I love these old training films I'm still pretty green so these are very helpful.
This WWII training film familiarizes radio operators with Vacuum Tubes. As the film explains, these are the devices used in radios that control electric current between electrodes in an evacuated container. Vacuum tubes mostly rely on thermionic emission of electrons from a hot filament or a cathode heated by the filament. The grid and filament forming the tube are explained, as are some basic circuits, and a couple of electronics packages are shown.