thumb|A Williams (or Williams-Kilburn) tube on display at [[The National Museum of Computing on Bletchley Park, Buckinghamshire]]

thumb|upright| [[James Pomerene with a Williams–Kilburn tube, a 5CP1A cathode ray tube, used in the memory array of the IAS computer ]]

thumb|upright|Williams–Kilburn tube from an [[IBM 701 at the Computer History Museum, in Mountain View, California]]

thumb|Memory pattern on [[SWAC (computer)|SWAC Williams tube CRT]]

The Williams tube, or the Williams–Kilburn tube named after British inventors Freddie Williams and Tom Kilburn, is an early form of computer memory. It was the first random-access digital storage device, and was used successfully in several early computers.

The Williams tube works by displaying a grid of dots on a cathode-ray tube (CRT). Due to the way CRTs work, this creates a small charge of static electricity over each dot. The charge at the location of each of the dots is read by a thin metal sheet just in front of the display. Since the display faded over time, it was periodically refreshed. It operates faster than earlier acoustic delay-line memory, at the speed of the electrons inside the vacuum tube, rather than at the speed of sound. The system was adversely affected by nearby electrical fields, and required frequent adjustment to remain operational. Williams–Kilburn tubes were used primarily on high-speed computer designs.

Williams and Kilburn applied for British patents on 11 December 1946, and 2 October 1947, followed by United States patent applications on 10 December 1947, and 16 May 1949.

Working principle

The Williams tube depends on an effect called secondary emission that occurs on cathode-ray tubes (CRTs). When the electron beam strikes the phosphor that forms the display surface, it normally causes it to illuminate. If the beam energy is above a given threshold (depending on the phosphor mix) it also causes electrons to be struck out of the phosphor. These electrons travel a short distance before being attracted back to the CRT surface and falling on it a short distance away. The overall effect is to cause a slight positive charge in the immediate region of the beam where there is a deficit of electrons, and a slight negative charge around the dot where those electrons land. The resulting charge well remains on the surface of the tube for a fraction of a second while the electrons flow back to their original locations. Rather than the Williams tube memory being designed for the Baby, the Baby was a testbed to demonstrate the reliability of the memory. Tom Kilburn wrote a 17-instruction program to calculate the highest proper factor of numbers as large as 2<sup>18</sup>. Tradition at the university has it that this was the only program Kilburn ever wrote.

Williams tubes tended to become unreliable with age, and most working installations had to be hand tuned. By contrast, mercury delay-line memory was slower and not random access, as the bits were presented serially, which complicated programming. Delay lines also needed hand tuning, but did not age as badly and enjoyed some success in early digital electronic computing despite their data rate, weight, cost, thermal and toxicity problems. The Manchester Mark 1, which used Williams tubes, was successfully commercialised as the Ferranti Mark 1. Some early computers in the United States also used Williams tubes, including the IAS machine (originally designed for Selectron tube memory), the UNIVAC 1103, IBM 701, IBM 702 and the Standards Western Automatic Computer (SWAC). Williams tubes were also used in the Soviet Strela-1 and in the Japan TAC (Tokyo Automatic Computer).

<gallery class="center" heights=220 widths=220>

Williams-tube.jpg|A Williams–Kilburn tube

WilliamsTubeFigure1.tiff|Diagram of Williams tube memory from the 1947 patent

File:Museum of Science, Boston, MA - IMG 3160.JPG|SWAC Williams tube assembly

File:SEACComputer 004.jpg|Diagram of SWAC Williams tube module

</gallery>

See also

  • Atanasoff–Berry computer&nbsp;– Used a type of memory called regenerative capacitor memory
  • Mellon optical memory

References

;Notes

;Bibliography

Further reading

  • The Williams Tube
  • Manchester Baby and the birth of Computer Memory
  • RCA 6571 Computer storage tube data sheet