23M Page 102
contain patches which are rich in particular properties. Such spanning can be done in conjunction with an examination of the Red Form, by correlating the spanning with pauses, but in practice it is found too much trouble to get the Red Form as a rule.
(k) Proving the motor setting.
When the limitation does not involve P5, that is when there is no autoclave it is easy to set the Ψ's. However, for this purpose it is nearly always necessary for the motor to be correctly set (not merely a good slide). In this sense the setting of the Ψ's is the most conclusive way of testing the motor settings. If, however, there are many different settings to choose between, it may be quicker to do a corroborative motor run and look for pick-ups; or simply to count a group of good (or bad) ΔD letters against motor dots at the rival settings of the motor.
Suppose, however, that there is still some uncertainty about the Χ's. Then a motor setting which is knew to be at least a good slide of the correct motor may be used for setting or resetting the Χ's, by means of runs against motor dots, just as if the motor were correctly set (R3 p.66). If in this way a new Χ setting is found it may be used to reset the motor. This is an example of a method of successive approximation. (R3 p.56). A motor setting can be identified as probably a good slide of the right setting by its sigma-age and by comparison of the relations between the settings that have turned up in the run. Observe that if a particular day's motor has a lot of good slides against itself, then there are effectively a lot less than 2257 independent settings possible and therefore a lower sigma-age may be significant. Thus the affect of the motor having good slides is double-edged.
(l) Providing mu61.
Sometimes a motor run is done with a provisional μ61 and the result used to clear up ambiguities in μ61.
23M Ψ-SETTING
(a) Setting Ψ1 as a motor run.
Suppose that the limitation is Χ2 + Ψ1'. Then the correct TM depends on the setting of Ψ1. Therefore it is possible to do a run for Ψ1 as a motor run, provided the BM and Χ2 are set correctly. Observe that the Ψ1 wheel is driven by a motor on which it itself has an influence, and in this respect the run differs from a TM run. A similar method can be used for Ψ5 when the limitation is Χ2 + P5 (R8, 32) but in this case the dangers of corruption are greater and the usual practice is to use stretches of 800 letters of the message for all the Ψ runs with an autoclave limitation.
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