A Curta calculator looks like a pepper mill that has been given a serious office job. Numbers are set with tiny sliders around its cylindrical body. A crank turns on top, and the result appears through little windows, all without a battery, screen, or microchip.
Inside are more than 600 parts.[2] Outside, it is small enough to sit in one hand.[1] This would be an irresistible piece of mechanical cleverness even if its history were ordinary.
Its history is not ordinary.
A machine made smaller by changing the arithmetic
Curt Herzstark grew up around calculating machines in Vienna. His father, Samuel, founded a calculator business, and Curt worked there as a precision engineer.[1] Early twentieth-century calculators could perform serious arithmetic, but they were desk machines. Herzstark wanted one a person could carry.[1]
The problem was not merely making every gear smaller. Conventional stepped-drum calculators spread digit mechanisms across multiple drums. Herzstark arranged them around one central drum instead, turning the whole layout into a cylinder.[3]
He also made subtraction behave more like addition.
His 1938 patent describes a drum with ordinary stepped teeth for addition and a second, complementary set of teeth for subtraction. Moving the drum along its axis selects one set or the other.[3] Pulling the crank upward before turning it is therefore not a decorative flourish. It shifts the mechanism into complement arithmetic.
The idea is familiar to computers, even if the teeth are not. In a fixed number of decimal places, you can subtract by adding a complement and discarding the overflow. For a two-digit example, 42 minus 17 can be handled as 42 plus 83: the result is 125, and dropping the leading 1 leaves 25. The Curta's mechanism handles the necessary decimal carry and complement details in metal.
Multiplication becomes repeated addition. Division becomes repeated subtraction. Rotate the upper carriage to move between decimal places, and the same compact mechanism can do all four basic operations. The Computer History Museum calls it the smallest all-mechanical four-function calculator ever built.[2]
The drawings became a means of survival
Herzstark had already patented the central idea and was developing the compact design before the Second World War interrupted the project.[1][3]
In 1943, the Nazis classified Herzstark as a "half-Jew" and deported him to Buchenwald concentration camp.[1]
His engineering skill put him to work in the camp's Gustloff factory. The Swiss National Museum's account says he became a supervisor there and used the position to save other prisoners.[1] In his supposed free time, he worked on drawings for the calculator. The camp commandant hoped the finished machine could be presented to Hitler.[1]
That grotesque plan made the unfinished calculator useful to his captors. Herzstark completed the production drawings in the camp.[1]
Buchenwald was liberated in April 1945. Herzstark carried the drawings out and eventually secured backing in Liechtenstein.[1] Production began in 1948, and roughly 140,000 Curtas were made before electronic pocket calculators displaced the mechanism in the 1970s.[1]
The machine Hitler was meant to receive never became his gift. It took the name of its inventor instead.
Do not let the object tidy the story
Precision machines invite a particular kind of admiration. We like the exploded diagram, the polished crank, the satisfying fact that hundreds of parts cooperate inside a cylinder. The Curta deserves that attention. Its layout is ingenious, and its complement drum is a lovely example of mathematics becoming mechanism.
But the smooth little machine can make its own history look smoother than it was.
The Curta was not simply "invented in a concentration camp," as if imprisonment were a severe version of a productive residency. Herzstark conceived it before his arrest. In Buchenwald, his captors exploited both his engineering ability and his need to survive. The design advanced under coercion because the people running the camp thought it might amuse a dictator.[1]
The Swiss National Museum's account keeps both parts of the story on the same page: an ingenious machine and the coercion under which its production drawings were completed.[1]
So there are two kinds of compression in the Curta. One is mechanical: a four-function desk machine reduced to a cylinder you can carry. The other is historical, and it is worth resisting. "More than 600 parts" is a marvelous fact. The circumstances that brought those parts together should not be reduced to one.