Fizeau's measurement of the speed of light in air

From 1848 to 1849, Hippolyte Fizeau used a toothed wheel apparatus to perform absolute measurements of the speed of light in air.

Subsequent experiments performed by Marie Alfred Cornu from 1872 to 1876 improved the methodology and made more accurate measurements.

[3] At the behest of the Paris Observatory under Urbain Le Verrier, Marie Alfred Cornu repeated Fizeau's 1848 toothed wheel measurement in a series of experiments from 1872 to 1876.

An electric circuit recorded the wheel rotations on a chronograph chart, which enabled precise rate comparisons against the observatory clock.

A telegraph key arrangement allowed Cornu to mark the precise moments when he judged that extinction had been entered on this same chart or exited.

Figure 1: Schematic of the Fizeau apparatus. The light passes on one side of a tooth on the way out, and the other side on the way back, assuming the cog rotates one tooth during transit of the light. ^{[

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Figure 2: Chronograph record from Cornu's speed of light determination shows wheel rotations, timing signals based on the observatory clock, and observer markings. ^{[

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