11B Page 7
The key-letter is the sum of the letter of the chi-key (Χ) formed by the five characters in the active positions of Χ1, Χ2, Χ3, Χ4, Χ5, and the letter of psi-key (Ψ') formed by the five characters in the active positions of Ψ1, Ψ2, Ψ3, Ψ4, Ψ5.
(d) Chi-key.
After each letter of the P-stream has been enciphered each chi moves on once. The pattern of characters added to each impulse of the P-stream has a period equal to the length of the corresponding chi-wheel, and since the lengths of these wheels are prime to each other, the stream of letters generated by the chis has a period of 41 x 31 x 29 x 26 x 23.
(e) Psi-key.
The motion of the psis is irregular and determined by the motor. After a letter has been enciphered either (i) each psi wheel moves on once and a new letter of psi-key is used for ciphering the next letter or (ii) all five psis remain still and the same letter of psi-key is used again. When (ii) happens there is said to be an extension of the psi-stream. The term EXTENDED PSI (or Ψ') stream is used for the actual sequence of letters added by the Psis to the P-stream, and the term Ψ-stream for the sequence of letters that the psis would generate if there were no extensions.
Example
| P-stream | 9 D I E 9 S C H O E N E 9 J U N |
| Ψ-stream | F L D E Q / K H B 4 |
| Ψ'-stream | F L L D E E E Q / K K H B 4 4 4 |
| (P, Ψ')-stream | D 5 H 3 S 9 K A O C A Y X D S C |
(f) Motors.
The dots and crosses arranged round the motor wheels do not mean the same as the symbols usually called dots and crosses.
A dot means STOP
A cross means GO.
Mu61 moves on once after each letter in enciphered. When mu61 has a cross in the active position (before moving) mu37 moves on once: when it has a dot in the active position (before moving) mu37 stays still. The character of mu37 in the earlier active position is the active character of the BASIC MOTOR (BM). In other words BM = Mu37 "extended by Mu61" = 'Mu37'.
Example of finding Basic Motor:
| Mu61: | x | . | x | x | x | . | x | x | . | x | x | x | x | x | . | x | x |
| Mu37: | x | . | . | x | . | . | x | x | . | x | . | x | . | . | x | x | . |
| (a) Number the characters of Mu61 repeating numbers wherever there is a dot: | |||||||||||||||||
| x | . | x | x | x | . | x | x | . | x | x | x | x | x | . | x | x | |
| 1 | 2 | 2 | 3 | 4 | 5 | 5 | 6 | 7 | 7 | 8 | 9 | 10 | 11 | 12 | 12 | 13 | |
| (b) Number the characters of Mu37 (without repeating) | |||||||||||||||||
| x | . | . | x | . | . | x | x | . | x | . | x | . | . | x | x | . | |
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | |
| (c) Replace the sequence of numbers given in (a) by their equivalents given in (b). | |||||||||||||||||
| 1 | 2 | 2 | 3 | 4 | 5 | 5 | 6 | 7 | 7 | 8 | 9 | 10 | 11 | 12 | 12 | 13 | |
| BM: | x | . | . | . | x | . | . | . | x | x | x | . | x | . | x | x | . |
The active character of the Basic Motor - in conjunction with the active character of the LIMITATION determines the character of the TOTAL MOTOR and this regulates the motion of the psis.
The limitation consists of a sequence of dots and crosses such that when there is a Basic Motor dot and a limitation cross in the active position there is a Total Motor dot and the psis do not move. At all other places (e.g. where there is a Basic Motor cross or a Basic Motor dot and a limitation dot) there is a Total Motor cross and every psi moves on once.
Example:
| Basic Motor | x | . | . | . | x | . | . | . | x | x | x | . | x | . | x | x | . |
| Limitation | . | x | . | x | x | . | . | x | x | x | . | . | x | x | . | . | x |
| Total Motor | x | . | x | . | x | x | x | . | x | x | x | x | x | . | x | x | . |
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