How Colossus was Built and Operated—One of its Engineers Reveals its Secrets

Flowers’ team, which included me, became involved with the design of the logic units of Newman’s proposed machine after Morrell’s Telegraph Group, which had been assigned the job, got into difficulties. For modulo-2 addition (‘exclusive-or’, or XOR) Morrell was proposing to use a type of frequency-modulator employed for voice-frequency telegraph signals. This might have been all right for adding only two signals, but it was useless for adding many signals, because the device was analogue in nature (i.e. not digital or discrete, but using continuously variable voltages). The small variations added up, with the result that the device often produced a wrong answer. After some clever work by Gil Hayward, it just about worked for the number of additions that were required. The Heath Robinson’s ‘bedstead’, containing the tape drive and the photoelectric tape-reader, was designed and built at Dollis Hill. Our people Eric Speight and Arnold Lynch had very recently used photoelectric cells to do what was required. Fighter Command had asked Dollis Hill for a fast means of recording the telegraphic signals from their aircraft observers. Speight and Lynch, working together with Morrell’s group, had designed some photoelectric equipment that would record these signals directly from the telegraphic punched tape. The device they built, called the ‘Auto-Teller’, was never in fact used, but this photoelectric technology formed the basis for the bedstead. When we finished our part of Newman’s machine at Dollis Hill I moved to Bletchley Park, and Alan Bruce from TRE accompanied their part of the machine, the counter and display rack. The Heath Robinson was installed in the wooden Hut 11—the Newmanry. I was privileged to be one of those present at the Heath’s inauguration before the VIPs—and I can confirm that smoke did rise from it at switch-on. I was able to deal with this. A large resistor had overloaded, which I bypassed, and we carried on. (The machine never did catch fire, on this or any other occasion, but as mentioned in Chapter 13, we had a benzene fire in our workshop, at a much later date, and this may have contributed to the erroneous stories of Heath Robinson catching fire.)