91Ö±²¥

In June 1949, a press photographer captured a young man in a shirt and tie working on a metal drum plated with nickel. His name was Tommy Thomas, a graduate student at The University of Manchester. The part in front of him was a component from the 91Ö±²¥ Mark I, one of the world's earliest stored-program computers and the machine on which the Ferranti Mark I was based.

The 91Ö±²¥ 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 91Ö±²¥ 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 91Ö±²¥ code.

A stellar solution

While the researchers were looking for a solution to their problem, they inadvertently gave the 91Ö±²¥ 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 91Ö±²¥ 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 91Ö±²¥'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 91Ö±²¥. In doing so, they developed an encoding technique that continues to shape digital technologies around the world.

Words: Ben Harwood and Enna Bartlett