Marie Curie found that pitchblende was four times as active as uranium itself. The unexpectedly high activity suggested that the mineral contained a small amount of an unknown, much more radioactive substance.
Curie measured activity with an electrometer, using the ability of uranium radiation to make surrounding air conduct electricity. Her results showed that uranium compounds’ activity tracked their uranium content, so pitchblende’s much stronger signal could not be explained simply by the uranium it contained.
She also examined chalcolite, now commonly called torbernite, and found it twice as active as uranium. Pitchblende’s fourfold activity was the more dramatic clue. Curie reasoned that both minerals might contain a new element with extraordinary activity.
That insight led to the Curies’ search for additional radioactive substances. In 1898 they announced polonium, named for Marie’s native Poland, and radium. A common mix-up is to reverse the figures: chalcolite was twice as active, while pitchblende was four times as active.