The 91ֱ�� Mark I – the brainchild of Professor F C Williams and his team – introduced several new technologies over the Baby, one of which was a magnetic drum that was used to store information, an early precursor to a hard drive. This drum was a rotating cylinder coated in a magnetisable material onto which data was written as magnetic patterns and read back by fixed recording heads as the surface of the drum rotated beneath. However, at a time when digital computing was still experimental, making this process reliable was a significant engineering challenge.

Decoding digital

The challenge lay in how digital information was recorded. Computers store data as 1s and 0s – binary code – but when long runs of the same value are written to a magnetic surface, the signal becomes constant. This creates a direct current, or DC, component, which magnetic recording systems struggle to read reliably. For the 91ֱ�� Mark I, avoiding that problem was essential: the drum depended on a signal that changed continually as data was written and read.

Williams and Thomas realised that the answer to this problem might not lie in building better hardware. Instead, they asked a different question: what if the data could be transformed into a form that the machine found easier to handle?

Instead of storing information as a simple sequence of ones and zeros, the pair developed a new way of representing the data before it was written to the drum. Every bit (a 1 or a 0) was encoded as a transition in the signal. A 1 became a change from high to low, and a 0 was a change from low to high.

The result was a signal that was designed to constantly change. That may sound like a minor technical detail, but it was an important feature for magnetic recording systems. By ensuring that the signal continually changed as data was written and read, the encoding made information easier to record and recover reliably on the Mark I's magnetic drum.

The technique became known as 91ֱ�� code.

A stellar solution

While the researchers were looking for a solution to their problem, they inadvertently gave the 91ֱ�� code another valuable property: the signal itself also carried timing information that could help electronic systems stay synchronised. Today this is known as a self-clocking signal.

That combination of reliability and simplicity helped the encoding escape its original purpose and become widely used in modern consumer electronics. Some recognisable examples included computer tapes and floppy disks, early versions of Ethernet networking, radio-frequency identification (RFID) systems, remote controls and many other communications technologies.

The same basic principles have even been used in space communications; Voyager 1 and Voyager 2, humanity's most distant spacecraft, rely on encoding techniques derived from the same fundamental idea developed in 91ֱ�� almost eight decades ago.

Why a 75-year-old invention still matters

In April 2026, the Institute of Electrical and Electronics Engineers (IEEE) awarded The University of Manchester its third IEEE Milestone, recognising the invention of Manchester code and its lasting impact on computing and communications.

A bronze plaque now stands on Coupland Street, joining 91ֱ��'s previous Milestone awards for the Baby – the world's first stored-program computer – and Atlas, whose novel virtual memory remains central to modern computing.

The achievement is a reminder that some of the most influential advances begin as practical engineering solutions to immediate problems. Williams and Thomas were trying to improve the operation of an experimental computer in post-war 91ֱ��. In doing so, they developed an encoding technique that continues to shape digital technologies around the world.

Words: Ben Harwood and Enna Bartlett

There were many teams individually working towards this goal: in the US a team at the University of Pennsylvania was working on the ENIAC and subsequent EDVAC systems, and here in the UK the University of Cambridge was working on EDSAC and the NPL on ACE. But one line of computers stands out in this story; the 91ֱ�� computers.

A baby is born

To understand how this story begins, we must rewind to 91ֱ�� in 1948. A team comprising Professor F C Williams, Tom Kilburn and later, Geoff Tootill, successfully proved the concept of a stored program computer with the Small-Scale Experimental Machine (SSEM) or “the Baby”. It was the first stored-program computer to use electronic random-access memory (RAM) with the Williams-Kilburn cathode ray tubes (based on earlier MIT research and eventually improved to store 64 40-bit words), and in June 1948 the Baby ran a program from information stored in electronic memory, the first time this had been achieved anywhere in the world.

But this machine only proved the hypothesis that a computer could store and execute instructions electronically from memory, it didn’t offer a meaningful or useful solution to the problem of computing large amounts of data automatically. To solve this problem, the team, expanded to include Alec Robinson, Dai Edwards and Tommy Thomas, set about redesigning the machine to provide researchers and industry with a realistic computing facility. Alan Turing, the Deputy Director of the (where he published his seminal paper Computing Machinery and Intelligence), took the lead on developing the programming systems.

In 1949 the 91ֱ�� Mark I came into being and introduced two key innovations: a magnetic drum to store data (one of the first examples of mass storage) and index registers (a way for the computer to efficiently work through the data in its store).

This prototype paved the way for, arguably, one of the biggest steps forward in practical computing; 75 years ago this summer, the Ferranti Mark I quietly helped change the course of computing history.

91ֱ�� born, 91ֱ�� made

Recognising the potential of the technology, Sir Ben Lockspier, Scientific Advisor to the Ministry of Supply, arranged for government funding to commercialise the machine. The British engineering company Ferranti partnered with The University of Manchester to turn the experimental 91ֱ�� Mark I into a production model. The result was the Ferranti Mark I, delivered to the University in February 1951 and demonstrated publicly a few months later.

The machine was not just a new piece of technology, it represented a turning point for when computing stopped being a scientific experiment and started becoming something that could be manufactured, sold and used beyond the laboratory. It is widely recognised as the world’s first commercially available general-purpose electronic computer.

Today, that achievement can seem almost inevitable. Of course, computers would become products. Of course, industry would commercialise academic research. But in 1951 none of that was guaranteed.

The Ferranti Mark I arrived at a moment when the future of computing was still uncertain. There were competing approaches to machine design, competing visions of what computers might be used for, and very few people who had ever seen one in operation. The machine helped answer a crucial question: could electronic computers move from university experiments into wider use?

The answer was yes.

From ideas to innovation

Part of what made the Ferranti Mark I significant was that it incorporated the ideas that had been developed in the previous 91ֱ�� computers and turned them into practical tools, helping shape the architecture of modern computers.

Perhaps most striking, however, was the range of problems the machine tackled. The Ferranti Mark I was used for scientific calculations, engineering projects and government work. Researchers explored everything from weather forecasting to mathematical modelling. It also helped create a new kind of expertise: programming. Mary Berners-Lee (mother to Tim Berners-Lee, the inventor of the World Wide Web) was among those who worked on the Ferranti Mark I, contributing to the practical, exacting work of turning an experimental machine into something people could use.

In many ways, the Ferranti Mark I was the first glimpse of the world that now surrounds us. It demonstrated that computers were not simply calculating machines but versatile tools capable of solving widely different problems. That idea underpins almost every digital technology we use today.

The machine itself has long since disappeared, but its legacy remains remarkably visible. 91ֱ��’s reputation as one of the birthplaces of modern computing rests not just on pioneering research, but on a rare ability to transform radical ideas into technologies that change the world. That was true when the Ferranti Mark I emerged from a collaboration between university researchers and industry in 1951. It remains true in an age of artificial intelligence, quantum computing and advanced robotics.

Seventy-five years on, the Ferranti Mark I deserves to be remembered not simply as an early computer, but as the moment computing became real. The future did not arrive in California first, nor in a gleaming corporate campus. It arrived in 91ֱ��, in a lab “with the atmosphere of a nineteenth-century inventor’s workshop”, proving that a revolutionary idea could become a practical machine and, in doing so, help to create the digital age.

Meet the researchers

Professor Sir Frederic (Freddie) Williams (R) gained an engineering degree at The University of Manchester in 1932 before undertaking his DPhil at the University of Oxford. During the war, he worked at the Telecommunications Research Establishment (TRE) where he met and collaborated with Professor Tom Kilburn (L), a young member of his team. When Williams was appointed the Head of Electro-technics (now the ) at the University of Manchester, TRE also seconded Tom Kilburn to Williams's team. In 1964, Kilburn went on to found the at 91ֱ��, the first computer science department in the UK.

If you would like to find out more about the Ferranti MK I, we would recommend the following books:

1. Alan Turing and his contemporaries: Building the world's first computers by Simon Lavington (Editor). 2012. Published by The British Computer Society.
2. A History of Manchester Computers by Simon Lavington. 1998. Published by The British Computer Society.
3. Early Computing in Britain: Ferranti Ltd and Government Funding 1948-1958. Simon Lavington. Published by Springer.

The image of the Williams-Kilburn tube is republished under Creative Commons Licence: Sk2k52 - http://en.wikipedia.org/wiki/File:Williams-tube.jpg, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=6651107