1. Tommy Flowers: The Telephone Engineer Who Designed Colossus
Tommy Flowers, who designed Colossus, was born in East London in 1905. He worked as an apprentice at the Royal Arsenal in Woolwich while attending night school [6]. In 1926, he joined the telephone department of the General Post Office (GPO) and worked at the Dollis Hill research station in northwest London [4]. At that time, the GPO managed the telephone network, so Flowers gained experience with devices that used many switches, like telephone exchanges. He designed Colossus based on requirements from mathematician Max Newman at Bletchley Park [2].
A technician raised in the field of telephony, not in a university lab, was given the task of building a machine to read codes. Flowers reportedly used some of his own money to build this machine at the GPO research station [4]. His experience in telephone work, where many switches had to work reliably every day, influenced how he later handled vacuum tubes. But what weaknesses did the earlier machines have?
2. The Weakness of the Earlier Heath Robinson Machine and How Colossus Fixed It
The machine built before Colossus, called the Heath Robinson, used a method involving two paper tapes. It was difficult to run the two tapes at the same speed and align them precisely, which was a major limitation [2]. Flowers proposed generating the pattern created by the cipher machine's gears using electronic circuits (ring circuits). That would remove the need for one of the tapes, and with it the problem of aligning two tapes [1].
There was also a difference in speed. Colossus is described as running about 5 times faster than the Heath Robinson, which used mechanical switches [4]. However, creating patterns with circuits requires many components that can switch electrical signals at high speed. These components were vacuum tubes. Here lay the first major doubt.
3. Why Flowers Left the Power On for 1,500 Vacuum Tubes
Colleagues were skeptical of Flowers' plan. They doubted whether 1,000 to 2,000 vacuum tubes could operate reliably together [2]. Flowers cited the British telephone network as an example. The network used thousands of vacuum tubes and was still reliable because the electricity remained on in the circuits [3]. Some sources state that keeping the power on to reduce thermal stress was the key to success.
Every time electricity is turned on or off, components heat up and cool down suddenly. It is explained that Flowers thought keeping the power on would reduce the number of these sudden temperature changes. However, this is an explanation passed down as reasoning, not a proven experiment. Still, as a result, Mark 1, which used about 1,500 vacuum tubes (sources vary, with some saying 1,600), worked. Mark 2, with 2,400 tubes, was also built [2]. So, what was this machine actually doing?
4. How Colossus Read 5,000 Characters Per Second and Counted Matches
Colossus read paper tapes of ciphertext using photoelectric cells (components that convert light to electricity) and processed 5,000 characters per second [1]. This equals 300,000 characters per minute (5,000 × 60). The basic task was to count how many times a complex logical function came out true between the ciphertext and the gear pattern generated by the ring circuits, across the entire text [1].
On the other hand, this machine was not a stored-program computer like modern ones. It was a special-purpose machine set up with switches and a plug board [2]. Therefore, sources use the term "one of the world's first electronic computers" rather than definitively calling it "the world's first computer." The design and construction period was about 11 months [5]. Mark 1 was running at Dollis Hill around November 1943, and it was delivered to Bletchley Park around early 1944 (sources differ on the delivery month) [3].
5. How Many Colossus Machines Were Built and the 1992 Reconstruction
About 10 Colossus machines were built in total (some sources say 12). They were placed in large rooms like Block F at Bletchley Park [1]. After the war, 8 of the 10 were dismantled at Bletchley Park. The remaining 2 were moved to GCHQ (the UK's signals intelligence agency) and dismantled in the 1960s [1].
The existence of the machines was secret until the mid-1970s. Flowers' work became known when the government released information in 1975 [6]. It is also reported in testimony that Flowers burned drawings and documents in a boiler. After the war, Flowers was refused a loan by a bank to build a similar machine because they did not believe it would work. A medal and a £1,000 reward are said to have been insufficient for the costs he had invested [3].
But the story does not end there. In 1992, Tony Sale and others began the work of rebuilding Colossus, relying only on fragments of drawings, old photographs, and people's memories. In the 2007 Code Challenge, the reconstructed machine cracked a code in 3.5 hours [5].
6. An Activity: Reading Aloud to Feel How Fast 5,000 Characters Per Second Is
Let us try to feel how fast 5,000 characters per second is. Open a book and read aloud for 1 minute, counting the characters you read (count punctuation marks as 1 character). Divide that number by 60 to get the characters per second. How many times faster is Colossus's 5,000 characters per second compared to your reading speed? Calculate it and write it in your notebook to see the machine's speed in numbers.