Mr. Newman's section began as a section for setting messages on wheels
broken from depths in Room 41. Wheel-breaking activities came later.
Bream started to use P5 limitation regularly in the middle of December,
1943, and as there seemed every chance that the use of this gadget would be
extended, research activities were devoted to the statistical solution of chis
from Z. Tutte's method of rectangles (see Ch.44) was elaborated and from January
1944 monthly keys were tackled operationally.
Significance tests were gradually instituted and methods improved. Soon
after Colossus 1 arrived in February 1944 it was discovered that it could be used
for chi-breaking. It was this discovery that made large scale wheel-breaking
possible oven after the introduction of the daily wheel change in July 1944.
(b) The period of expansion.
Between July and November 1944 the number of computers increased from 4 to
about 16 a watch, and the number of Colossi from three to six, of which three were
fitted with a rectangling device. New Garbos, Miles and arrival terminals from
Knockholt were installed in Block H which opened in September and housed all
wheel-breaking operators from the middle of November onwards.
From August onwards extensive rectangling was rarely applied to any
particular day's messages. A few long tapes on each day were rectangled and it
was assumed that when the dottage was high and the interception good the
rectangle would be significant. Colossus work on significant rectangles largely
replaced the more laborious method of the conditional rectangle, and from the end
of August a machine and a man to supervise operation could be spared most of the
From the middle of November 1944 to May 1945 the number of machines and
trained staff continued to increase, and about 15 sets of wheels broken on
rectangles each week. In 1945 there were about 15 Computers per shift, whose
main job was to converge rectangles on paper. The head of Computers was
called the Rectangles Registrar. A man, called the Wheel Man (WM) was in charge
of wheel-breaking operators and there were other men called wheel-breakers, each
of whom took charge of one wheel-breaking job on a Colossus.