How it began
Henri Becquerel held the chair of applied physics at the National Museum of Natural History in Paris and studied phosphorescence and uranium compounds. After hearing of Röntgen's X-rays in January 1896, he tested whether phosphorescent uranium salts exposed to sunlight emitted similar rays. When the weather turned overcast in late February, he put his uranium crystals and wrapped photographic plates away in a drawer. On March 1 he developed the plates and found a clear image. The next day he reported it to the Academy of Sciences.
Before it existed
Becquerel's starting idea, which turned out to be wrong, was that uranium salts absorbed sunlight and re-emitted it as X-rays. According to the American Physical Society, about 40 years earlier the photographer Abel Niépce de Saint-Victor had made the same accidental observation with uranium compounds and photographic paper kept in a dark drawer.
How it works
Becquerel placed crystals of uranium salt on photographic plates wrapped in black paper. The plates darkened where the crystals lay, even without sunlight, and objects such as coins placed in between left outlines. He found that non-phosphorescent uranium compounds did the same, and in May 1896 he announced that the element uranium itself emitted the radiation. Unlike X-rays, his rays could be deflected by electric or magnetic fields.
How it spread
Many scientists were still absorbed in X-rays, but in 1898 Marie and Pierre Curie in Paris began to study the uranium rays. They worked out how to measure their intensity and identified further radioactive elements, including polonium and radium. Marie Curie coined the term radioactivity. Ernest Rutherford then separated the rays into alpha, beta, and gamma radiation. In 1902 Rutherford and Frederick Soddy explained radioactivity as a spontaneous transmutation of elements. Becquerel and the Curies shared the 1903 Nobel Prize in Physics for their work on radioactivity.
What lasted
Becquerel's finding started a series of discoveries rewarded with numerous Nobel prizes. Irène Joliot-Curie and Frédéric Joliot later produced radioactivity artificially, and the ability to make radioisotopes of different elements opened tracer studies in biology and medicine.
