total number of 0s generated, with a high count indicating a greater probability of the starting position of the chi key sequence being correct. Counting Wynn-Williams had obtained his PhD at Cambridge University for his work at the Cavendish Laboratory with Sir Ernest Rutherford. In 1926 he had constructed an amplifier using thermionic valves (vacuum tubes) for the very small electrical currents arising from detectors in their nuclear disintegration experiments. Rutherford had got him to devote his attention to the construction of a reliable valve amplifier and methods of registering and counting these particles. The counter used gas-filled Thyratron tubes which are bi-stable devices. The counters that Wynn-Williams designed for Heath Robinson, and subsequently for the Colossus computers used thyratrons to count units of 1, 2, 4, 8; high speed relays to count units of 16, 32, 48, 64; and slower relays to count 80, 160, 240, 320, 400, 800, 1200, 1600, 2000, 4000, 6000, and 8000. The count obtained for each run-through of the message tape was compared with a pre-set value, and if it exceeded it, was displayed along with a count that indicated the position of the key tape in relation to the message tape. The Wren operators initially had to write down these numbers before the next count that exceeded the threshold was displayed – which was "a fruitful source of error", so a printer was soon introduced. Robinson developments The original Heath Robinson was a prototype and was effective despite a number of serious shortcomings. All but one of these, the lack of "spanning" ability, were progressively overcome in the development of what became known as "Old Robinson". However, Tommy Flowers realised that he could produce a machine that generated the key stream electronically so that the main problem of keeping two tapes synchronised with each other would be eliminated. This was the genesis of the Colossus computer. Despite the success of Colossus, the Robinson approach was still valuable for certain problems. Improved versions were developed, nicknamed Peter Robinson and Robinson and Cleaver after department stores in London. A further development of the ideas was a machine called Super Robinson or Super Rob. Designed by Tommy Flowers, this one had four bedsteads to allow for running four tapes and was used for running depths and "cribs" or known-plaintext attack runs. References and notes Bibliography in That version is a facsimile copy, but there is a