What a difference 200 years makes


Recently, I had an opportunity to see the Babbage Difference Engine No. 2 (serial #2) in action. It’s an impressive piece of machinery, weighing in at about five tons, consisting of 25,000 parts. Mostly metal. It’s on display at the Computer History Museum in Mountain View through December, when Nathan Myhrvold takes it home and installs it in his living room, next to the T-Rex. Babbage built a few smaller models, but never saw the completion of the project after a falling out with his master builder and subsequent loss of funding from the government. Still, he had something like 12 years of funding to attempt to build the device. (He also made money on other inventions such as the cowcatcher at the front of steam engines.)

The Science Museum in London built Difference Engine No. 2 serial #1 in the late 1980s to commemorate the 200th anniversary of Babbage’s birth.

Front view showing the registers

Front view showing the registers

The problem the engine was designed to solve was the high error rate in the manual creation of logarithm tables. Babbage solved the problem by creating a difference engine that solved polynomials, solutions to which would be used to approximate the logarithms. But since the traditional technique of creating logarithm tables had several sources of error introduced by people (transcription and typesetting), Babbage sought to remove those errors by automating the process.

Thus the device not only calculates the solution, but also typesets it (one line at a time) and then creates a print mold for each page. The mold can be filled with lead to create an page image for printing. It even creates two sizes of page at a time, a small and a large one!

Printing mechanism

Printing mechanism. Paper roll is the "print preview"; the rectangular tray at the bottom contains the plaster mold that is imprinted with the computed numbers.

This brute display of hardware ingenuity is particularly impressive to me in contrast to building things in software. The physical manifestation of dependencies is truly awesome, as it clanks away, generating the next set of coefficients as a skilled operator turns its crank. (Skill is required to turn it at a reasonably-constant rate despite variations in load over the course of its compute cycle.) In fact, the crank was one of a few small departures from the original design (this second machine was completed last year): an extra gear was added to the crank to make it possible for a normal person to operate it!

Crank and cams for synchronizing operation of various subsystems

Crank drives gear on top right, which transfers motion to cams for synchronizing operation of various subsystems

The history of this device — from plans in the middle of the 19th Century to a first full build according those plans at the end of the 20th (and second in the first years of the 21st) — is testament both to strength of Babbage’s vision and to the frustrations of implementing that vision, something familiar to researchers in many disciplines. Imagine having to wait for the 200th anniversary of your birth to have your idea vindicated. Makes the normal tech transfer cycle seem positively instantaneous!

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  1. […] event will be held on October 5th in the Computer History Museum (the current home of the Babbage Difference Engine No2 Serial #2), and will feature a great lineup of researchers reporting on their accomplishments in a variety of […]

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