They were crucial to the development of radio, television, radar, sound recording and reproduction, long-distance telephone networks, and analog and early digital computers. These devices became a key component of electronic circuits for the first half of the twentieth century. Adding one or more control grids within the tube allows the current between the cathode and anode to be controlled by the voltage on the grids.
Electrons can only flow in one direction through the device-from the cathode to the anode. The simplest vacuum tube, the diode, invented in 1904 by John Ambrose Fleming, contains only a heated electron-emitting cathode and an anode. Not shown are the impedances (resistors or inductors) that would be included in series with the C and B voltage sources. Illustration representing a primitive triode vacuum tube and the polarities of the typical dc operating potentials.
