"Nine's complement arithmetic can be implemented in hardware by inverting the cogs on the step drum. Herzstark cleverly realized that a single step drum can be used to implement both normal addition and nine's complement addition by interleaving the normal and inverted cogs and moving the step drum up or down slightly to switch between them. [...] An additional row of ten teeth is added to the step drum to implement one plus the nine's complement of zero."
How the Curta Works
Tags: computers, mad science, mpegs, retrocomputing, toys
You've seen this old collection of U.S. Navy instructional videos detailing how mechanical fire-control computers worked, I hope.
Those "previously" links are there to assist with your personal growth.
I had never heard of the Curta until reading William Gibson's "Pattern Recognition" which uses one as a plot device, offhandedly called a "math grenade". The book also talks about Stephen King's Wang. And yes, Gibson understands how that sounds:
There's an long biographical interview online someone did with Curt Herzstark, the man who created the Curta. His life story was ridiculously sad. He designed the Curta while he was in a concentration camp, and ultimately the design was stolen by the people in Liechtenstein who promised to fund the factory to make the things. He was a really smart and interesting man who had a pretty awful life. :(