Selectron tube

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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. Development started in 1946 at the behest of John von Neumann of the Institute for Advanced Study with a planned production of 200 by the end of the year, but production problems meant that they were still not available by the middle of 1948. By that time von Neumann's IAS machine, was forced to switch to the Williams tube for storage, and RCA eventually had to scale down the Selectron from storing 4096 bits, to 256. This smaller version saw only two design wins; eventually RCA gave up on the concept.

The original 4096-bit Selectron was a 10-inch long by 3-inch diameter vacuum tube with a 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 an enclosing metal cylinder called the signal plate. The bits were discrete physical locations on the signal plate.

The two sets of orthogonal grid wires were normally "biased" slightly negative, so that the electrons from the cathode could not flow through the grid and reach the dielectric. To select a bit, two adjacent wires on each of the two grids were biased positive, allowing current to move to the dielectric.

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 off of (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.

To read from the device a bit locations 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 "production" device was in a similar vacuum tube envelope, but built as a series of discrete "eyelets" on a rectangular plate. The glass envelope of the 256-bit tube was of approximately the same physical dimensions as the earlier cylindrical model. The pin count was reduced from 44 for the 4096-bit device down to 31 pins and two coaxial signal connectors. The 256-bit Selectron was projected to cost about $500 to build full production, and while they were more reliable and faster than the Williams tube, that cost meant they were used only in one computer, the RAND Corporation's JOHNNIAC.

Both the Selectron and the Williams-Kilburn CRT memories were superseded in the market by the more compact and cost effective core memory, in the early 1950s.