Cathode ray tubes are a remarkable technology that
incorporate many seemingly magic principles of physics. Thermionic emission causes electrons to
“boil” off a cathode, high voltage electric fields accelerate and focus them, and
magnetic fields steer them to the anode screen where they energize phosphor
molecules, which then re-release that energy as visible light!
While developing the electronics to control the CRT and make
all this magic happen, we often had to “bring up the spot”, showing the
electron beam in one static location, where it could be examined visually and
measured with various instruments.
Those who know me would be stunned to learn that I have a
gun collection. I acquired them in the course
of my work trying to make computer images on film in the 1990s. They are electron guns, the mysterious
workings at the business end of a cathode ray tube.
This is the first of three posts describing a now-(nearly)-obsolete technology.
Thomas Edison nearly discovered them. In his experiments with heated filaments in evacuated glass bulbs trying to find a suitable incandescent lamp, there were hints. He noticed depositions of material on the walls of the glass tubes. Many scientific discoveries are preceded not by the expression “Eureka”, but instead by the comment: “Hmm, that’s funny”. If he had followed up on this odd result, he might have also invented the vacuum tube amplifier.