Henri Becquerel

Henri Becquerel discovered radioactivity by leaving a photographic plate in a drawer because the weather in Paris was too dull to run the experiment he had planned. He was the third of four generations to hold the same chair of physics at the Museum of Natural History: his grandfather Antoine Cesar worked on electrochemistry, his father Edmond on phosphorescence and solar radiation, and his son Jean followed him in turn. He trained at the Ecole Polytechnique and the Ecole des Ponts et Chaussees, worked as an engineer, and inherited both the chair and his father's collection of phosphorescent minerals. When Roentgen announced X-rays in January 1896, a natural guess was that phosphorescent substances stimulated by sunlight might emit them too, and Becquerel had the materials to hand to test it. He wrapped photographic plates in thick black paper, laid uranium salts on top, and put them in the sun. Overcast days at the end of February stopped the experiment, and he put plates and salts together in a dark drawer. On 1 March he developed the plates anyway and found them strongly fogged: the uranium was emitting something continuously, with no sunlight and no stimulation of any kind. He established within months that the emission depended only on how much uranium was present, not on its chemical compound or physical state, which is what made it a property of the atom rather than of a substance. He showed the rays were deflected by a magnetic field and were therefore partly charged particles. Marie Curie took up the phenomenon for her thesis and named it radioactivity, and the three shared the Nobel Prize in Physics in 1903. He carried a tube of radium in his waistcoat pocket for six hours in 1901 and described the burn it left, one of the first accounts of biological damage from radiation. He died suddenly at fifty-five. The SI unit of radioactivity is the becquerel.

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