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.
[...] Today, we take number-crunching for granted. Our smartphones have calculator apps, and most of us have a pocket calculator somewhere in our home or office. But it wasn't always so easy. For centuries, anything more than simple addition was painfully time-consuming. The first slide rules appeared in the 17th century, not long after John Napier's invention of the logarithm, but they could only handle a couple of positions beyond the decimal place. There were also various kinds of mechanical adding machines, but most were crudely built and unsuited to scientific work. By the late 19th century, more reliable desktop calculators began to appear, but they were heavy and expensive.
The shortcomings of these machines were very much on the mind of the young Curt Herzstark, whose family was in the business of making and selling calculating machines and other office equipment. Born in 1902 in Vienna, Herzstark was running the family business by the 1930s. He traveled extensively across Austria, Hungary, and Czechoslovakia, selling mechanical calculators to banks and factories.
[...] "People said again and again, 'Yes, that is nice, but isn't there anything smaller?'" Herzstark recalled. Slide rules were not good enough; his customers wanted precise figures, not approximations. Simply taking existing designs and making all of the various parts smaller wouldn't do the trick; the keys and knobs would be too small to use. A radical redesign was needed.
[...] Herzstark began to experiment with "sliders" that wrapped around a cylinder so that numbers could be entered by moving a thumb or finger. He also reasoned that there only needed to be a single calculating mechanism, so long as each input digit could access it. At the heart of the device would be a single, rotating "step-drum"; the drum would have two sets of teeth, one for addition and one for subtraction. A central hand crank would turn the drum, and shifting the drum's position by a few millimeters was enough to switch between the adding and subtracting functions. Multiplication and division were slightly more complicated, but they still required just a few flicks of the sliders and a few turns of the crank.
By 1937, Herzstark had the essentials of the design worked out; after that, it was just a matter of machining the parts and building a prototype.
And then Hitler came to power.
It's a long-format story complete with pictures. Many videos are available on-line of the device in operation and even YouTube videos 3D-printing one of your own.