Elizabeth Zimmerman, age 15, of Pocatello, Ida., for her question:
WHO DISCOVERED THE X RAY?
An X ray is a penetrating electromagnetic radiation, having a shorter wave length than light and produced by bombarding a target, usually made of tungsten, with high speed electrons. X rays were discovered accidentally in 1895 by a German physicist named William Roentgen noticed that a barium platinocyanide screen, inadvertently lying nearby, emitted fluorescent light whenever the tubes were in operation.
After conducting further experiments, Roentgen determined that the fluorescence was caused by invisible radiation of a more penetrating nature than ultraviolet rays.
Roentgen named the invisible radiation "X ray" because of its unknown nature. Subsequently, X rays were known also as Roentgen rays in his honor.
X rays are electromagnetic radiation ranging in wavelength from about 100 A to 0.01 A. An A is equivalent to about 4 billionths of an inch. The shorter the wavelength of the X ray, the greater is its energy and its penetrating power.
Longer wavelengths, near the ultraviolet ray band of the electromagnetic spectrum, are known as soft X rays. The shorter wavelengths, closer to and overlapping the gamma ray range, are called hard X rays.
A mixture of many different wavelength is known as "white" X rays, as opposed to "monochromatic" X rays, which represent only a single wavelength.
Both light and X rays are produced by transitions of electrons that orbit atoms, light by the transitions of outer electrons and X rays by the transitions of inner electrons.
X rays are produced whenever high velocity electrons strike a material object. Much of the energy of the electrons is lost in heat and the remainder produces X rays.
The first X ray tube was the Crookes tube, a partially evacuated glass bulb containing two electrodes, named after its designer, the British chemist and physicist Sir William Crookes.
An improvement in the X ray tube was the introduction of a curved cathode to focus the beam of electrons on a heavy metal target, called the anticathode, or anode. This type generates harder rays of shorter wavelengths and of greater energy than those produced by the original Crookes tube, but the operation of such tubes is erratic because the X ray production depends on the gas pressure within the tube.
The next great improvement was made in 1913 by an American physicist named William Coolidge. The Coolidge tube is highly evacuated and heated filament and a target.
Most X ray tubes in present day use are modified Coolidge tubes. The larger and more powerful tubes have water cooled anticathodes to prevent melting under the impact of the electron bombardment.
The widely used shockproof tube is a modification of the Coolidge tube with improved insulation of the envelope (by oil) and grounded power cables.