24B Page 119
(f) Colossus Rectangling.
Colossus rectangling is the most highly developed method in use.
The necessary rectangle gadgets have been fitted to Colossi 2, 4, 6, 7, 9. Colossus 6 has a bedstead for tapes 26,000 long, and is used almost continuously for rectangling.
The basis of Colossus rectangling is as follows:
put one cross in chi 1, one cross in chi 2 and switch the condition Χ1 = x, Χ2 = x: This will select a set of places on the cipher spaced at intervals 1271, i.e. the places in a cell of the rectangle. If these wheels step through all settings, they will select all cells of the rectangle in turn. Chi wheels move backwards when their settings increase, and therefore the rectangle is made backwards. If the wheels were to step uniformly the rectangle would be made backwards diagonally.
On Colossus, however, it is possible to produce the rectangle row by row. Step chi 1 fast, chi 2 slow (i.e. chi 2 steps only when chi 1 reaches the setting plug in Χ1): chi 1 steps and a row of the rectangle is produced; when chi 1 reaches its setting plug, chi 2 steps and another row is produced and so on.
It is impossible to make Colossus rectangling fully intelligible without a detailed account of the operations perform by the machine. For this reason the instructions are given here baldly, the explanation being postponed to 53M.
It may be remarked at once that the "rectangling gadget" modifies the operation of Colossus in many ways. Optionally, if the depth is constant, it can be made to perform the subtraction pippage = 2 x (score of ΔZ12 = .) - depth: rectangling which uses this facility is known as "Normal" as opposed to "Print Scores". This reduction of the length to a multiple of 1271 may cause a serious loss of evidence on a short text.
The instruction for a 1+2 rectangle are:
Spanning. Span 04 to (04 + 1271 x depth)
Count text.
Chi-patterns. (triggers) Crosses in 02, 02, of Χ1, Χ2: on rectangling Colossi one trigger has this perminantly set up.
Selection switches. Q = Χ
Q Panel. Χ2 = x in all counters.
Multiple test impulses R1, R2, R3, R4, R5 = x in counters 5, 4, 3, 2, 1
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