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The Curta is a small, hand-cranked mechanical calculator introduced by Curt Herzstark in 1948. It can be used to perform addition, subtraction, multiplication, division, and (with more difficulty) square roots and other operations.
The Curta's design is a descendant of Gottfried Leibniz's Stepped Reckoner and Thomas's Arithmometer, accumulating values on cogs, which are added or complemented by a stepped drum mechanism. It has an extremely compact design: a small cylinder that fits in the palm of the hand.
Curtas were considered the best portable calculators available until they were displaced by electronic calculators in the 1970s.
The Curta was conceived by Curt Herzstark (1902–1988) in the 1930s in Vienna, Austria. By 1938, he had filed a key patent, covering his complemented stepped drum, Deutsches Reichspatent (German National Patent) No. 747073. This single drum replaced the multiple drums, typically around 10 or so, of contemporary calculators, and it enabled not only addition, but 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, the son of a Catholic mother and Jewish father, was taken into custody in 1943, eventually finding himself at the Buchenwald concentration camp. Ironically, it was in the concentration camp that he was encouraged to continue his earlier research: "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."
Herzstark worked hard to move his invention from his knowing how to build the device "in principle" to concise working drawings for a manufacturable device.
The department head's celebration plan did not materialize, but Herzstark's construction plans did. Between April 11, 1945, when Buchenwald was liberated by the United States, and the following November, Herzstark was, after making only a few "detailed improvements" to the design, able to locate a factory in Sommertal, near Weimar, where machinists were skilled enough to work at the necessary level of precision, and walk away with three working models of the calculator.
The forces of the USSR had arrived in July, and Herzstark feared being sent to Russia, so, later that same month, he fled to Austria. He began to look for financial backers, at the same time filing continuing patents as well as several additional patents to protect his work. The Prince of Liechtenstein eventually showed interest in the manufacture of the device, and soon a newly-formed company, Contina AG Mauren, (aka Contina Ltd Mauren) began production in Liechtenstein.
It was not long before the financial backers, apparently having gotten from him all they thought they needed, contrived to force him out by reducing the value of all existing stock to zero, including his one-third interest in the company. These were the same people who, earlier, had elected not to transfer ownership of the Herzstark's patents to the company, so that, should anyone sue, they would be suing Herzstark, not the company, thereby protecting themselves at Herzstark's expense. This ploy now backfired: without the patent rights, they could manufacture nothing. Herzstark was able to negotiate a new agreement, and money continued to flow to him.
Curtas were considered the best portable calculators available until they were displaced by electronic calculators in the 1970s. Herzstark continued to make money from his invention until that time, although, like many inventors before him, he was not among those who profited the most from his invention. The Curta, however, lives on, being a highly-popular collectible, with thousands of machines working just as smoothly as they did 40, 50, and 60 years ago when they were manufactured.
The Curta Type I was sold for $125 in the later years of production; the Type II was sold for $175. While only 3% of Curtas were returned to the factory because of repairs under warranty, a small, but significant number of buyers returned the Curta in pieces. Many purchasers attempted to disassemble the Curta. Reassembling the machine was more difficult as assembly required intimate knowledge of the orientation and installation order for each part and sub-assembly, plus special guides designed to hold the pieces in place during assembly. Many identical looking parts, each with slightly different dimensions, required test fitting and selection as well as special tools to adjust tolerances.
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 it performs a subtraction instead of an addition. Multiplication, division, and other functions require a series of crank and carriage-shifting operations.
The Curta was affectionately 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.
The Type I Curta has 8 digits for data entry (known as "setting sliders"), a 6-digit revolution counter, and an 11-digit result counter. According to the advertising literature, it weighs only 8 ounces (230 g). Serial number 70154, produced in 1969, weighs 245 grams (8.6 oz).
The larger Type II Curta, introduced in 1954, has 11 digits for data entry, an 8-digit revolution counter, and a 15-digit result counter. It weighs 13.15 ounces (373 g), based on weighing serial number 550973, produced in early 1966.
An estimated 140,000 Curta calculators were made (80,000 Type I and 60,000 Type II). According to Curt Herzstark, the last Curta was produced in 1972.
The Curta was popular among contestants in sports car rallies during the 1960s, 1970s and into the 1980s. Even after the introduction of the electronic calculator for other purposes, they were used in time-speed-distance (TSD) rallies to aid in computation of times to checkpoints, distances off-course and so on, since the early electronic calculators did not fare well with the bounces and jolts of rally racing.
Contestants who used such calculators were often called "Curta-crankers" by those who were limited to paper and pencil, or who used computers linked to the car's wheels.
The Curta was also favored by both commercial and general-aviation pilots, before the advent of electronic calculators, because of both its precision and the user's ability to confirm the accuracy of his or her manipulations via the revolution counter. Because calculations such as weight and balance are critical for safe flight, precise results free of pilot error are essential.
- Stoll, Cliff (January 2004). "The Curious History of the First Pocket Calculator". Scientific American 290 (1): 92–9. doi:10.1038/scientificamerican0104-92. PMID 14682043.
- "An Interview with Curt Herzstark" conducted by Erwin Tomash, Charles Babbage Institute, University of Minnesota, Minneapolis. Herzstark interview in German
- Curt Herzstark and his Pocket Calculator, Peter Kradolfer, backup 6/88 pp. 5–9
- ICES Insight 48. International Council for the Exploration of the Sea. September 2011. p. 13. ISBN 978-87-7482-097-0. Retrieved 2012-08-06.
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