How was the radiation detected?

In 1896, the French physicist Henri Becquerel (1852-1908) discovered the emission of unknown penetrating radiation by uranium salts, Becquerel studied the phenomenon of luminescence, in particular, of uranium salts, irradiated by sunlight. On a photographic plate, wrapped in thick black paper, he put grains of uranium salt and exposed it to sunlight. After development, the photographic plate turned black in the places where grains of uranium salt lay. The experiment took place on a bright sunny day, so Becquerel believed that the uranium salt, under the influence of direct sunlight, created an unknown radiation acting on the photographic plate. The researcher failed to conduct another experiment because it was a cloudy day. Becquerel put the photographic plate into a drawer, placing a copper cross covered with uranium salt on it. Having developed a photographic plate, he found on it a blackening in the shape of a cross. It followed from experience that in the absence of sunlight, uranium salt spontaneously creates unknown radiation.

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