What did uranium rays cause the air around Marie Curie's samples to conduct?

Uranium rays caused the air around Marie Curie’s samples to conduct electricity. This ionization effect gave Curie a measurable way to compare radioactive materials rather than relying only on photographic darkening.

Curie studied uranium radiation soon after Becquerel’s discovery and used a highly sensitive electrometer developed by Pierre Curie and his brother Jacques. The instrument detected electrical changes produced when radiation made nearby air conductive. This method became the foundation of her systematic measurements.

Her early results showed that the activity of uranium compounds depended on the amount of uranium present. That observation suggested that the radiation came from the atom itself, not from a chemical reaction or a temporary interaction between molecules. It helped undermine the older idea that atoms were indivisible.

The effect should not be confused with magnetism, heat, or sound. Radiation ionizes air by producing charged particles, allowing electricity to pass more readily. Curie used this property to identify unusually active minerals, including pitchblende and chalcolite.