# Curta

The Curta is a very small mechanical calculation machine. It was invented by Curt Herzstark, and made by Contina AG Mauren, based in Mauren, Liechtenstein. It was in production from 1948 to 1972.

It is shaped like a pepper mill, a small cylindrical metal body with a crank handle centred on the top. Along the side are a number of vertical sliders which allow you to set the digits of one of your numbers, which are shown just above the sliders.

On the top face, along the outside, are shown two registers. The shortest of the two is the revolution counter, the longest is the result counter.

On the bottom plate of the cylinder is printed the type and serial number of the machine.

## Calculation

When you crank the handle clockwise once, the input number from the sliders on the side will be added to the result counter on the top. The revolution counter is incremented. You can shift the whole top section one column to the right, and if you then crank the handle again, ten times the input number is added to the result, and ten is added to the revolution counter. In this way you can add any multiple of the input to the results, and the multiple that you have added is shown in the revolution counter.

If you pull up the crank so that a red ring around its base becomes visible, then turns of the crank subtract the input from the result, and reduces the revolution counter appropriately. A turn of the crank in this position is the exact inverse of a turn in the normal position.

With the clearing ring you can clear one or both of the counters. You have to lift up the top slightly to make the ring free to move, and then pull the clearing ring around across either counter, in either direction.

There is a switch on the side of the Curta, opposite the input number sliders. Flipping this switch changes the direction in which the revolution counter is changed. Instead of counting how often the input has been added to the result, it then counts how many times the input has been subtracted from the result. This allows you to do division as follows. Put the numerator in the result counter (clear it, input the numerator, add it once, clear the revolution counter). Input the denominator. Pull up the crank for subtraction, and flip the switch to reverse. Now subtract multiples of the input to get the result counter as close to zero as possible. The revolution counter shows how often it was subtracted, i.e. it gives the quotient. Whatever is left in the result counter is the remainder of the division.

## Models

There are two models.

Type I 1948-1970 000000-080427
• 8 digit input
• 6 digit revolution counter
• 11 digit results counter
• Slim, matte black metal body
Type II 1954-1970 500000-569944
• 11 digit input
• 8 digit revolution counter
• 15 digit results counter
• Wide, black or grey metal body

Mechanically the Curta has hardly changed over its production run. The main changes between early and later versions are the writing and markings, and changes in the material and shape of the crank, the clearing ring and the storage can.

## My Curta type I

My Curta is a type I with serial number 57064. This means it was made in 1964.
It is in good working condition, but the clearing ring has a crack on one side.
It has a metal storage can. The can shows a lot of wear.

## Video

Here's a video where I demonstrate how the Curta works.

## Books

I own the following books:

Instructions for use of the CURTA, Contina Ltd. / Vaduz, Liechtenstein

This is a modern reproduction of the original booklet. It describes how to handle the Curta, the four basic arithmetic operations, a few examples, and square roots.

Computing Examples for the CURTA Calculating Machine, Contina AG., Vaduz, Principality of Liechtenstein

This too is a modern reproduction of the original booklet. It continues where the instruction booklet left off, starting with a recap of divisions and square roots. It then describes various calculations such as those involving percentages, pre-decimal English currency, statistics, angles in a triangle given the sides, distances between coordinates, and linear interpolation.

Curta Square Root Tables
This leaflet contains a table for calculating square roots, and English instructions explaining how to use it. The leaflet is printed on only one side, and is folded zig-zag fashion into 6 parts. There is no copyright year.

Mathematical Handbook, The CURTA Company, Van Nuys, California
This is a modern reproduction of the original booklet. It is a small promotional booklet, with 18 pages describing the Curta and its advantages (essentially the same as the booklet below), followed by 46 pages of formulas and tables for various mathematical problems.

CURTA - Die kleinste Universal-Rechenmaschine.
Original advertising booklet, 14 pages, 103mm × 149mm, German language, dated from about 1954. It describes the Curta models, and the Curta's advantages over other calculators.
(PDF, 2.87 MB)

Promotional leaflet for the Curta, dated from about 1954, in English.

Promotional leaflet for the Curta, dated from about 1954, in German.

Invoice for a type I Curta, dated 29th August 1953.

Handwritten quote of Curta model prices on paper with Contina letterhead, probably from 1953 or 1954.

Business card of Baron A. F. de Gerliczy, publicity manager of Contina, probably dated around 1953 or 1954.

Carbon copy of a letter from Colonel A.R. Hercz to Curt Herzstark, dated 24 November 1954. It mentions the idea of joining two Curtas together, which could be useful in calculations for artillery.

A photographic copy of a technical drawing which shows two Curtas connected to each other to make one mechanism. Only one of the Curtas has a crank handle. On the back is written "Only one turn indicator dial needed" and "Only one reversing lever needed".

An Italian article from La Scienze, February 2004:

## Patents

PatentFiling datePublish dateNameDescription
DE 747,07319-08-193827-01-1944Curt HerzstarkPrototype of Curta
DE 747,07413-04-193927-01-1944Curt HerzstarkPrototype of Curta
US 2,525,3529 Jan 194810 Oct 1950Curt HerzstarkCurta
US 2,533,37220 Jan 194912 Dec 1950Curt HerzstarkZeroizing mechanism
US RE 23,55320 Jan 194930 Sep 1952Curt HerzstarkZeroizing mechanism (Reissue of previous patent with additional claims)
US 2,566,83527 Apr 19504 Sep 1951Curt HerzstarkLocking mechanism (locks crank when lifting top for clearing ring)
US 2,588,83527 Oct 194911 Mar 1952Curt HerzstarkIndependent actuator tens-transfer mechanism
US 2,661,15522 Mar 19521 Dec 1953Franz MarkAccumulator-carriage and drive-shaft interlock for miniature-type calculating machines
AT 195,14715-12-195425-01-1958Curt HerzstarkDual Curta