What Are X Rays?
At the end of 1895, Wilhelm Röntgen (there is a drawing of him in Figure 1.2) was experimenting using a vacuum tube (a glass tube from which the air has been removed) which had a positively-charged plate at one end and a negatively-charged plate at the other. It was well known at that time that in this setup, rays, then called cathode rays, would travel from one plate to the other. It was only later, in 1897, that Joseph John Thomson discovered that these rays were in fact electrons. Röntgen observed that, on hitting the positive plate, the electrons produced some penetrating radiation, which could be detected outside the glass tube by its ability to blacken a photographic film. He called these new rays “X rays”, with the “X” standing for something unknown.