Difference Engine - History

History

J. H. Müller, an engineer in the Hessian army, conceived of the idea of a difference machine. This was described in a book published in 1786, but Müller failed to obtain funding to progress with the idea.

In 1822, Charles Babbage proposed the use of such a machine in a paper to the Royal Astronomical Society on 14 June entitled "Note on the application of machinery to the computation of astronomical and mathematical tables". This machine used the decimal number system and was powered by cranking a handle. The British government was interested, since producing tables was time consuming and expensive and they hoped the difference engine would make the task more economical.

In 1823, the British government gave Babbage ₤1700 to start work on the project. Although Babbage's design was technically feasible, no one had built a mechanical device to such exacting standards before, so the engine proved to be much more expensive than anticipated. By the time the government killed the project in 1842, they had given Babbage over ₤17,000, without receiving a working engine. What Babbage did not, or was unwilling to, recognize was that the government was interested in economically produced tables, not the engine itself. The other issue that undermined the government’s confidence in the difference engine was Babbage had moved on to an analytical engine. By developing something better, Babbage had rendered the difference engine useless in the eyes of the government.

Babbage went on to design his much more general analytical engine, but later produced an improved "Difference Engine No. 2" design, between 1847 and 1849. Babbage was able to take advantage of ideas developed for the analytical engine to make the new difference engine calculate more quickly while using fewer parts. Inspired by Babbage's difference engine plans, Per Georg Scheutz built several difference engines from 1855 onwards, one of which was sold to the British government in 1859. Martin Wiberg improved Scheutz's construction but used his device only for producing and publishing printed logarithmic tables.

During the 1980s, Allan Bromley, an associate professor at the University of Sydney, Australia, studied Babbage's original drawings for the Difference and Analytical Engines at the Science Museum library in London. This work lead the Science Museum to construct a working difference engine No. 2 from 1989 to 1991, under Doron Swade, the then Curator of Computing. This was to celebrate the 200th anniversary of Babbage's birth. In 2000, the printer which Babbage originally designed for the difference engine was also completed. The conversion of the original design drawings into drawings suitable for engineering manufacturers' use revealed some minor errors in Babbage's design (possibly introduced as a protection in case the plans were stolen), which had to be corrected. Once completed, both the engine and its printer worked flawlessly, and still do. The difference engine and printer were constructed to tolerances achievable with 19th-century technology, resolving a long-standing debate whether Babbage's design would actually have worked. (One of the reasons formerly advanced for the non-completion of Babbage's engines had been that engineering methods were insufficiently developed in the Victorian era.)

Although the "printer" is here referred to as such, its primary purpose is to produce stereotype plates for use in printing presses; Babbage's intention being that the Engine's results be conveyed directly to mass printing. Babbage recognized that errors in previous tables were not the result of human calculating errors but from human error in the printing process. The printer's paper output is mainly a means of checking the Engine's performance.

In addition to funding the construction of the output mechanism for the Science Museum's Difference Engine No. 2, Nathan Myhrvold commissioned the construction of a second complete Difference Engine No. 2, which is currently on exhibit at the Computer History Museum in Mountain View, California.