What was the Difference Engine?

It was an automatic calculating machine designed by Charles Babbage (1791– 1871), English mathematician and polymath. He conceived the machine in 1821, in response to a dismaying number of errors in mathematical tables calculated by hand. The supposed infallibility of machinery would eliminate the risk of human error in the calculation and production of printed tables on which science, engineering and commerce heavily relied. On 14 June 1822 – 200 years ago this month – Babbage announced his invention, in a paper read to the Royal Astronomical Society.


When was a Difference Engine successfully built?

For a complex variety of reasons, Babbage failed to build a complete Difference Engine. His engineer, Joseph Clement, completed one-seventh of the full machine in 1832; it worked impeccably, but the project was abandoned following a dispute about payment. The first complete working Babbage engine is Difference Engine No 2, designed 1847–49 and completed at London’s Science Museum in 2002 – built faithfully to the original 19th-century designs. It weighs 5 tonnes, has 8,000 mechanical parts and is some 3.4 metres long.

How did it shape the future of computing?

Barely, if at all; pioneers of modern computing reinvented the principles and practice of computing almost entirely in ignorance of the detail of Babbage’s work. But the legend of what he had done endured, and the viability of machine computation was afterwards little doubted.

A group of formerly enslaved African-Americans around the time of the US Civil War

Why should we remember this invention today?

The Difference Engine was the first complete design for a computing engine. The small section completed in 1832 was the first successful automatic calculating machine to be built, and is the single most-celebrated icon in the prehistory of computing. A user, by cranking a handle, and without necessarily understanding how it worked, could achieve results that up to that time could be accomplished only by mental effort. “He [Babbage],” wrote a contemporary, “had taught wheel work to think.”

So Babbage’s engine represents the start of automatic computing, and also the start of machine intelligence, for which autonomous action is the first prerequisite. Though not itself a general-purpose machine (we would now call it a special-function calculator), the Difference Engine led directly to one that was – the Analytical Engine. This was conceived by Babbage by 1834, and its designs contain just about every logical feature of the modern digital computer.

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Doron Swade is a museum professional specialising in the history of computing


This article first appeared in the June 2022 issue of BBC History Magazine