X rays were discovered in Germany in 1895 by Wilhelm Roentgen (1845-1923) quite by accident while he was studying the conduction of electricity through gases at low pressure. The discovery was made when these mysterious "X" rays were observed to light up a fluorescent screen a few meters from the source. Roentgen soon found that these rays were quite penetrating and was actually able to insert his hand between the source and the screen and see on the screen the faint shadow of the bones in his hand. This indicated that more dense materials such as bone absorbed more x rays than less dense material such as human flesh. He soon found that photographic plates were sensitive to x rays and was able to make the first crude x-ray photographs.
Roentgen had been experimenting with what was called a cathode ray discharge tube, i.e., a partially evacuated glass tube with metal electrodes at each end. When a high electrical voltage was applied between the electrodes a discharge took place in the tube. One effect of the discharge was to produce electrons which acquired high velocities as they were attracted to the positive electrode. When they hit this metal electrode the x rays were produced. It was not until 1913 in the United States that W. D. Coolidge (1873-1975) invented the xray tube similar to those still used today. Coolidge removed as much air from the tube as possible and used a hot tungsten filament as the source of electrons. This permitted more careful experiments in which the high voltage applied to the tube and the rate at which electrons hit the target could be varied independently.