156 | 14 TUNNy: HITlER’S BIGGEST fISH
Colossus used Tutte’s method to cleanse the ciphertext of the part of the key that the chi-
wheels had contributed. The result of stripping away the contribution of the chi-wheels was
called the ‘de-chi’ of the message: this consisted of the plaintext obscured only by the contri-
bution of the psi-wheels. De-chis could almost always be broken by hand, because of distinc-
tive patterns in the component of the key contributed by the psi-wheels. These patterns were
due to the irregular movement of the psi wheels, the Tunny machine’s greatest weakness.
De-chis were broken in a section called simply the ‘Testery’, after its head, Major Ralph
Tester. Captain Jerry Roberts, one of the Testery’s codebreakers, explains this part of the process
in Chapter 16.
The first computer centre
By the end of the war Newman had nine Colossi working round the clock in the Newmanry,
and another stood in the factory almost ready for delivery. The nine computers were housed
in two vast steel-framed buildings. It was the world’s first electronic computing facility, with
job-queues, teams of operators working in shifts, specialized tape-punching crews, and engi-
neers on hand day and night to keep the machinery running smoothly. Not until the 1960s was
anything like it seen again, when the first modern computing centres started to develop around
large mainframe computers.
figure 14.8 Colossus. Notice the early computer printer in the foreground. The long punched tape containing the
message to be analysed is mounted on aluminium wheels.
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.