Freddie Williams (left) and Tom Kilburn at the control panel of the Manchester Baby computer, 1948
Credit: University of Manchester

The Williams Tube was the first random-access computer memory. Invented by F. C. “Freddie” Williams and Tom Kilburn, initially as a moving target indicator for WWII radars, it worked by storing information as electric charges on the face of a cathode ray tube (CRT). These charges remained in place for about a fifth of a second, during which time they were detected and rewritten in place. The fact that the CRT produces a spot of light in the process is coincidental and could not be seen because the tube was covered with a metal plate used to detect the charges.

Williams Tube memories worked, but they were quite unreliable. Nevertheless the world’s first stored program computer, the Manchester “Baby,” operational in June 1948, used Williams Tube memory, as did many first generation computers including von Neumann’s IAS machine and the IBM 701.

Competing memory technologies included mercury delay lines, which were slow and could not be randomly accessed, and the

RCA Selectron tube, which used a hard-to-manufacture mesh of small charged iron rings. All of these approaches, including the Williams Tube, were made obsolete by the invention of magnetic core memory.