In 1896 Henri Becquerel discovered that uranium salts emitted rays that resembled X-rays in their penetrating power. Madame Curie decided to look into uranium rays as a possible field of research for a thesis. She found that the activity of the uranium compounds were characteristic of other substances, such as thorium. Marie named this phenomenon radioactivity.
To measure radioactive emission, Marie used an instrument called electrometer. Her husband and his brother had invented the instrument 15 years earlier. The instrument enabled them to measure event the smallest quantity of electric power. Marie showed that the radiation came from the atom itself.