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ID number:  BIRRC-P0880
Institution:  Research and Cultural Collections
Named collection:  Historic Physics Collection
Artist / Maker:  Ducretet et Roger
Title / Object name:  Foucault apparatus for determining the speed of light
Object type:  Optical instrument
Date made:  c. 1909
Materials:  Mainly brass and steel
Measurements:  20 x 15 x 25 cm

Léon Foucault devised a technique for determining the speed of light using a mirror that was spinning rapidly about an axis in its plane. A beam of light was reflected from this mirror to a distant mirror and back. On its return the mirror had moved slightly and the beam reflected from it was deflected by an amount proportional to the speed of rotation. In 1862 he successfully measured the speed of light over a path of 40 m with a speed of rotation of 400 revs per second.
Our apparatus is very similar to that used by Foucault, the mirror being rotated by an air-driven turbine situated just above the mirror. It has very good bearings, with facility for supporting the lower one with compressed air. It would have been sold for teaching purposes.

Inscriptions / Translations:  F. Ducretet & E. Roger
B’ham. Univ. Physics

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