General Report on Tunny

23L Page 100

lower it is harder to set the Χ's, but if they can be set then it is easier to set the motor. Fortunately machine and hand methods are complementary in this respect. When d is high, the psis are easy to set by hand and then the setting of the motor is a routine job.

(e) Pick-ups.

The formula of the expected sigma-age can be used for deciding between alternative motor runs, but of course, it is possible to do more than one independent motor run and look for pick-ups between the runs. (This is a reason for using a set total of not more than 2½ sigma in motor runs.)

(f) Switching of a motor run on Colossus.

The motor run is usually of the form

BM = . | ΔD ε ζ

or BM = . | ΔD ε ζ ; Χ₂ ≠ x

where ζ is a class of teleprinter letters. When the conditions to the right of the vertical line are switched by themselves the score obtained, which is called r, provides a check of the Χ settings and patterns and of the correctness of the tape etc. The routine of counting r before doing the run is the same as in the case of Χ setting. The run is done quintuple testing on μ₃₇ and takes under 10 minutes for a tape of length 5000. For further details as to switching see 53L (h)(1). The best score on the motor run is always checked even if it is not good enough to use.

(g) Good slides of the motor.

Quite often a top score of as much as 5σ on a motor run may not be certain due to strong competition arising from good slides of the basic motor against itself. These good slide settings do not all agree with one or other of the settings corresponding to the top score. See for example R0, 58 and R3, 9. In this way good slides of the motor are rather different from those of the Χ's and Ψ's. In particular it is not a good policy to stop motor runs in the middle when a good score comes up and then cross run for μ₃₇ and μ₆₁ as short runs.

(h) Motor runs with not all the Χ's net.

Suppose Χ_1,2,3,4 are set but Χ₅ has given difficulty. Then we may sometimes be able to set the motor and to use the new information for setting Χ₅. The expected score for a motor run with not all the Χ's set can be obtained in the same way as in the case when all five Χ's are set, but it so happens that we are more likely to run into trouble due to the use of good motor

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