Crookes Tube - How A Crookes Tube Works

How A Crookes Tube Works

Crookes tubes are cold cathode tubes, meaning that they do not have a heated filament in them that releases electrons like the later electronic vacuum tubes usually do. Instead, electrons are generated by the ionization of the residual air by a high DC voltage (from a few kilovolts to about 100 kilovolts) applied between the electrodes, usually by an induction coil (a "Ruhmkorff coil"). The Crookes tubes require a small amount of air in them to function, from about 10−6 to 5×10−8 atmosphere (7×10−4 - 4×10−5 torr or 0.1 - 0.005 pascal).

When high voltage is applied to the tube, the electric field accelerates the small number of electrically charged ions always present in the gas, created by natural processes like radioactivity. These collide with other gas molecules, knocking electrons off them and creating more positive ions in a chain reaction. All the positive ions are attracted to the cathode or negative electrode. When they strike it, they knock large numbers of electrons out of the surface of the metal, which in turn are repelled by the cathode and attracted to the anode or positive electrode. These are the cathode rays.

Enough of the air has 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 fluorescence, 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 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.