General Report on Tunny
23E PAGE 96
from which the "Certain, Good" tables [23C(b)] may be derived.
If there is competition, 
can be found for each competitor and the results compared. [R3 p. 134; R4 p. 1, 70; R5 p. 1, 3, 7, 113.]
23X CHECKS ON SETTINGS
(a) General.
This does not deal with the complete system of checks [35 D, E], but only with checks applied during setting on Colossus.
Setting yields D = Χ + Z.
Both Χ and Z are checked beforehand; D is checked afterwards. Each score used is checked as it occurs [See 23D].
(b) Χ Tests.
The Χ to be tested is the pattern set up on the triggers: this is not checked for each message, but only when patterns are set up afresh or are subject to suspicion.
(c) Χ Test Tapes.
The obvious method of checking Χ triggers is to make a tape Z ≡ Χ at some definite settings, and count /'s in Z + Χ = Χ + Χ = /. The correct score is of course the text length, or span length. It is better to count /'s in Δ Z + ΔΧ which checks Colossus Δ'ing simultaneously: this of course reduces the score by 1.
Such a tape not only checks the trigger; but, by being spanned, enables a fault to be located either on the trigger or on the tape itself.
Χ test tapes are made to a standard length of 2002 and spanned 0001-2001.
(d) Χ Test Runs.
To avoid the need for putting on the special Χ test tape, counts are made depending only on the Χ trigger and the span, so that any sufficiently long tape which happens to be on Colossus can be used. The Χ test tape is required once only, as early as possible: if it checks, Χ test runs are done at once and the scores recorded. Thereafter the trigger can be checked by repeating these runs and seeing that the same scores are obtained.
The actual form of the test is to count ΔΧ1 + ΔΧ2 + ΔΧ3 + ΔΧ4 + ΔΧ5.
Back to General Report on Tunny. Contents.