The coming of the machines

The Tunny machine

Once the complete logic of the cipher machine had been worked out by Bill Tutte and the codebreakers, the next step was to try to mechanise some of this to enable the enciphered texts to be read in a faster time than the current manual methods.

Firstly, in 1942, the Post Office Research Labs at Dollis Hill were requested to build a machine capable of duplicating the logic worked out by Tutte. Frank Morrell produced a rack of uniselectors and relays which emulated the logic. This machine was called "Tunny" and could be connected to a teleprinter. Once the settings for the pins and start positions had been worked out by hand and entered, typing in the original cipher text would print out the plain text message. The hand methods though could take four to six weeks to generate so something faster was required.

Fully working Tunny Machine rebuild at The National Museum of Computing

Heath Robinson

At this point, a mathematician called Max Newman though it would be possible to automate the process for finding the settings using a machine. He approached TRE at Malvern to design an electronic machine to run the double-delta method worked out by Tutte to find the wheel start positions. The machine was built at Dollis Hill and quickly became known as Heath Robinson after the cartoonist and illustrator best known for drawing complex contraptions.

Heath Robinson worked well enough to prove the concept but was a constant struggle to run consistently. It had two paper tapes which ran at 1,000 characters per second which had to be kept matched together in sync. One tape had the wheel patterns and the second had the enciphered message. Each run through of tape would calculate the double-delta and then the relative position of the tapes would be adjusted by one and the measurement repeated. The main problems included the tape ripping or stretching which meant the counts were no longer valid.

"Heath Robinson" rebuilt at The National Museum of Computing

Newman went to Dollis Hill and met with a brilliant electronics engineer, Tommy Flowers, with the hope that a better solution could be provided.

Colossus is born

Tommy Flowers
Tommy Flowers came back to Bletchley Park with a solution based on a machine that rather than using mostly slower relays, would use a huge number of electronic valves (or Vacuum tubes). He proposed having the wheel patterns generated electronically which meant removing one of the tapes and therefore solving the tape synchronisation problem.

In those days, valves were generally only used a few at a time as they tended to be unreliable and required replacing quite often. Tommy Flowers had been testing valves in relation to his work in telephone exchange switching and knew that valves would mainly break when being switched on and off so as long as they were kept running, they were very stable.

His solution, rather than using the couple of dozen valves used by Heath Robinson called for 1,500 valves working together! We can only imagine the looks of astonishment and disbelief from those he was trying to persuade. Bletchley Park therefore decided not to go ahead with the construction of this.

On returning to Dollis Hill in March 1943, he managed to gain the full backing of this project anyway from the director, W.G.Radley. With a team of helpers, he managed to build the first "Colossus" in 11 months. The Mark 1 Colossus ran for the first time at the end of 1943 and was then dismantled and reassembled at Bletchley Park in January 1944. On it's first demonstration, it successfully completed it's first test against a real enciphered message tape and Bletchley Park pretty soon ordered more Colossus machines.

The amazing machine that Tommy Flowers built was far ahead of anything available at the time and with a huge amount of forethought, rather than building it to just run a specific task, he made it so that the calculation being run could be altered using logic switches and plugs. This meant that once installed, as new solutions were come up with, the Colossus could be reprogrammed to run new tests and calculations. He also came up with solutions to run using a clock pulse to keep all the control systems working together and even a basic shift register - these ideas are still in use today on our modern-day computers!

Colossus reduced the time to break a Lorenz message from weeks to hours. It was up and running in time to decipher messages prior to D-Day which gave much needed confirmation that Hitler had fallen for the deceptions that the attack was to happen across the Pas-de-Calais rather than Normandy which meant a number of divisions of troops were kept away from the real site of attack.

After the successes of the Mark 1, Tommy Flowers already had many ideas for several upgrades for the next model and the Mark 2 Colossus was delivered in June 1944. Another 8 Colossus were built and delivered, amazingly at the rate of one per month, thereafter until the end of the war. The Colossus Mark 2's main feature was that it could remember five previous bits each run and check against each thereby speeding up the rate from 5,000 characters per second to the equivalent of 25,000 per second.

The end of Colossus

After VJ Day, it was all over. The requirement came down to dismantle and destroy the Colossus machines and all notes and drawings to keep its existence secret. Two machines were sent to Eastcote in North London and then to GHCQ at Cheltenham and these ran until 1960 when they finally got dismantled.