The Curta is a small mechanical calculator developed by Curt Herzstark. The Curta's design is a descendant of Gottfried Leibniz's Stepped Reckoner and Charles Thomas's Arithmometer, accumulating values on cogs, which are added or complemented by a stepped drum mechanism. It has an extremely compact design: a small cylinder that fits in the palm of the hand. Curta's were considered the best portable calculators available until they were displaced by electronic calculators in the 1970's.
NB: I really loved illustrating the Curta: it was a commission from Scientific American journal: They were very pleased with it, and I loved and enjoyed every moment of the project . . . a GREAT and fun project to work on!