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The hand-cranked calculator invented by a Nazi concentration camp prisoner

The Curta mechanical calculator.
Enlarge / The Curta mechanical calculator.

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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.

“Isn’t there anything smaller?”

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.

Thanks to an extensive interview conducted for the Charles Babbage Institute many years later, we have Herzstark’s own recollections of those busy years. He recalled that as sophisticated as his company’s machines were, “something was missing in the world market.” He remembered meeting with architects, foremen, and customs officers who needed calculating machines that were not only accurate and reliable but also portable.

“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.

“What does this kind of machine really have to look like so that someone could use it? It cannot be a cube or a ruler; it has to be a cylinder so that it can be held in one hand,” Herzstark mused. “And if one can hold it in one hand, then if it is miniaturized, you could adjust it with the other hand… I started to design the ideal machine from the outside first, before I designed the insides.”

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.

Surviving the war

On March 12, 1938, Austria was annexed by Nazi Germany (during the event known as the Anschluss). Herzstark, the son of a Jewish father and Catholic mother, feared the worst, though for the next few years, the factory was allowed to continue to operate, so long as it produced machines and tools for the German army. But the situation quickly deteriorated. Two colleagues were arrested for listening to British radio stations, and when Herzstark was offered to testify on their behalf, he, too, was arrested.

“I was accused of supporting Jews, aggravation, and having an erotic relationship with an Aryan woman… it was all fabricated,” Herzstark said. He was sent to the Pankratz prison in Prague and later transferred to the Buchenwald concentration camp in central Germany.

Conditions were horrific. “When they hung someone,” Herzstark recalled, “we had to watch until he finally died. Terrible. They hung people so they died slowly, a wretched death.” Herzstark was put to work in an adjacent factory that built components for Germany’s V2 rockets. Eventually, a senior German engineer took him aside.

For the first time, Herzstark began to imagine that he might survive the war.

“See, Herzstark, I understand you’ve been working on a new thing, a small calculating machine,” the engineer said. “Do you know, I can give you a tip. We will allow you to make and draw everything. If it is really worth something, then we will give it to the Fuhrer as a present after we win the war. Then, surely, you will be made an Aryan.”

For the first time, Herzstark began to imagine that he might survive the war. “And then and there I started to draw the Curta the way I had imagined it,” he said.

Buchenwald was liberated by US troops on April 11, 1945. A few days later, Herzstark walked to the city of Weimar, some four miles away, with the plans for his calculating machine in his pocket. He found a factory that was still functioning, and before long, he had a prototype of the machine.

Soon, however, the Soviet army arrived. Herzstark retreated back to Vienna, carrying only a box containing the disassembled parts of the device. With European industry struggling in the post-war years, Herzstark was happy to find that the government of tiny Liechtenstein was interested in his machine. A company called Contina AG Mauren was set up, with Herzstark serving as technical director. The first batch of Curtas went on sale in 1948. A slightly larger model that could display more digits, the Type II Curta, appeared in 1954.

The Curta was popular with accountants, engineers, and surveyors. Rally car navigators liked it because it could be used by touch; an experienced user hardly even needed to look at the device. Peter Boyce, a retired astronomer who worked for many years at the Lowell Observatory in Arizona, used a Curta when he was a graduate student at the University of Michigan in the 1960s. He remembers it as “a wonderful, precision machine,” one that was especially useful outside the office. “It was good to take to the telescope, where I used it instead of pencil and paper if I needed to calculate something at 2:00 am.”

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