Coolidge X-ray tube, 1913-1923.

Wilhelm Roentgen (1845-1923) was experimenting with cathode rays in 1895 when he noticed that his tubes could make a screen glow, even when there was something in the way. He had discovered X-rays. X-rays are emitted whenever fast-moving electrons are stopped in their tracks. Later X-ray tubes, like the one shown here, used a heated cathode.