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August Kundt made standing wave patterns of sound waves visible through the dust patterns in Kundt's tube. Neighboring repeat marks in the ideal pattern are separated by half a wavelength, so a measured spacing $d$ and known frequency $f$ give speed of sound $c=2fd$. The tube turns an oscillation too rapid to follow directly into a spatial ruler. Comparing resonance conditions in a solid rod and a gas also connects the method with measurements of sound propagation in different materials.^[ref]

A physicist whose appointments took him from Zürich in 1868 through Würzburg and Strasbourg to the Berlin chair of experimental physics in 1888.^[ref]