Bletchley Park unveils rebuilt Tunny code-cracking machine

The National Museum of Computing at Bletchley Park is home to Europe’s largest collection of functioning historic computers, and another machine has just joined the ranks already populated by the likes of the reconstructed Colossus and Harwell.

This newcomer is the Tunny machine, and while it might look like the abandoned lovechild of a kitchen dresser and a filing cabinet, it was an extremely important part of the Second World War, being used to decipher the encrypted messages of the German’s Lorenz cipher machine.

The Tunny was first operational and cracking enemy codes in 1942, and it used wheel settings painstakingly found by hand along with some help from sluggish and unreliable Robinson machines. This meant unlocking a message could take several weeks of hard graft.

Fortunately when Colossus was finished in 1944, it provided the wheel settings far more quickly than the earlier Robinson machines, reducing cracking times to a more respectable four days.

The amount of Tunny machines was also increased by 12 to 15, and by the closing stages of the war, they were all working around the clock cracking around 300 messages a week, which was obviously a vital part of British intelligence.

John Pether and John Whetter led the team who rebuilt the Tunny, and they did so with minimal evidence – just a few photos, some partial circuit diagrams and the ailing recollections of some of the original operators (“During the war…”).

John Pether commented: “We’ve succeeded in rebuilding Tunny with scraps of evidence, and although we are very proud of our work it is rather different from the truly astonishing achievement of Bill Tutte’s re-engineering of the Lorenz machine.”

“Sourcing 200 suitable relays and dealing with the complex wiring schedules was difficult, but we really got in tune with the original team when we had to set up the electronic timing circuits. They were a continuous source of problems then as they are even now for the rebuild team – except the original team didn’t even have the benefit of digital storage oscilloscopes!”

The new Tunny Gallery opens to the public this weekend at Bletchley Park, Milton Keynes.