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punched on the tapes and become adept at reading them: this alphabet was the same as the
GPO (General Post Office) teleprinter alphabet. On either side of each sprocket hole there was
space for two larger holes to be punched above and three below: the letter ‘A’, for instance, was
two holes above the sprocket hole and nothing below.
We were taken round the section and shown what everyone was doing. We saw the room
where the messages came in on teleprinter tape on two separate machines. Most of the messages
came from Knockholt in Kent and also Kedleston in Derbyshire, I learned later. We were shown
into a long room where tapes were cut and joined, and tapes that had split on the machines
were repaired. Then we went to the Registry itself (or ‘Ops’, as it was called), where all tapes
were registered and tabulated and put into a series of cubbyholes. We were also shown the two
Colossus computing machines the section had at that time. I was overawed by them, a mass of
switches, valves, and whirring tape: I thought they were incredible—quite fantastic. It is now
known that the computers were built to take over codebreaking work originally done by means
of a hand method invented by Alan Turing himself, and it is also known that although Turing
had no hand in designing Colossus, some of the ideas involved in his hand method were funda-
mental to the algorithms used by Colossus. But, of course, we humble computer operators were
told nothing about any of this at the time.
At the end of the fortnight we were tested on our knowledge and, depending on how well
we performed, were selected for various tasks—administration, dealing with the tapes as they
came in, and so forth. I was delighted to be chosen to operate Colossus, which I considered the
plum job (Fig. 15.2). A Wren named Jean Bradridge taught me how to operate the machine.
She explained what all the switches were for, and showed me how to peg a wheel pattern on the
grid at the back of the machine, using pins that looked like very large and very strong hairpins,
copper–nickel plated.
The tape was shut into position in front of the photoelectric cell, which had a small gate
for the tape to slide through. Metal wheels supported the tape, and depending on the length
of the tape we would use as many wheels as necessary to make the tape completely taut. This
figure 15.2 Colossus with two operators.
From ‘General Report on Tunny’, National Archive ref. HW 25/5 (Vol. 2). Crown copyright and reproduced with permission of the
National Archives Image Library, Kew.