Selectron tube
The Selectron was an early form of digital computer memory developed by Jan A. Rajchman and his group at the Radio Corporation of America (RCA) under the direction of Vladimir K. Zworykin.
Development of Selectron started in 1946 at the behest of John von Neumann of the Institute for Advanced Study,[1] who was in the midst of designing the IAS machine and was looking for a new form of high-speed memory.
As development dragged on, the IAS machine was forced to switch to Williams tubes for storage, and the primary customer for Selectron disappeared.
RCA lost interest in the design and assigned its engineers to improve televisions[2] A contract from the US Air Force led to a re-examination of the device in a 256-bit form.
Rand Corporation took advantage of this project to switch their own IAS machine, the JOHNNIAC, to this new version of the Selectron, using 80 of them to provide 512 40-bit words of main memory.
As a result, RCA assigned their engineers to color television development, and put the Selectron in the hands of "the mothers-in-law of two deserving employees (the Chairman of the Board and the President).
To read the display, the beam scanned the tube again, this time set to a voltage very close to that of the secondary emission threshold.
The main difference was the holding gun, which fired continually and unfocussed so it covered the entire storage area on the phosphor.
[6] In comparison, the Williams tube used much higher voltages, producing a pattern that could only be stored for a short period before it decayed below readability.
[6] The grid also had the advantage of breaking the display into individual spots without requiring the tight focus of the Williams system.
The Selectron further modified the basic holding gun concept through the use of individual metal eyelets that were used to store additional charge in a more predictable and long-lasting fashion.
It had an indirectly heated cathode running up the middle, surrounded by two separate sets of wires — one radial, one axial — forming a cylindrical grid array, and finally a dielectric storage material coating on the inside of four segments of an enclosing metal cylinder, called the signal plates.
To select a bit to be read from or written to, all but two adjacent wires on each of the two grids were biased negative, allowing current to flow to the dielectric at one location only.
This not only made operation faster due to the lack of required pauses but also meant the data was much more reliable as it was constantly refreshed.
This version included visible green phosphors in each eyelet[citation needed] so that the bit status could also be read by eye.
