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The Williams tube or the Williams-Kilburn tube (after inventors Freddie Williams and Tom Kilburn), developed about 1946 or 1947, was a cathode ray tube used to electronically store binary data.

Williams Tube

Computer memory types Volatile DRAM, e.g. DDR SDRAM SRAM Upcoming Z-RAM TTRAM Historical Williams tube Delay line memory Non-Volatile ROM PROM EAROM EPROM EEPROM Flash memory Upcoming FeRAM MRAM PRAM SONOS RRAM NRAM Historical Drum memory Magnetic core memory Bubble memory

When a dot is drawn on a cathode ray tube, the visible spot lasts for a time (called "persistence") that depends on the type of phosphor used in the tube. The operation of the Williams tube is due to a completely unrelated effect (in fact some Williams tubes were made with no phosphor), caused by secondary emission, such that the area of this dot becomes slightly positively charged and the area immediately around this dot becomes slightly negatively charged (creating a charge well). Also a positively charged dot is erased (filling the charge well) by drawing a second dot immediately adjacent to the one to be erased (most systems did this by drawing a short dash starting at the dot position, the extension of the dash erased the charge initially stored at the starting point). By later drawing a dot at that spot and measuring the charge, by means of a metal plate placed over the outside of the front of the tube, you have a simple form of memory that lasts for a time depending on the electrical resistance of the inside surface of the face of the tube. Reading a memory location destroyed its contents (creating a charge well), so any read had to be followed by a write (most systems did this by drawing a short dash starting at the dot position if the positive charge created needed to be erased). Also, because the charge gradually leaked off, it was necessary to scan the tube periodically and rewrite every dot (similar to the memory refresh cycles of DRAM in modern systems).

Williams tubes stored roughly 500 to 1,000 bits of data.

Developed at the University of Manchester in England, it provided the medium on which the first ever electronically stored-memory program was written in the Manchester Mark I computer. Tom Kilburn wrote a 17-line program to calculate the highest factor of a number. Tradition at Manchester University has it that this was the only program Tom Kilburn ever wrote.

The Williams tube tended to become unreliable with age, and most working installations had to be "tuned" by hand. By contrast, mercury delay line memory was slower and also needed hand tuning, but it did not age as badly and enjoyed some success in early digital electronic computing despite its speed, weight, cost, thermal and toxicity problems. However, the Manchester Mark I was successfully commercialised as the Ferranti Mark I and some early computers in the USA also used the Williams tube, including the IAS machine, originally designed for Selectron tube memory, and the UNIVAC 1103, IBM 701 and IBM 702. It was also used in the Soviet computer, Strela-1.

• Regenerative capacitor memory
• Mellon optical memory

• Lavington, Simon H. (1980). Early British Computers. Manchester University Press. ISBN 0-932376-08-8. 
• Bashe, Charles J. (1986). IBM's Early Computers. MIT Press, p. 105. ISBN 0-262-02225-7. 

• The Williams Tube