The World’s Smallest Mechanical Parallel Calculator: Discovery of Original Drawings and Patent Documents from the 1950s in Switzerland

. The Austrian engineer Curt Herzstark invented the world-renowned mechanical pocket calculator Curta . The books and the papers on the history of computing mention two models, Curta 1 and Curta 2. In November 2015 original drawings and patent documents on a multiple Curta were discovered in Switzerland. This device is supposed to be the world’s smallest mechanical parallel calculator.


Herzstark's Legacy: Discovery of Unknown Machine
On 14 November 2015 I examined the legacy of Curt Herzstark. It is located at the typewriter museum Beck at Pfäffikon near Zurich. To my great surprise I found original drawings and patent documents of an unknown multiple calculating machine: the world's smallest mechanical parallel calculator. Curt Herzstark, a highly gifted Austrian engineer, was forced by the Nazis to design the construction drawings for the world-famous Curta at the concentration camp of Buchenwald. The Curta is the smallest and most successful mechanical calculating machine which is able to perform all four basic arithmetic operations. Today, these pretty "pepper mills" are still working. They inspire specialists as well as laymen. From 1947 to 1971 this engineering marvel was produced in large quantities at Mauren (Liechtenstein). Up to now we knew two marks of the stepped drum machine, the Curta 1 and the Curta 2.
According to the patent documents recently turned up any number of mechanical cylindrical calculators may be combined. Herzstark describes the following forms:  two Curtas side by side,  two Curtas one upon another,  four Curtas side by side,  five Curtas in a circle.

Characteristic Features of the Multiple Calculating Machine
The parallel calculator excels in the following properties:  All machines are driven by one single, common crank.  All devices may be switched (by axial shifting of the driving shaft) in common or alone to another basic arithmetic operation. This is possible even if the individual machines are set up for different arithmetic operations.  All registers may be shifted (decade by decade) in common or alone in both rotating directions.  All result registers and/or revolution counters may be cleared in common or alone.  The multiple calculation machine can be equipped with an electric drive.

Double, Fourfold and Fivefold Curta
The extraordinary find proves that the Viennese inventor did not only design single but also multiple Curtas.

Acceleration of the Arithmetic Operations
The

The World's Smallest Mechanical Parallel Calculator
To my knowledge the multiple Curta is the smallest mechanical calculating machine in the world, the first pocket-sized mechanical parallel calculator. The patent documentation contains comprehensive descriptions. Curt Herzstark presumably built some prototypes. I conducted several oral history interviews with six elderly eye witnesses in Liechtenstein, Austria, Germany and Switzerland. Unfortunately they do not remember the inventor's corresponding research work. The multiple calculating machine was not mass-produced. Herzstark who was defrauded of his lifework by the Nazis had already left his manufacturing firm Contina AG at this time. There is a plans to rebuild the multiple Curta. In order to simplify the handling the Austrian engineer designed another type of construction for of the double Curta with a base plate (additional patent). The patent application does not mention the following variation with four cylindrical machines.   4. Fivefold Curta. The five pocket calculating machines stand on a revolving pedestal which is attached on a circular base plate. Thus the setting mechanisms of the five devices can be comfortably manipulated. The principal driving shaft (in the middle) with the crank is connected through spur wheels with the five stepped drum shafts (© Österreichisches Patentamt, Wien, Stammpatent 1954/57).