Telling someone 100 years ago that we all have powerful calculators in devices in our pockets today would’ve absolutely blown their mind. Back then, calculators were large mechanical devices that sat on desktops and were used by the technical elite. The common person would have nearly no way to access a calculator of any means, nonetheless a portable one.

However, in the mid-1930s, a man named Curt Herzstark had an idea that changed the world of calculators forever. He worked as a mechanical calculator salesman in the former Austrian Empire and consistently heard customers complain about how bulky these mechanical monoliths were. They were expensive and just too large to be practical in many applications. Many even desired something completely new: a calculator that could fit in a pocket.

For 10 years, Curt worked on a mechanical calculator concept that would drastically reduce their size and allow him to create a device that perfectly met the needs of the market.

He struggled for years to compress the already stressed mechanics of calculators of the day. In 1937, he had a breakthrough. Instead of trying to design better internal mechanics, he started with the external user interface. He wanted the device to be able to do simple arithmetic but was running into trouble working out the mechanics to do every operation. So, he decided to use the method of complements in his design, essentially allowing him to subtract using addition.

For example, the complement of 9 is 0, of 8 is 1, of 7 is 2, and on it goes. If you wanted to subtract 210 from 782 (782-210) using complementing numbers, you would convert the larger number to its complement, which is 217, and then add that number to 210, equalling 427. Finding the complement of  427, you get 572, which is the final answer to the problem. This allowed Hertzstark to simplify every mathematical operation down to addition from a mechanical standpoint.

Instead of having registers of numbers in his mechanical calculator like other designs, he created a step drum where each step corresponds to a number. He added another drum on top of the first, but in reverse, allowing a simple shift of the device to switch between addition and subtraction through the use of mechanically imposed complimentary calculation.

Herzstark was taken prisoner by the Nazis in the second world war due to being a Jewish engineer. Luckily, his idea for a compact pocket mechanical calculator ended up saving his life. An SS guard took an interest in his designs and ultimately believed that allowing Herzstark to create it could be advantageous to the Nazi regime. Ultimately, the Allies liberated the camp and Herzstark decided now was the time to bring his calculator to the market.

He worked with local factories to produce prototypes, but now fearing capture from the Soviet Red Army, he was only able to escape with 3 working calculators, which he decided to name, the Curta.

Image Source: Curta.li

After working on the device for over a decade, in 1946, the Prince of Lichtenstein decided to back the device and have Curt manufacture Curtas exclusively in his country.

The Curta Mark 1 was introduced to the market in 1949 for \$149 and it completely blew away the idea of mechanical calculators for nearly everyone in the industry. Engineers and traveling accountants loved the device. Over 20 years, 150,000 of them were built and today they still sell for over \$1000. With an accuracy from 11 to 15 decimal places and a robust design, their simple yet masterful ability to quickly solve calculations brought them into the premiere spot exemplifying mechanical calculators. Their run into calculator dominance was only halted by the emergence of electronic calculators in the 1960s and 1970s.

Even still, the Curta calculator is one of the most impressive mechanical engineering designs to this day. It was the world’s first, last, and only pocket-sized mechanical calculator. It was truly a masterfully designed unicorn of the engineering realm.

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