A blackened photographic plate provided the clue.

Watch carefully to see a plate blacken.

Notice how the invisible radiation blackened the photographic plate gradually over time.

Becquerel discovered that the rays were more penetrating than x-rays and that they discharged an electroscope.

Pierre & Marie Curie led much of the early research into radioactivity.

The Curies established that 1 gram of radium gives off about 420 Joules of energy every hour.

The Curies discovered the elements Radium and Polonium introduced the term radioactivity.

In 1896 Henri Becquerel discovered radioactivity. It happened by accident when he was investigating phosphorescence. Becquerel was aware of Roentgen’s discovery of x-rays in 1895 and thought there might be a relationship between phosphorescence and x-radiation.
In February 1896, Becquerel covered a photographic plate with black paper and coated the paper with a uranium salt. Following several hours exposure to sunlight, he removed the black paper and found that the photographic plate was clouded.

Two days later he prepared another photographic plate in the same way but the weather was cloudy and he put everything away in the one draw. On 1st March, Becquerel developed this second plate and found it to be cloudy. Whatever it was that had come from the uranium salt, penetrated the black paper and clouded the photographic plate, had nothing to do with sunlight or phosphorescence.

Becquerel had discovered some new kind of radiation that had come from the uranium salt spontaneously and without outside influence. This contrasted with x-rays where large voltages were needed to release the radiation.

Pierre and Marie Curie

Many scientists began investigating the new phenomenon. Marie and Pierre Curie also working in Paris did much of the pioneering work in this new branch of physics.
Becquerel had already noted that the radiation emitted by uranium ionised the surrounding air. Using this characterisation of ionisation Pierre and Marie Curie began searching for other materials that could emit radiation. When they tested an ore of uranium, called pitchblende, for its ability to ionise air. The Curies found that the pitchblende produced a current 300 times stronger than that produced by pure uranium.

The Curies believed that a very active unknown substance must exist within the pitchblende. After a lengthy and gruelling search the Curies identified two new elements present in pitchblende in tiny quantities both of which were capable of emitting penetrating radiation. In reporting on their findings, they introduced the new term: "radio-active." The two new elements they called Polonium (after Marie’s native country) and Radium.

More than one type of radiation

Ernest Rutherford, discovered in 1898 that two different types of radiation were produced by radioactive materials. They called them alpha and beta rays. Later a third type of radiation was identified by and called gamma rays.

More Information

Work of the Curies
Work of Rutherford
History of radioactivity