X-rays were discovered in 1895 by Wilhelm Conrad Roentgen. By 1896 X-rays were being used by battlefield physicians to locate bullets in wounded soldiers. X-rays were used primarily in medicine and dentistry until 1912. When high-vacuum tubes were invented by William Coolidge, which produced up to 100,000 volts, higher voltages produced rays of adequate penetrating power for industrial applications.
A 200,000-volt X-ray tube was created in 1922 that allowed radiographs of thick steel parts to be produced in a reasonable amount of time. In 1931 General Electric Company developed 1,000,000 volt X-ray generators, providing an effective tool for industrial radiography. That same year, the American Society of Mechanical Engineers (ASME) permitted X-ray approval of fusion-welded pressure vessels, further opening the door to industrial use. In 1975 Robert Ledley patented the CAT scan, using X-rays in a spiral pattern to compile multiple images of the body in "slices" 3.5 millimeters apart.
The first warning of possible adverse effects of X-rays came from Thomas Edison, William J. Morton and Nikola Tesla, who each reported eye irritations from experimentation with X-rays and fluorescent substances. Today, the negative side effects of radiation have been thoroughly investigated. Radiation levels are specified and controlled, so that medical, scientific and industrial uses may continue with risks no greater than any other technology.