Henri Becquerel discovered radioactivity on March 1, 1896

Henri Becquerel discovered radioactivity in Paris on March 1, 1896, when he developed photographic plates that uranium salts had darkened inside a closed drawer, with no sunlight involved. He reported the finding to the French Academy of Sciences the next day, March 2.

The breakthrough grew out of a wrong idea. Weeks after Röntgen announced X-rays, Becquerel, a phosphorescence specialist like his father and grandfather, suspected that uranium salts soaked up sunlight and re-emitted it as X-ray-like rays. On February 24 he told the Academy that sunlit crystals left outlines on plates wrapped in black paper. Then Paris clouded over, and he put his potassium uranyl sulfate and the plates away in a drawer.

On March 1 he developed them anyway, expecting a faint image. It was sharp. Nobody knows quite why he bothered: the American Physical Society suggests curiosity, pressure to report something at the next day’s meeting, or simple impatience.

Not quite the first

• 1857–1861: photographer Abel Niépce de Saint-Victor finds uranium salts exposing photographic material in the dark, and describes “a radiation that is invisible to our eyes”.
• 1868: Henri’s father, Edmond Becquerel, mentions the finding in his book on light. Nobody pursues it.
• March 1896: Henri repeats essentially the same drawer experiment, and this time follows it up.
• May 1896: he shows the element uranium is the source, since non-phosphorescent uranium compounds do it too.
• 1898: Marie and Pierre Curie take up the “uranium rays”, and Marie coins the word radioactivity.

What made Becquerel the discoverer was the follow-through: he showed the rays needed no light, did not fade with time and, unlike X-rays, could be bent by electric and magnetic fields. He shared the 1903 Nobel Prize in Physics with the Curies, and the SI unit of radioactivity, the becquerel, is one decay per second.