Braun tube (cathode ray tube)

What it is

The cathode ray tube, still called the Braunsche Röhre in German, was published by Ferdinand Braun in February 1897 in the Annalen der Physik, while he was professor of physics in Strasbourg, then part of the German Empire. Cathode rays had been studied for decades, but Braun's tube was the first to turn a controlled electron beam into a measuring and display instrument. He used it to visualise alternating currents, making it the first oscilloscope.

How it works

A cathode in an evacuated glass tube emits electrons, which are accelerated towards the far end and narrowed into a beam. Where the beam hits a fluorescent coating, the screen glows. Magnetic coils or electric plates bend the beam, so a changing signal traces a moving line. Braun's assistant Jonathan Zenneck added refinements, and later engineers added a second deflection axis and scanned the beam line by line to paint a picture.

Why it stands out

• It was the basis of every television set and computer monitor until flat panels took over in the 2000s.
• Braun also discovered, in 1874 at the age of 24, that a metal wire touching a sulphide crystal conducts current in one direction only, the first semiconductor rectifier effect. He never built a device on it, but it became the crystal radio detector.
• He shared the 1909 Nobel Prize in Physics with Guglielmo Marconi for work on wireless telegraphy.
• He went to New York in 1914 as a witness in a radio patent case, could not return because of the war, and died in Brooklyn in April 1918.

Where you meet it

Today mainly in museums and in vintage oscilloscopes, radar and arcade cabinets. Its successors, LCD and OLED panels, rely on German chemistry too: liquid crystals were first identified at Karlsruhe and commercialised by Merck KGaA.

Sources

• IEEE Milestones: Ferdinand Braun
• University of Würzburg archive: Ferdinand Braun
• Computer History Museum: 1874 rectifier effect
• Wikipedia: Ferdinand Braun