Williams Tube
The postwar period witnessed a frantic quest to solve the thorny problem of data storage in early electronic computers. While Presper Eckert and John Mauchly toiled over mercury delay lines in Philadelphia and Jay Forrester began his research on magnetic core memory at MIT, Frederic Williams charted a different course at the Telecommunications Research Establishment in Great Malvern.
Williams’s visit to the ENIAC project in 1946 gave him two key insights: the concept of the stored program and the vital need for fast electronic storage. His background in radar circuits during the world war led him, along with his doctoral student Tom Kilburn, to explore cathode ray tubes as memory devices. Computers required approximately 32,000 binary bits organized into 1,024 words of 32 bits. Electronic flip-flops would have demanded over 64,000 vacuum tubes, making any reliability impossible.
Williams and Kilburn’s breakthrough rested on a subtle physical phenomenon: electron bombardment on the screen created a positive potential well. A second bombardment nearby generated secondary electrons that partially filled the first well. Detection of this filling during a subsequent scan determined whether a 1 or 0 had been written. A metal plate against the screen captured these minute charge variations.
Five variants of this principle emerged. The dot-dash system encoded a 0 as a dot and a 1 as a dash. The dash-dot inverted this logic. The defocus-focus and focus-defocus systems played with beam sharpness. The anticipation system exploited a predictive signal during charge destruction.
In early 1947, now at the University of Manchester, Williams and Kilburn stored their first stable bit. By autumn, they maintained 1,024 digits for several hours. Their stroke of genius was introducing a regeneration mechanism. Since the charge persisted for only a few tenths of a second, the content was constantly read and rewritten—a technique that survives in our current DRAM memories.
June 1948 saw the birth of the Manchester Baby, the nickname for the Small-Scale Experimental Machine. This prototype used three tubes: one for main storage (32 words), one for the accumulator (calculation register), and a final one for control (program counter and current instruction). Its architecture allowed random access to memory, whereas delay lines imposed sequential access. Its instruction set was limited to seven operations: JMP (jump), JRP (relative jump), LDN (load negative), STO (store), SUB (subtract), CMP (compare), and STP (stop).
Two new versions emerged in 1949. The first offered 128 words of 40 bits with an electronic multiplier. It served for sophisticated calculations such as verifying the Riemann hypothesis and testing the primality of Mersenne numbers. The second combined Williams memory with a magnetic drum storing an additional 40,960 bits, thus creating a two-level memory hierarchy that foreshadowed the organization of future computers.
IBM seized this technology in 1951, acquired a license, and launched the manufacture of Williams tubes for its 701, the company’s first commercial computer. This industrialization required remarkable achievements; the company created ultra-clean environments ahead of its time. Personnel wore orlon smocks to avoid textile particles. Sticky mats trapped dust from shoes. These practices anticipated those later adopted for semiconductors.
The final version of the Manchester computer, delivered in 1951, aligned eight Williams tubes each storing 1,024 bits (total memory of 10,240 bits), supplemented by a magnetic drum of 150,000 bits. It processed instructions in 1.2 milliseconds and performed multiplications in 2.16 milliseconds. Its applications touched biology, optics, crystallography, chemical engineering, and even chess.
The technical constraints of Williams tubes proved numerous. The quality of the phosphorescent screen was critical: the slightest imperfection—carbon particle or microscopic hole—compromised storage. The beam required perfect uniformity across the entire surface. The scanning speed necessitated absolute precision control.
Despite these challenges, the technology equipped several machines of the 1950s: the Ferranti Mark I (commercial successor to the Manchester machine), the IBM 701, and the MANIAC I at Los Alamos. By mid-decade, it yielded to Forrester’s magnetic core memory, which was more reliable and denser. In 1998, to celebrate the fiftieth anniversary of the Manchester Baby, Chris Burton led the construction of a functional replica, shedding new light on the problems faced by the 1948 team.