Babbage Difference Engine

[Versión en castellano - Spanish Version]

Charles Babbage [1791-1871] is widely recognized as the father of the modern computer. His design for the Analytical Engine contains much of the basic features still found in a modern computer. The Analytical Engine has received much attention and is covered elsewhere in the web.  A good site is John Walker's. His previous work on the Difference Engine brings up very few references, that of the The Science Museum at London where a Difference Engine was built according to Babbage's blueprints and Ed Thelen's website which includes excelent descriptions and original documentation provided by the Science Museum.

Babbage never completed any of his designs. His image is often seen as that of a frustrated inventor. I was surprised to find that he was not only a member of the Royal Society, but held the Lucasian chair of mathematics in Cambridge for 11 years. That position has been held by Isaac Newton and currently Stephen Hawking, hardly a position granted to a failure.

In a visit to London, early in 1991, there was a section in the ground floor of the Museum where a couple of people were assembling an impressive collection of gears, shafts and cams into which would be the first full size Difference Engine ever to be finished to commemorate the bi-centenary of Babbage's birth. It is based on Babbage's Difference Engine No2. From now on, when I say Difference Engine, or just Engine, I mean this model. Late in 1998, in another visit to London, I was able to see the final model, though in a static display. Even though I knew the principles under which it operates, a couple of animations that were shown by the exhibit were very helpful in understanding the basics of how the mechanism works. Thus I got the idea to do the same to show on-line, while at the same time, building the virtual model helped me to understand some details that still escaped me.

In doing so, I contacted the people at the Science Museum and Doron Swade, Assitant Director and then Curator of the collection of computers and control at the Museum was very kind to provide me with a copy of his paper on the design of the Engine and the construction of the model at the Museum, which helped me understand several issues that were not clear to me, with the little information I had at the time.

By the time I received the paper, my virtual model was very advanced, with a few errors and some estimated dimensions instead of true ones, and I decided to publish it on the web as is, instead of waiting to fix it, which I'll do in the hopefully near future.

I'd appreciate any comments, I can be reached at:

Table of Contents

  1. The need for an automatic calculator
  2. Mathematical principle
  3. Overall design
  4. Figure wheel
  5. Adding a digit
  6. Restoring the value of a number wheel
  7. Carry detection
  8. Carry processing
  9. Secondary Carries
  10. Miscellaneous