Two cloudy days in Paris led Henri Becquerel to radioactivity
Henri Becquerel expected sunlight to make uranium salts give off X-ray-like rays. When overcast skies in late February 1896 halted his tests, he left the wrapped plates in a drawer, then developed them anyway. The images were just as sharp as before. The uranium had been radiating all by itself.
Becquerel was born in Paris in 1852 into a physics dynasty: his grandfather and father were notable physicists, and so was his son Jean. He trained as an engineer and wrote his doctoral thesis on polarised light and the way crystals absorb it and glow. At the natural history museum in Paris he became professor of applied physics in 1892, the same post his father and grandfather had held.
His long interest was phosphorescence, in which a substance lit by one colour afterwards shines in another. In January 1896 Henri Poincaré read Wilhelm Röntgen's paper on X-rays to the French Academy, and Becquerel wondered whether glowing materials also emitted penetrating rays. He wrapped photographic plates in thick black paper, laid coins or metal cut-outs and phosphorescent crystals on top, and set the bundles in the sun. The developed plates showed the shapes, and he reported this on 24 February.
Then the weather turned. On 26 and 27 February he stored the prepared plates in a dark cabinet, and on 1 March developed them to find the silhouettes as distinct as ever, with William Crookes and his 18-year-old son watching. By May, tests with uranium salts that did not glow at all confirmed that the radiation came from the uranium itself, without any outside energy. He published seven papers on it that year, and later showed that magnetic fields bend different emissions differently, revealing positive, negative and neutral kinds.
The discovery had nearly been made decades earlier. In 1857 Abel Niépce de Saint-Victor noticed uranium salts darkening photographic emulsions, and Becquerel's own father mentioned the effect in an 1868 book. Marie and Pierre Curie followed up by finding polonium and radium, and the three shared the 1903 Nobel Prize in Physics. Becquerel died in 1908.
Source: Henri Becquerel
