the Curta Calculator

Thinking about computers - here’s for something completely different, and one heck of a story to boot. A few years ago I managed to lay my hands on a Curta calculator. The Curta is an early mechanical computer, that was favored by pilots and navigators in the fifties, partly because you can use it single handedly. Even if it does get dropped it’ll survive it, and it’s unlikely that your calculations have been messed up, so you can continue where you were. Flying with an iPad is not quite the same! 

Here’s the story behind the calculator. The original inventor was a chap called Curt Herzstark, who ran a machine shop in Austria in the late thirties, just before the war. The Curta accumulates values on cogs, which can then be added or subtracted using a stepped drum mechanism. The result is an amazingly small device, that fits in one hand, and it became a beloved device of pilots and navigators as it was (relatively) easy to use even while flying in turbulent air. 

Originally the Curta was conceived in 1930s Vienna, in Austria. Curt Herzstark filed a key patent in 1938, which covered the stepped drum. This single drum replaced the multiple drums, typically around 10 or so, of contemporary calculators, and it not only made addition possible, but also subtraction through nines complement math, essentially subtracting by adding. The nines' complement math breakthrough eliminated the significant mechanical complexity created when "borrowing" during subtraction. This drum would prove to be the key to the small, hand-held mechanical calculator the Curta would become. His work on the pocket calculator stopped in 1938 when the Nazis forced him and his company to concentrate on manufacturing measuring instruments and distance gauges for the German army.

Herzstark mother was Catholic, and his father was Jewish. After the “Anschluss” whereby Germany took over control of Austria in March of 1938, this became a problem. He managed to file two patents in Germany before he was picked up by the Germans due to some staff problems - and being Jewish, he was ultimately taken to Buchenwald, a notorious concentration camp. Strangely enough it was there that the main research and development work that led to the Curta was done after being encouraged by some of the elders on the basis that he might be able to save his life by presenting a functional device to Hitler… 

"While I was imprisoned inside Buchenwald I had, after a few days, told the people in the work production scheduling department of my ideas. The head of the department, Mr. Munich said, 'See, Herzstark, I understand you've been working on a new thing, a small calculating machine. 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 Führer as a present after we win the war. Then, surely, you will be made an Aryan.' For me, that was the first time I thought to myself, my God, if you do this, you can extend your life. And then and there I started to draw the CURTA, the way I had imagined it.Numbers are entered using slides (one slide per digit) on the side of the device. The revolution counter and result counter appear on the top. A single turn of the crank adds the input number to the result counter, at any position, and increments the revolution counter accordingly. Pulling the crank upwards slightly before turning performs a subtraction instead of an addition. Multiplication, division, and other functions require a series of crank and carriage-shifting operations.

Herzstark survived the war with detailed drawings of his Curta calculator concept, and had three prototypes made by a factory in what became East Germany. He figured out that the Russians were on their way and he’d easily become a prisoner all over again. He left to head back to Austria where his younger brother was running the company and wanted a part of the proceeds of Herzstark’s Curta invention - something he was not willing to accept. He found investors in Lichtenstein, a tiny country between Austria and Switzerland, to help him with his idea for the Curta, and they started a company to start production of the Curtas. The investors didn’t want to spend money on the patents that he wanted to file, but he filed a US patent anyway, on his own accord. A few years in, his investors decided to push him out of the company, as they did not feel that had a further need for Herzstark when they hit some funding problems- but they had not taken the patents into account, which were filed in his own name as the sole inventor. Ultimately, control of the patents for the Curta allowed Herzstark to finally make some money out of his invention - and 140,000 Curta calculators were produced until 1972, when he stopped production as electronic calculators started to arrive on the scene. 

The Curta was also known as the "pepper grinder" or "peppermill" due to its shape and means of operation. It was also termed the "math grenade" due to its superficial resemblance to a certain type of hand grenade. Herzstark had actually called it the Liliput (and the three prototypes were marked with that name). It was only when the investors did not like that name during a trade fair in 1948 when one of their trade correspondents, the Dutch Miss Ramakers, mentioned that if the calculator was the daughter of a man called Curt, it should be called Curta. The name changed, and the devices have been known as Curta calculators ever since. 

I’m lucky enough to have been able to acquire a very nice Curta I Calculator - the original model I. The Curta I used 571 components to create a device that has a capacity of 8 positions for the multiplicand, 6 positions for the revolutions register for the multiplier, and 11 positions in the results register. About 80,000 of these units were produced. The later model, the Curta II, has 11 positions for the multiplicand, 8 positions in the revolutions register and 11 positions in the results register - it was slightly larger than the original version, and has 719 parts. 

It’s an amazing machine, and if you ever get a chance to play with one, do. The mechanics are so beautifully made that you can just feel the quality. Once you get a feel for the device it’s so quick and easy to use - I certainly wish I’d had one of these to complement my trusty slide rule when I was at college! Failing that - have a play with this on-line simulator by clicking over this image.

Oh - and if you ever get curious: no, do not even think about taking one to bits. Apparently there’s no way to re-assemble these units without a bunch of specialist tools. However, there are some 3D printed versions around, so if you have a printer, there's a alternative possibility to re-create one of these amazing machines.