Wilhelm Conrad Röntgen discovered X-rays in 1895. He was a German mechanical engineer and physicist who produced and detected electromagnetic radiation in a wavelength range known as X-rays or Röntgen rays. Röntgen studied cathode radiation, which occurs when an electrical charge is applied to two metal plates inside a glass tube filled with rarefied gas. While experimenting with electric current flow in a partially evacuated glass tube, he observed that a screen of barium platinocyanide far from the tube gave off light when the tube was in operation. He theorized that when the cathode rays (electrons) struck the glass wall of the tube, some unknown radiation was formed that traveled across the room, struck the chemical, and caused the fluorescence. Further investigation revealed that paper, wood, and aluminum, among other materials, are transparent to this new form of radiation. He found that it affected photographic plates, and, since it did not noticeably exhibit any properties of light, such as reflection or refraction, he mistakenly thought the rays were unrelated to light. In view of its uncertain nature, he called the phenomenon X-radiation, though it also became known as Röntgen radiation.