had to be periodically refreshed using the same basic method. As the data is read and then immediately rewritten, this operation can be carried out by external circuitry while the central processing unit (CPU) was busy carrying out other operations. This refresh operation is similar to the memory refresh cycles of DRAM in modern systems. Since the refresh process caused the same pattern to continually reappear on the display, there was a need to be able to erase previously written values. This was normally accomplished by writing to the display just beside the original location. The electrons released by this new write would fall into the previously written well, filling it. The original systems produced this effect by writing a small dash, which was easy to accomplish without changing the master timers and simply producing the write current for a slightly longer period. The resulting pattern was a series of dots and dashes. There was a considerable amount of research on more effective erasing systems, with some systems using out-of-focus beams or complex patterns. Some Williams tubes were made from radar-type cathode-ray tubes with a phosphor coating that made the data visible, while other tubes were purpose-built without such a coating. The presence or absence of this coating had no effect on the operation of the tube, and was of no importance to the operators, since the face of the tube was covered by the pickup plate. If a visible output was needed, a second tube connected in parallel with the storage tube, with a phosphor coating, but without a pickup plate, was used as a display device. Development Developed at the University of Manchester in England, it provided the medium on which the first electronically stored-memory program was implemented in the Manchester Baby computer, which first successfully ran a program on 21 June 1948. In fact, 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-line program to calculate the highest proper factor of 218. 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 "tuned" by hand. By contrast, mercury delay-line memory was slower and not truly random access, as the bits were presented serially, which complicated programming. Delay lines