These were added shortly after Colossus 2 came into use, and afterwards
fitted, with technical modifications, to several Colossi. Score meters were added
later; Colossus 6 has some special gadgets for key rectangles.
Colossus rectangling has been slightly disappointing; although the
rectangle is produced in the required form, it has been found necessary to copy it
onto squared paper for convergence; as a single operation it cannot be used with
(h) The use Of Colossus for wheel-breaking : not 99.
Colossus was designed for chi and psi setting, not for breaking. The first
attempts to use it for chi-breaking consisted of setting up some provisional
wheels and changing the characters one by one : if the score improved the
character remained changed, otherwise it reverted. It was soon realized that this
was equivalent to the more rapid process of putting only one cross in a trigger,
and stepping it, thus in effect using the trigger to select the characters
of the wheel one by one. Essentially the same method had already been used on
That Colossus (including Colossus 1) should prove suitable for wheel
breaking, justified the policy of making it as flexible as possible, but
immediately demanded further improvements.
(i) Longer bedsteads, because breaking requires longer
texts than setting.
(ii) Uncertain letters replaced by 9's are a nuisance in
chi-setting and breaking from cipher, even if there
is no slide, but it is in chi-breaking from depth
key, where missing letters are a substantial part
of the text that the problem becomes acute. It was
found necessary to use the Q panel for the condition
Z not = 9 , there being no "not" facility on the plug-
board, and plugging all runs, which was intolerably
In consequence "not 9" was fitted, a device which
imposed Z not = 9 but this lost all genuine 9's also
(about 1/17 of the text after differencing), and was
replaced by "not 99".
(iii) Multiple testing on doubted wheels is obviously
of great value when setting long messages during
(iv) Intolerable delays and mistakes during wheel-breaking
were caused by the need for setting up pins at the
back of Colossus and complaints finally extorted
the wheel-breaking panel on the front of some machines.
(i) Objections to specialised gadgets.
The clamour for specialised gadgets continues : the objection to it is
the difficulty of maintaining Colossi unless they are all alike : a device worth
fitting to all Colossi is much more welcome.
Colossus soon replaced Robinson for setting and breaking, but Robinson
remained indispensable for crib runs in which two tapes (derived from Z and P),
must be compared in all