A reading that should have been smaller
After Becquerel found that uranium salts darkened photographic plates, Marie Curie measured the electrical conductivity produced when the rays ionized air. The activity depended on the amount of uranium, not on its chemical form. She concluded that the radiation came from the atom itself.
Pitchblende was more active than pure uranium. Something unknown had to be hiding in the ore.
Diagram description
Static physics diagram: curie_activity_measurement.
From a number to tons of residue
In 1898 Marie and Pierre Curie announced polonium and radium. Naming them was quick. Isolating them meant crushing, dissolving, stirring, crystallizing, and repeatedly measuring tons of mine residue in a rough shed.
They called the spontaneous emission radioactivity. It broke the old picture of permanent atoms: atoms could transform and release immense energy. The danger of prolonged exposure was not yet understood.
The discovery began not with glowing radium, but with a number that refused to fit. Marie Curie followed the discrepancy all the way to a new element.