helmholtz (1821-1894) of the earlier optical comparator by Jules Antoine Lissajous (1822-1880). The device allows one to determine the frequency of a tuning fork or other vibrating object with respect to a fork of known frequency, by way of Lissajous figure analysis.
Wooden sticks, when dropped on the floor, sound a variety of tones. While the bars of a xylophone are varied in tone by changing their length, these “tone bars” are all of the same length and width, but have different thicknesses and different densities and elastic properties.
Helmholtz described in his 1862 [sic] book, On the Sensations of Tone, an apparatus able to pick out specific frequencies from a sound. The Helmholtz resonator, as it is now called, consists of a rigid container of a known volume, nearly spherical in shape, with a small neck and hole in one end and a larger hole in the other end to admit the sound.
André Prosper Crova was a faculty member at the University of Montpelier. He invented this acoustic wave model in the 1860s and commissioned the prominent Parisian acoustic instrument maker Rudolph Koenig to manufacture it for him. It was first shown at the 1867 Paris World Fair.
„This instrument was used in an Ohio high school and probably dates from the late 19th or early 20th centuries. The replication of classic experiments was a common way to teach science at this time, and Chladni’s figures were considered to be both instructive and beautiful.
These instruments demonstrate the “communication of vibrations” between connected plates. This was a topic first investigated by the French scientist Felix Savart, in the 1820s. Savart experimented with a pair of identical glass disks that were connected by only a single glass rod. When the two disks were sprinkled with sand and the first one vibrated, both disks formed identical patterns.
Source: Steven Turner; Curator, Physical Sciences, National Museum of American History, Smithsonian Institution.
This rack has 3 square and 3 circular brass plates of varying dimensions. It was used to demonstrate the effect of changes in the size and thickness of plates on both their tone and the Chladni figures that they produce. A plate that is the same size as the one next to it, but double the thickness, will produce a note twice as high, while a plate that is half the area of the one next to it, but double the thickness, will sound a note that is four times higher.
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