English: Experiment with Crookes tubes illustrating that cathode rays travel in straight lines, performed by William Crookes in 1879. Shows two identical Crookes tubes, with a high DC voltage of several thousand volts from an induction coil applied between the concave cathode (a) (negative electrode) on the right side and three anodes (b, c, d) (positive electrodes). The lefthand tube has a low vacuum of around 10^-3 atm. The high voltage ionizes the gas atoms in the tube, making it conductive, allowing electrons to flow from the cathode (a) to the anodes. Because of the higher density of gas atoms in this tube, the electrons constantly collide with gas atoms and are confined to curving arcs of conductive gas between the cathode and the three anodes. The righthand tube is evacuated to a higher vacuum of 10^-6 atm. Since most of the gas atoms are removed, electrons released by ion bombardment of the cathode travel unobstructed at high speed in straight lines, called cathode rays. The cathode rays are emitted perpendicularly from the surface of the cathode. Since the cathode is in the form of a concave surface, the cathode rays are focused to a point.