The First Column Turned

1991 · Difference Engine No. 2 · Work 516 of 1,871

The First Column Turned, 1991, Difference Engine No. 2

A single column of a newly made difference engine standing on a workshop bench, a stack of bright brass figure wheels one above the other on a steel shaft, the metal freshly machined, brass swarf on the bench surface beside it, the bench top grey and plain, a flat grey wall behind, the teeth of the wheels sharp and the shaft ends bright, even flat light from directly in front, the column alone in the frame.

What it was used for
In June 1991 the calculating section of Difference Engine No. 2 was finished, not the machine. So the object record of the Science Museum Group for inventory number 1992-556 states: designed 1847 to 1849 by Charles Babbage, built by the Science Museum, the main part completed in June 1991 for the bicentennial year of Babbage's birth; the printing mechanism was constructed and added only in 2002. The calculating section is driven by a human operator turning a large crank handle at one end. Doron Swade gives the run-in speed of operation as ten complete engine cycles per minute, that is one tabular calculation every six seconds, equivalent to forty turns of the handle per minute with the four-to-one reduction gear in use.
What was new
The machine has a window of working speed, and Swade's build report says so: run faster, and the detent wheel of the intermittent drive of the carry axes tends to overshoot and jam the drive; run erratically or slower, and the lowered sectors tend not to mesh cleanly with the figure wheels, and jams occur again. One fault appeared only in the multiple assembly: on the trial piece the figure-wheel drive arms did not foul the internal nibs, but with multiple stacks lowering the axes produced consistent jamming, especially with wheels set at 0 or 9; chamfering the undersides of the arms solved it. And the price of the deadline: Swade writes that they reluctantly abandoned the idea of building the printing apparatus for the bicentenary and lowered the target to the calculating section only — Babbage had done the same. Without the printer, which is integral to the engine, it remained incomplete.

Further reading