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Arago, Dominique François Jean

disc needle spun experiment

[aragoh] (1786–1853) French physicist.

Beginning his career as a secretary at the Bureau de Longitudes, Arago went with to Spain in 1806 to complete the geodetic measurements of an arc of the meridian. The return journey was eventful as the ship was wrecked and he was almost enslaved at Algiers. He made distinguished researches in many branches of physics, and in 1838 suggested a crucial experiment to decide between the particle and wave theories of light, by measuring its speed in air and in water. The experiment was tried by in 1850 and pointed to the wave theory.

Arago was the first to discover that substances other than iron have magnetic properties. He discovered in 1820 the production of magnetism by electricity; a piece of iron, surrounded by a coil of wire, was briefly magnetized by passing a current from either a capacitor or a voltaic cell through the coil.

The device called Arago’s disc consists of a horizontal copper disc with a central vertical spindle; the disc can be spun by a belt and pulley. Above it, and separately mounted, is a pivoted compass needle. When the disc is spun, the needle follows it; if the rotation of the plate is reversed, the needle slows, stops and also reverses. The effect is due to eddy currents in the disc, although Arago did not know this. The value of his experiments in electricity and magnetism is that they later inspired to make his major discoveries.

Arago was a close friend of for half a century; the latter wrote of Arago as ‘one gifted with the noblest of natures, equally distinguished for intellectual power and for moral excellence’.

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