9 - Bombe Rebuild Machine
The target machine was a three-wheel, 36-Enigma version, which had high speed, 104 Siemens-type double sense relays. During the war 69 were built by BTM. The target rebuild machine was modelled partly on No. 297 – Atlanta – used in the US bay at Eastcote and delivered in July 1944.
The Bombe mechanism was not a true cyclometer and could not carry between adjacent commutator segments of the upper drums and at the same time continue sensing. The carry takes from 13 character times or points (39-point machine) down to four points (30-point machine), depending on the model. A point is one-26th of the rotation of the top drums or one sense point. The rebuild is a 39 point or 13 point-carry machine, whereas the original Atlanta machine was a 30 point or 4 point-carry machine.
On the rebuilt 39 point Bombe, sensing takes place on 26 steps of the top drums followed by 13 steps of no sensing whilst the carry mechanism operates to step on the middle drums by one position. A further 26 sensings takes place followed by 13 more non-sensing whilst a second carry takes place. There are therefore two carries for every three revolutions of the top drums.
Regardless of being a 39 point or 30 point machine, the time that a Bombe takes to sense all 17,575 combinations of drum positions is dependent on the rate of operation of the carry mechanism. The Bombe is designed for 65 carries per minute, which means that a full, uninterrupted run should take 26x26/65 = 10.4 minutes.
(c) John Jackson