William Conrad Roentgen

The Discovery of X-Rays

In 1895 William Conrad Roentgen became interested in the ultraviolet radiation emitted by cathode-ray tubes. Because the eye cannot detect ultraviolet radiation, experiments of this type require a UV detector. Roentgen used a screen coated with barium tetracyanoplatinate [BaPt(CN)₄] because this compound emits light, or fluoresces, when exposed to UV radiation.

One evening in November, 1895, Roentgen was working with a cathode-ray tube that had been carefully wrapped with black cardboard. Much to his surprise, the BaPt(CN)₄ screen next to the cathode-ray tube gave off light when the tube was switched on. Obviously, something had hit the screen to make it emit light. It was equally obvious that it couldn't have been either UV radiation or cathode rays because neither of these substances could pass through the opaque cardboard.

In an intense series of experiments over a seven-week period, Roentgen found that this new kind of radiation -- which he called x-rays -- passed through solid objects placed between the cathode-ray tube and the detector. He even found that he could see the image of the bones in his hand when he held it between the tube and the screen. Roentgen eventually discovered that he could capture such images on film and one of his first x-ray images is reproduced below.