At the time the discovery of X-rays was announced by Prof. Wilhelm Conrad Rontgen of the University of Wurzburg, Germany, he was not sure of their exact nature, and so he named them "X-Rays," because "X" has always been understood to be the symbol for an "unknown quantity."

They are invisible rays transmitted through the air in a manner similar to light. They are produced by passing unidirectional electric current of from twenty to one hundred thousand volts pressure through a specially constructed high vacuum tube, within which rays radiating from the surface of a concave cathode (the negative electrode of a galvanic battery), are focused upon and bombard a target of refractory material such as tungsten, iridium, platinum, from which focus spot the X-rays radiate in all directions.

They are used in medicine and surgery, to photograph the skeleton and all the internal organs of the human body, as an aid in diagnosis; also to destroy diseased tissue without the aid of surgery. Cancers and tumors of certain kinds and a number of skin diseases are said to be made to disappear by their use. When the apparatus is used, the subject is placed on a long table and the X-ray tube, in its lead glass shield container, is brought over the part of the body to which the rays are to be applied.

The most up-to-date apparatus consists of a high-tension transformer and rectifier, driven by a rotary converter, which derives power from direct-current electric service and delivers alternating current to the high-tension transformer.

Modern X-Ray Apparatus.