X-RAYS. A radiation proceeding out from the interior of a highly exhausted glass bulb through which a discharge of eleetrieity is taking place. This radiation was discovered in 1895 by Professor INintgen (q.v.) of the University of Wiirzburg, who was performing some experiments involving the discharge of electricity ,through glass bulbs exhausted until they contained only traces of air. This radiation was discovered, and, as far as could be told, differed in all respects from light and other known radiations. To it Professor Riintgen gave the name of N-rays, signifying that its cause was unknown.
Professor lhhitgen discovered that the essen tial feature iu its production was an exhausted bulb or Crooke tube (q.v.) into which entered two metal conductors serving to introduce and to withdraw the electric current. The conductor which introduces the current may be an ordinary wire or it may have at its end inside the bulb a small metal plate; it is called the 'anode.' The conductor by which the leaves the ex hausted bulb consists of a wire carrying at its end inside the bulb a concave metal surface: it is called the 'cathode.' Professor I intgen found that the centre from which the X-rays proceeded was the points of the solid walls of the bulb which were struck by the cathode rays proceed ing out from the cathode, and he therefore de vised the plan of introducing in the bulb a square plate of platinum directly facing the cathods so that it received practically all the cathode rays, and served, therefore, as the source of X-rays for all outside points. It was found later that
the intensity of the radiation could be increased by joining the anode to this small piece of plati num, which is called the 'anti-cathode.' Ili5ntgen observed that as the electrical discharge was con tinued through such a tube the intensity and character of the radiation of N-rays varied. This is owing in the main to a change in the vacuum, i.e. in the pressure of the traces of gas left in the bulb, occasioned by the electric current. In order, therefore, to maintain constant conditions in the tube it is necessary to keep the gas at a constant pressure, and the principle involved in the construction of all modern forms of X-ray tubes is to have such side attachments to the main bulb as will permit the evolution of minute amounts of gas whenever the vacuum in the bulb becomes too perfect. Bulbs satisfying all these requirements are furnished by instrument-makers under the name of 'X-ray self-acting tithes.' The main properties of X-rays were discovered by Professor PaMtgen himself immediately after his first observation. These may be described briefly under the following heads: