Crookes tube
The electronic vacuum tubes invented later around 1904 superseded the Crookes tube. These operate at a still lower pressure, around 10−9 atm (10−4 Pa), at which there are so few gas molecules that they do not conduct by ionization. Instead, they use a more reliable and controllable source of electrons, a heated filament or hot cathode which releases electrons by thermionic emission. The ionization method of creating cathode rays used in Crookes tubes is today only used in a few specialized gas discharge tubes such as thyratrons.