Earth and the Moon

Cosmic Rays

How were cosmic rays shown to be charged particles?

In 1925 American physicist Robert A. Millikan (1868–1953) lowered an electroscope deep into a lake and detected the same kind of powerful radiation that Victor Franz Hess had found in his balloon experiments. Millikan was the first to call this radiation cosmic rays, but he did not know what they were made of. In 1932, the American physicist Arthur Holly Compton (1892–1962) measured cosmic-ray radiation at many points on Earth’s surface and found that it was more intense at higher latitudes (toward the north and south poles) than at lower latitudes (toward the equator). He concluded that Earth’s magnetic field was affecting the cosmic rays, deflecting them away from the equator and toward Earth’s magnetic field. Since electromagnetism was now shown to affect the rays, it was clear that cosmic rays had to be electrically charged particles.


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