The Bombe's Drums
1940 · Bombe · Work 471 of 1,871
A close view of coloured drums on a machine face, each drum a cylinder of bakelite ringed with a band of capital letters around its rim, drums in reds, greens and browns, brushes of fine wire pressing against their contact plates, one drum lifted half off its spindle showing the circle of brass contacts beneath, the spindles arranged in a grid on a bronze coloured panel, a single work lamp from above at an angle, brass and coloured bakelite catching the light, the machine face receding into darkness at the edges.
- What it was used for
- The drums were the moving part of the bombe. Each machine carried 108 of them — Carter describes 36 sets of three drums in three banks of twelve — each wired like an Enigma rotor. The Bombe Rebuild Project gives 104 steel brushes per drum; for one machine that is 11,232 sliding contacts. The drums set the pace: the electrical test itself is fast — Tony Sale writes that the network stabilises in a few microseconds — yet a whole run still took about 18 minutes according to Carter.
- What was new
- The trick is in the wiring: Tony Sale describes how each drum carries the wiring of an Enigma rotor both forwards and backwards; Carter gives the purpose: current must be able to pass through the drums in either direction. Nothing of it survives: according to Carter, none of the wartime machines lasted. What a reconstruction costs is shown by the Bombe Rebuild Project. In 1995 GCHQ released some 2,000 BTM drawings and documents to the Bletchley Park Trust; John Harper then began the rebuild, his team grew to more than 60 volunteers, and 900 new drawings plus 100 assembly drawings were produced. The machine was officially opened in July 2007 by the Duke of Kent.
Connections
Every connection names its kind. “Forerunner” on its own would be a claim.
- Works withThe Turing–Welchman Bombe1940Each vertical set of three drums was the equivalent of one Enigma, and thirty-six such sets ran in a single Bombe. In one run the drums were advanced through 17,576 positions, and at each position the machine tested the 26 possible stecker connections.Source: Bombe Rebuild Trust: Bombe Description
- Follows fromThe Rotor Set1938Each bombe drum carried two sets of internal wiring matching the internal wiring of its corresponding Enigma wheel; the connections are the same as those between the right- and left-hand rings of an Enigma wheel's terminals. Each vertical set of three drums made up one Enigma, and a bombe held 36 such sets.Source: Turing Welchman Bombe Rebuild Project: Bombe Description · Center for Cryptologic History (NSA): Solving the Enigma - History of the Cryptanalytic Bombe