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Computer memory types

The Selectron was an early form of digital computer memory developed by Jan A. Rajchman and his group at the Radio Corporation of America under the direction of Vladimir Zworykin, of television technology fame. The team was never able to produce a commercially viable form of Selectron before core memory became almost universal, and it remains practically unknown today.



4096-bit Selectron tube
256-bit Selectron tube

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. RCA responded with the Selectron with a capacity of 4096 bits, with a planned production of 200 by the end of the year. They found the device to be much more difficult to build than expected, and they were still not available by the middle of 1948. As development dragged on the IAS machine was forced to switch to Williams tubes for storage, and the primary customer for Selectron disappeared.

RCA continued work on the concept, re-designing it for a smaller 256-bit capacity. The 256-bit Selectron was projected to cost about $500 each when in full production. While they were more reliable and faster than the Williams tube, that cost and the lack of availability, meant they were used only in one computer: the RAND Corporation's JOHNNIAC.[2]

Both the Selectron and the Williams tube were superseded in the market by the more compact and cost effective magnetic core memory, in the early 1950s.


The original 4096-bit Selectron[3] was a 10-inch long by 3-inch diameter vacuum tube configured as 1024 by 4 bits. 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. The bits were stored as discrete regions of charge on the smooth surface of the signal plates.

The two sets of orthogonal grid wires were normally "biased" slightly positive, so that the electrons from the cathode could flow through the grid and reach the dielectric. The continuous flow of electrons allowed the stored charge to be continuously regenerated by the secondary emission of electrons. 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.

Writing was accomplished by selecting a bit, as above, and then sending a pulse of potential, either positive or negative, to the signal plate. With a bit selected, electrons would be pulled onto (with a positive potential) or pushed from (negative potential) the dielectric. When the bias on the grid was dropped, the electrons were trapped on the dielectric as a spot of static electricity.

Selectron cross section

To read from the device a bit location was selected and a pulse sent from the cathode. If the dielectric for that bit contained a charge, the electrons would be pushed off the dielectric and read as a brief pulse of current in the signal plate. No such pulse meant that the dielectric must not have held a charge.

The smaller capacity 256-bit (128 by 2 bits) "production" device[4] was in a similar vacuum tube envelope. It was built with two storage arrays of discrete "eyelets" on a rectangular plate, separated by a row of eight cathodes. The pin count was reduced from 44 for the 4096-bit device down to 31 pins and two coaxial signal output connectors.


  1. ^ Metropolis N, Rajchman, JA (1980) Early Research on Computers at RCA A History of Computing in the Twentieth Century pp 465-469, ISBN 0124916503
  2. ^ Greuenberger JF (1968) The History of the JOHNNIAC pp 25-27
  3. ^ Rajchman JA, (1947) "The Selectron -- A Tube for Selective Electrostatic Storage", Mathematical Tables and Other Aids to Computation 2 (20):359-361
  4. ^ Rajchman JA (1951) "The Selective Electrostatic Storage Tube", RCA Review 12 (1):53-97


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