Cathode ray tube
- 1 - CERN Teachers Lab
Quick reference guide
Introduction
The Cathode Ray Tube or Braun’s Tube was invented by the German
physicist Karl Ferdinand Braun in 1897 and is today used in computer
monitors, TV sets and oscilloscope tubes. The path of the electrons in
the tube filled with a low pressure rare gas can be observed in a
darkened room as a trace of light. Electron beam deflection can be
effected by means of either an electrical or a magnetic field.
Functional principle
• The source of the electron beam is the electron gun, which produces a stream of electrons through
thermionic emission at the heated cathode and focuses it into a thin beam by the control grid (or
“Wehnelt cylinder”).
• A strong electric field between cathode and anode accelerates the electrons, before they leave the
electron gun through a small hole in the anode.
• The electron beam can be deflected by a capacitor or coils in a way which causes it to display an
image on the screen. The image may represent electrical waveforms (oscilloscope), pictures
(television, computer monitor), echoes of aircraft detected by radar etc.
• When electrons strike the fluorescent screen, light is emitted.
• The whole configuration is placed in a vacuum tube to avoid collisions between electrons and gas
molecules of the air, which would attenuate the beam.
UA
Cathode Anode
Flourescent screen
Control grid
