Henri Becquerel - French Physicist
(1852-1908)
January 20, 1896
At the meeting of the French Academy of Sciences in Paris, Röntgen's discovery of X-rays is discussed. Becquerel immediately sees a connection between X-rays and fluorescence, which he had been studying. He performs a number of experiments on fluorescent materials and on March 2, 1896 announces that he had found that certain compounds of Uranium gave off new rays regardless if they fluoresced.
Becquerel tested a number of phosphorescent or fluorescent substances to see if they emited X-rays. His experimental procedure involved wrapping photographic plates with thick black paper and then placing the substance to be tested on the paper. This was then exposed to sunlight for several hours. If X-rays were emitted, they would pass through the paper and fog the plate. Becquerel's first tests gave negative results. However he eventually tested uranyl potassium sulfate, a uranium salt, and achieved positive results. At the end of February, Becquerel attempted to repeat this experiment, but it was cloudy in Paris so he placed his experimental setup in a drawer until March 1. He proceeded to develop the plate and found that the fogging was just as intense as the experiment in which the uranium salt had been exposed to sunlight. Becquerel realized that the fogging of the plate had nothing to do with the fluorescent properties of the salt or X-rays. He had discovered the phenomenon we now know as radioactivity.
The first plate fogged by "Becquerel's rays" - Courtesy of French CEA
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