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Switches to finesse the decryption
Underneath the jack-board was a telegraph relay. The bank of switches was used to finesse the output of the machine. Occasionally the decryted messages would turn into gobbledegook, so the switches would be used to go back and restart the machine from a specific point, without having to decode parts of the message that had already been decoded.
Below the relay, shielded by two grey boxes, were the matrix control relays for the circuitry. There were at least 120 relays that controlled the operation of the machine, Whetter said.
The entire machine was rebuilt using spares from BT telephone exchanges that were remodelled in the 1980s. All of the engineers that worked on the rebuild were ex-BT employees and had contacts at other museums, and so could lay their hands on the components, Whetter added.
Photo credit: Bletchley Park
See ZDNet UK's gallery of Acorn computers at Bletchley Park.