Difference Engine

(computer, history)Charles Babbage's design for the firstautomatic mechanical calculator. The Difference Engine was aspecial purpose device intended for the production ofmathematical tables. Babbage started work on the DifferenceEngine in 1823 with funding from the British Government. Onlyone-seventh of the complete engine, about 2000 parts, wasbuilt in 1832 by Babbage's engineer, Joseph Clement. This wasdemonstrated successfully by Babbage and still worksperfectly. The engine was never completed and most of the12,000 parts manufactured were later melted for scrap.

It was left to Georg and Edvard Schuetz to construct the firstworking devices to the same design which were successful inlimited applications. The Difference Engine No. 2 was finallycompleted in 1991 at the Science Museum, London, UK and is ondisplay there.

The engine used gears to compute cumulative sums in a seriesof registers: r[i] := r[i] + r[i+1]. However, the additionhad the side effect of zeroing r[i+1]. Babbage overcamethis by simultaneously copying r[i+1] to a temporary registerduring the addition and then copying it back to r[i+1] at theend of each cycle (each turn of a handle).

Difference Engine at the Science Museum.

Difference Engine

An early calculator designed by Charles Babbage and subsidized by the British government. Employing wheels and rods, which others had experimented with earlier, the project was started in 1821 but failed its test in 1833. Babbage then turned his attention to the Analytical Engine and completely abandoned the Difference Engine by 1842. Although never completed, it did improve the precision of Britain's machine-tool industry. In 1991, the National Museum of Science and Technology built a working model of the Difference Engine.

In 1879, Babbage's son reassembled a section of the Difference Engine from parts, and in 1995, Christie's auction in London auctioned off that section to the Power House Museum in Sydney for USD $282,000. The other known sections are owned by Harvard and Cambridge Universities. See Analytical Engine.

The Difference Engine

This impression from a woodcut was printed in 1853 showing a portion of the Difference Engine that was built in 1833. Babbage later turned his attention to the Analytical Engine. It, too, was never finished. (Image courtesy of Charles Babbage Institute, University of Minnesota, www.cbi.umn.edu)