General Report on Tunny

13B Page 25


The machines used in Tunny-breaking may be classified as :-

      (1) Counting and Stepping Machines.
      (2) Copying Machines.
      (3) Miscellaneous simple machines.


(a) Counting and stepping machines.

These machines are given two teleprinter patterns, combine them in some way and count the number of places of the combined pattern in which a certain condition is satisfied.

As essential feature is that these counts must be made with the two patterns in all possible relative positions i.e. one pattern must "step".

For example, chi-setting consists of adding ΔΧ + ΔZ in all possible relative positions, and counting for each position the number of places in which a condition such as ΔΧ1 + ΔΧ2 + ΔZ1 + ΔZ2 = dot is satisfied.

At each setting the answer is, of course, a number.

(b) Copying Machines.

These combine one or more teleprinter patterns. They differ from "Counting and Stepping" machines in that

      (i) there is no stepping.
      (ii) the result is not a number, but a sequence of letters.

The sequence of letters may either be a punched tape or a print-out.

These machines vary greatly in complexity, from the hand-perforator in which a pattern tapped out on a keyboard letter by letter is reproduced on a tape, to the decoding machine in which Chi, Mu, Psi set up electrically are combined with Z to produce P.

Of all machines "Counting and Stepping" machines are by far the most spectacular: both cryptographically and electrically they are notable achievements. For producing results they are dependent on humbler machines, especially tape-making machines.


There are three versatile machines:-

The fundamental difference between Colossus and Robinson is that on Robinson all patterns are punched on tapes, whereas on Colossus only one pattern is on a tape, the other being represented electrically.

5202, the photographic machine, is essentially a Robinson, using film instead of tape, but working many times faster, first making an

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