Electromagnetic radiation with wavelengths between ultraviolet and gamma rays. It was discovered in 1895 by German physicist W.K. Roentgen, hence it is also called the Roentgen ray. X-rays with wavelengths less than 0.1 angstroms are called super-hard X-rays, those within the range of 0.1 to 1 angstroms are called hard X-rays, and those within the range of 1 to 10 angstroms are called soft X-rays. In the laboratory, X-rays are produced by an X-ray tube, which is a vacuum tube with a cathode and anode. The cathode is made of tungsten wire that emits thermoelectrons when electrified. The anode (also known as the target electrode) is made of high melting point metal (usually tungsten; for X-ray tubes used in crystal structure analysis, materials such as iron, copper, and nickel can also be used). Electrons are accelerated by a high voltage of tens of thousands to hundreds of thousands of volts, and the electron beam bombards the target electrode, causing X-rays to be emitted from the target electrode. When electrons bombard the target electrode, high temperatures are generated, so the target electrode must be water-cooled. Sometimes, the target electrode is also designed to rotate.