1895

Wilhelm Roentgen discovered x-rays. While studying the luminescence (light) produced by cathode rays, Roentgen had placed a cathode ray tube in a box in a darkened room. (A cathode ray tube is a vacuum tube in which a cathode, or negatively charged electrode, sends out a stream of electrons.) A sheet of paper coated with a barium compound happened to be near the box. Roentgen noticed that when the tube was switched on in the closed box, the paper glowed brilliantly. He concluded that some sort of ray had penetrated the box and caused the paper to glow. Because he didn't know what they were or where they came from, he called them x-rays (x for unknown). He also noticed the rays caused photographic plates, even when wrapped in paper, to darken or fog. This led him to take x-ray photographs of objects such as his hand. The photographs revealed the inner structure of the objects.

The world immediately appreciated the medical potential of x-rays. X-rays revolutionized medicine because they enabled doctors to see the interior of the body without surgery. Within five years of the discovery, for example, the British Army began using a mobile x-ray unit to locate bullets and shrapnel in wounded soldiers in the Sudan.