It’s no bigger than a drinking glass, and it fits easily in the palm of the hand. It resembles a pepper grinder—or perhaps a hand grenade.

The diminutive “Curta” is a striking machine, a mechanical calculator that combines the complexity of a steamship engine and the precision craftsmanship of a fine pocket watch. It first appeared in 1948, and for the next two decades—until it was displaced by the electronic calculator—it was the best portable calculating machine on the planet. And its story is all the more compelling in light of the extraordinary circumstances in which it was invented.

The idea of the Curta came to its Austrian-born inventor in the darkness of the Buchenwald concentration camp.