Williams tube

The Williams tube, or the Williams–Kilburn tube after inventors Freddie Williams and Tom Kilburn (1921–2001), developed in 1946 and 1947, was a cathode ray tube used as a computer memory to electronically store binary data. It was the first random-access digital storage device, and was used successfully in several early computers.

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

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 light up, however, 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. They travel a short distance before being attracted back to the 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. The lifetime depends on the electrical resistance of the phosphor and the size of the well.

The process of creating the charge well is used as the write operation in a computer memory, storing a single binary digit, or bit. A collection of dots or spaces, often one horizontal row on the display, represents a computer word. There is a relationship between the size and spacing of the dots and their lifetime, as well as the ability to reject crosstalk with adjacent dots. This produces an upper limit on the memory density, and each Williams tube could typically store about 1024–2560 bits of data. Because the electron beam is essentially inertia free and can be moved anywhere on the display, the computer can access any location, making it a random access memory. Typically the computer would load the address as an X and Y pair into the driver circuitry and then trigger a time base generator that would sweep the selected locations, reading from or writing to the internal registers, normally implemented as flip-flops.

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