History of the Williams-Kilburn Tube

As a result of a trip to the U.S.A. in June 1946, Dr F.C. (Freddie) Williams started active investigation at TRE into the storage of both analog and digital information on a Cathode Ray Tube. Storage of analog information could help solve the problem of static objects cluttering the dynamic picture on a radar screen (because). Storage of digital information could solve the problem holding up the development of computers worldwide, i.e. lack of a storage mechanism that would work at electronic speeds. By November 1946 he was able to store a single bit (with the "anticipation" method), based around a standard radar CRT, and filed a provisional patent for the mechanism in December 1946.

In December 1946 Freddie Williams was appointed to a chair at the University of Manchester, and left TRE. However both he and TRE wanted the research to continue, so Tom Kilburn, who was in his group at TRE, was seconded to the University of Manchester to continue the work with Freddie Williams on digital CRT storage. A Scientific Officer from TRE was also seconded full time to help him, initially Arthur Marsh, who left after a few months, and was replaced in the summer of 1947 by Geoff Tootill.

By March 1947 Tom Kilburn had discovered a different and better method of storing information, more suited to storing a large number of bits on the same tube. By November 1947 they had succeeded in storing 2048 bits for a period of hours, having investigated a number of variations on storing a set of bits (dot-dash, dash-dot, defocus-focus, focus-defocus).

The general principle behind the storage of binary information was to plant charge in one of two different ways at an array of spots on a CRT using standard techniques. The type of charge at any spot, representing a 0 or 1, could be sensed by a metal pick-up plate on the outside of the CRT screen, thus "reading" the "value" of the spot. However, the charge dissipated very quickly, so values were preserved indefinitely by continuously reading their value and resetting the charge as appropriate to the value.

Although the phosphor on the CRT would glow at charged points, in a way that might be distinguishable (between 0 and 1), the current contents of a CRT storage tube could not be viewed directly in practice. The front of the tube would be obscured by the pick-up plate. (At first this was a wire mesh, but later it was a metal plate.) Also the CRT had to be screened from outside electrical interference, e.g. local trams or close and aggressive motor cycles, so it was enclosed in a metal box. So typically the information on a Williams-Kilburn Tube would be displayed on a separate Display Tube, which would be updated synchronously with the refresh cycle of the Williams-Kilburn CRT Store. However the opportunity would also be taken to lay the values out in a manner most convenient to the onlooker.

The memory storage system nearest to a successful realisation elsewhere was the Mercury Acoustic Delay Line store, which was chosen as the basis for the earliest active stored-program computer projects (EDVAC, the EDSAC and ACE). The most significant advantage of the Williams-Kilburn CRT Store over a Delay Line store was that the CRT store allowed fast random access to short strings of bits, e.g. 20-bit or 40-bit addressable strings corresponding to "lines" or "words" in RAM. This contrasts with the sequential access mechanisms inherent in Delay Line stores, which would store say 1024 bits in a single delay line and a smaller string could only be read when it "came round" on a 1024-bit cycle. The Williams-Kilburn CRT Store also had the practical advantages that it was made of standard components, was cheap and compact, and did not require temperature control or accurately controlled power supplies.

When in Autumn 1947 the group had successfully stored 2048 digits, they had the problem of proving that the store would operate successfully inside a computer. They could only alter bits at the rate of around 1 a second, which was 100,000 times slower than the store's capability. In the end they decided that the simplest way to test that the CRT storage system was suitable for use in computers was to build a small computer round it. So the way was clear for the construction of the SSEM, the "Baby", "to subject the system to the most searching test possible." The Baby was therefore built to show that Williams-Kilburn CRT Stores could work dynamically in a working computer, and to demonstrate the feasibility of building a much larger machine using them. That it also was the first machine to realise the stored-program computer, and prove its feasibility was a secondary consideration.

Although by 1948 standards the Williams-Kilburn CRT Store was acceptably reliable, it was not 100% so, and there was regular experimentation and development for many years from 1947 onwards to improve its reliability. The principles are described here is the "dot-dash" method used on early machines.

The Williams-Kilburn CRT Store was used in several models of computer including the IBM 701 and 702 computers. It was eventually superseded in new systems in about 1955 by a cheaper random access store called the "magnetic core store."