been removed from the tube that most of the electrons can travel the length of the tube without striking a gas molecule. The high voltage accelerates these low-mass particles to a high velocity (about 37,000 miles per second, or 59,000 km/s, about 20 percent of the speed of light, for a typical tube voltage of 10 kV). When they get to the anode end of the tube, they have so much momentum that, although they are attracted to the anode, many fly past it and strike the end wall of the tube. When they strike atoms in the glass, they knock their orbital electrons into a higher energy level. When the electrons fall back to their original energy level, they emit light. This process, called cathodoluminescence, causes the glass to glow, usually yellow-green. The electrons themselves are invisible, but the glow reveals where the beam of electrons strikes the glass. Later on, researchers painted the inside back wall of the tube with a phosphor, a fluorescent chemical such as zinc sulfide, in order to make the glow more visible. After striking the wall, the electrons eventually make their way to the anode, flow through the anode wire, the power supply, and back to the cathode. The above only describes the motion of the electrons. The full details of the action in a Crookes tube are complicated, because it contains a nonequilibrium plasma of positively charged ions, electrons, and neutral atoms which are constantly interacting. At higher gas pressures, above 10−6 atm (0.1 Pa), this creates a glow discharge; a pattern of different colored glowing regions in the gas, depending on the pressure in the tube (see diagram). The details were not fully understood until the development of plasma physics in the early 20th century. History Crookes tubes evolved from the earlier Geissler tubes invented by the German physicist and glassblower Heinrich Geissler in 1857, experimental tubes which are similar to modern neon tube lights. Geissler tubes had only a low vacuum, around 10−3 atm (100 Pa), and the electrons in them could only travel a short distance before hitting a gas molecule. So the current of electrons moved in a slow diffusion process, constantly colliding with gas molecules, never gaining much energy. These tubes did not create beams of cathode rays, only a colorful glow discharge that filled the tube as the electrons struck the gas molecules and excited them,