The adventures of Arago

One of the first scientists to study eddy currents, François Arago, also served as the 25^th Prime Minister of France in 1848 (a provisional position, Arago held the post for just over a month) and campaigned for the abolition of slavery. Arago noted that when a copper disc was placed beneath a compass, the oscillations of the needle were damped. His name is commemorated in a piece of apparatus, Arago’s disc, which consists of a copper disc, rotated with a hand crank, situated beneath a compass needle. The motion of the disc relative to the needle induces eddy currents in the copper, which in turn, cause the needle to move. Arago was unable to account for the motion, and the explanation of the phenomenon was later supplied by Michael Faraday.

In addition to his scientific and political work, Arago lived an adventure-filled life which he described in vivid detail in his autobiography. He grew up close to the Pyrenees and reported an early encounter with Spanish soldiers:

The Spanish troops in their retreat had partly mistaken their road… I ran immediately to the house to arm myself with a lance which had been left there by a soldier of the levée en masse, and placing myself in ambush at the corner of a street, I struck with a blow of this weapon the brigadier placed at the head of the party. The wound was not dangerous; a cut of the sabre, however, was descending to punish my hardihood, when some countrymen came to my aid, and, armed with forks, overturned the five cavaliers from their saddles, and made them prisoners. I was then seven years old.

With fellow French physicist Jean-Baptiste Biot, Arago was commissioned to carry out a survey of the meridian line, and, on a visit to Valencia, he met the city’s archbishop. For some reason the meeting soured, and the archbishop hit Arago with a “movement, which was very near breaking my teeth, a gesture which I might justly call a blow of the fist”.

Elsewhere, Arago and Biot were attacked by two men in an argument over a woman and, while fleeing in his carriage, Arago accidentally killed one of his assailants.

During the peninsula war between Spain and France, Arago was captured by locals in Majorca who assumed that his triangulation equipment was evidence of espionage activities. He was imprisoned but escaped by boat to Algeria, from where he returned to France bearing two lions as a gift from the ruler of Algiers to Napoleon.

Just as Marseilles was in sight, his boat was captured by a Spanish vessel and he was returned to Spain where he was imprisoned: first in a windmill, then a fortress and finally a prison ship. Arago wrote to the Dey of Algiers, to inform him that one of his lions had died during the Spanish attacks. In response, the Dey threatened Spain with war unless the Frenchman was released.

Liberated, Arago took a boat to Marseilles, but a storm damaged his ship, driving it to Sardinia and then onto Algeria. Arago set out over land, again hoping to seek help from the Dey of Algiers. However, in the interim, the Dey had been beheaded and the new ruler imprisoned Arago as a slave and demanded a ransom for his release.

Fortunately for Arago, the new Dey was himself deposed and hanged, enabling Arago to set out for France. Again in sight of Marseilles, the hapless physicist’s ship was intercepted by a British vessel blockading the port. But this time, happily, Arago was allowed to proceed to Paris where he received a hero’s welcome.

Arago became friends with Jules Verne who used the physicist’s accounts of his experiences as the basis for two novels set in the Balearic Islands.

Arago and Biot’s survey is commemorated by a series of 135 bronze medallions marking the Paris meridian running from Montmartre in the north to the Paris Observatory in the south (though not all of the medallions are currently visible).