Schematic diagram of a Crookes tube. This was a cold cathode discharge tube invented by William Crookes and other physicists around the 1870s in which cathode rays (electrons) were discovered. It consisted of a partially evacuated glass tube with two electrodes. When a high potential of several thousand volts is applied between the electrodes, cathode rays are emitted by the negative electrode (right) and travel in straight lines through the tube. When they hit the glass wall of the tube (left) they cause it to glow or fluoresce. In this common example, a metal cross is placed in the tube in front of the cathode, which blocks the rays. The shadow it throws on the glowing back wall demonstrates that the cathode rays travel in parallel lines.