A report entitled Cryptanalytic Report #2, Yellow Machine written by Roy D. Johnson in 1943 is located at NR 3175, Box 1009, Record Group 457, Historic Cryptographic Collection, National Archives and Records Administration, Washington, D.C. The text on this web page, which concerns the machine gun attachment for the bombe, is taken from pages 33-35 of this report.
The most readily apparent source of additional menu strength is the case discussed on page 31 wherein two letters on the menu are steckered to the same letter not on the menu. Although different details may be involved in various manners of eliminating such answers there is only one fundamental method. It begins when the bombe has stopped by the normal processes previously described for the ordinary bombe. The next step is to try each point on the entry line for the insertion of current until the point at which the current passes only through that point in the line, in other words, the single stecker box. On reaching this point it stops and maintains this current as it searches each other line of the reciprocal board by inserting a line current.
The electrical method of searching each line is indicated in example 17 which represents the test of a line of the reciprocal board. If current goes through any point in the line it activates a relay indicated for that point a, b, c, . . . . z, causing both arms to move up. When one such relay is activated the current is moved up to level one. The activation of one or more additional relays will move the current up to level two.
[diagram17 shows a series of 26 branches connected in parallel left to right with line current entering along a bus at bottom; a relay arm marked 2 for each branch at the start of each branch, a relay arm marked 1 for each branch above it, then the branches all connecting together at a bus marked level 2 at the top. Level 1 is indicated as a point between arms 2 and 1 on each branch]
Current emerging from level two causes the bombe to continue its run. To summarize, we may say if the current enters any line at more than one point, the machine is started to continue its run, and if the current enters no line at more than one point, the machine is satisfied. A second search down each line is used to print all the actual stecker obtained.
[Footnote: The method herein described in used on the British model] Addition of this feature permits the use of a 13-letter chain with no closures as compared to a similar chain of 16 letters needed for the ordinary bombe. Similar reductions are made for menus with closures. The mechanical search is accomplished in two or three seconds, and it does not seriously impede the speed of the bombe.
In example 16 [which was A/g B/p C/p D/d E/h] we have two stecker for P, namely P/B and P/C. Thus the points b and c of reciprocal board line P will have current, and these two points will cause the bombe to resume its run. We have eliminated a stop in two to three seconds and we need not examine it by hand.
The stacatto sound accompanying the search of reciprocal board lines is not unlike that of a machine gun, and the additional equipment is accordingly called a "machine gun" by the British. The addition of the "machine gun" gives the bombe greater size as well as a larger capacity, and the complete bombe is consequently known as a "Jumbo Bombe." The complete position and stecker printed by the Jumbo Bombe is known as a stecker story or, more simply, a story, in contrast to the stop of the ordinary bombe indicating position only.
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