Jaap's Mechanical Calculators Page


Conto C, in opened case

My Conto C
History and Models

My Conto C

This is a Conto Model C, made in Switzerland. This brass adding machine has 8 dials, but unlike for example the Calcumeter they are not discs turned by using a stylus. Instead they consist of a pointer that is turned by hand.

A further difference from other dial machines is that the dials are merely the input mechanism, and not the register itself. After you enter a number on the dial pointers, the number can be read in the input register located below the dials. Shifting the crescent-shaped lever to the right and back to the left will add the number to the totals register, and clear the input register and dials. There is a separate clearing knob which clears the totals register.

Conto C, closed case
Conto C, in opened case
Conto C, with opened case
Conto C, front
Conto C, back
Conto C, right
Conto C, left
Conto C, levers
Conto C, levers
Conto C, inscription
Conto C, serial number
Conto C, dials
Conto C, dials
Conto C, dials
Conto C, in action
Conto C, reset
Conto C, seal warning
Conto C, bottom
Conto C, sealed screw

The serial number located on the crescent-shaped lever is 2578. The model C was made from 1914 until the early 1930s, but this one must have been one of the latest ones produced.


Below is a video I made that demonstrates the Conto C.

History and Models

The Conto was invented by Karl Karasek und Johannes Aumund, and patented in 1905 and 1906. Johannes Aumund produced the Conto model A in his engineering workshop in Zurich from about 1906. This model did not have a clearing lever for the totals, and no input register display.
Production was transferred to Carl Landholt in Thalwil, located about 10 km south of Zurich. It is unclear when this happened - some sources say 1910 because that was apparently when Landolt set up a company. However Landolt was already involved in selling the Conto from 9 October 1906, because in 1908 Aumund sued Landolt for access to the records of all Landolt's sales of the Conto starting from that date. Whatever the dispute was, it seems to have been settled because Landolt eventually fully took over the production of the Conto. Before his involvement with the Conto, Carl Landolt had been a statistician working in various places such as Bern and Zurich, generally on housing surveys.

Landolt designed a reset mechanism for the totals register, which was activated by pulling the input-clearing lever to the right instead of the left. He start started producing this version, the model B, in 1912.

A final improvement in about 1914 resulted in the model C. On this model there is a separate clearing lever for the totals. Landolt died in September 1923 at the age of 54. At some point his business passed to his successor, Alfred Müller, who then also produced Conto machines. Müller was advertising them in 1932, but from the serial numbers it seems that he produced far fewer than Landolt. This could mean that there was a hiatus in the production, or simply that Müller was not able to make and sell as many.

The standard capacity was 8 digits, but variants with 10 and 11 digits were made as well.

Here is a table setting out the details of the three models.

Model A Model B Model C
Year: 1906-1912 1912-1914 1914-1932?
Manufacturer: Johannes Aumund,
(1910?) Carl Landolt
Carl Landolt Carl Landolt,
(1924) Alfred Müller
Totals Register: Above and to left of dials Above and to left of dials Directly above dials
Input Register: No Yes Yes
Totals Clearing: No Yes, same lever Yes, separate lever
Shape & Size: Rectangular
5 × 38 × 2.5 cm
5 × 34 × 4 cm
Rectangular with sloping top face
5 × 34 × 3.5-4.5 cm
Known Serial Nos.: 8-digit: 50, 66, 81, 84, 151, 209, 309. 8-digit: 1029, 1123, 1302, 1342, 1655*. 8-digit: 1406, 1596, 1724, 1988, 2129, 2184, 2366, 2578
10-digit: Unknown, Unknown. 10-digit: 6119 10-digit: 6249, 6306 6319, 6352 6518
11-digit: - 11-digit: 4118
Estimated # made: 8-digit: 400 8-digit: 400 8-digit: 1200
10-digit: ? 10-digit: 200 10-digit: 350
11-digit: 100 11-digit: 100
Total: 400 Total: 700 Total: 1650

I tried to find as many serial numbers mentioned online as possible, in order to get a rough idea of how many Contos of each model were produced. The model A were numbered starting from 1. Probably about 400 were made, but certainly less than a thousand because it seems that the numbering for the model B started at 1000. The special 10 and 11 digit variants of the model B were given their own numberings, probably starting from 6000 and 4000 respectively. It looks like all variants of the model C were numbered consecutively to those of the model B. I have decided to ignore the model B number marked with an asterisk (*) as it does not fit the pattern and is not backed by photographic evidence.

In Ernst Martin's book it is claimed that there are also variants with 6 dials, and ones for Sterling currency with 9 or 10 dials. I have not seen any images of those, so it is likely that very few, if any, of these variants were actually produced.

The engravings on the Model C vary. The ones made by Carl Landolt have his CL logo and bear the text:
     Modell C | CARL LANDOLT Rechenmaschinen | THALWIL b/ZÜRICH | PATENT
When Müller continued, he at first sold the already constructed machines and simply covered part of the text with a plate bearing a different text:
     Modell C | Alfr. Müller vorm. C. LANDOLT | THALWIL b/ZÜRICH | PATENT
Later versions such as mine have the text:
     Modell C | ALFRED MÜLLER | Feinmech. Werkstätte | THALWIL b/ZÜRICH | C. LANDOLT's Nachfolger | PATENT


I have not found any advertisements relating to the Conto in online newspaper and magazine archives, and the only articles are minor mentions of Karl Karasek and Johannes Aumund. Included here is the judgement to an appeal in the court case in which Aumund wanted to see Landolt's sales records of the Conto. Lastly there is the entry for the Conto in Ernst Martin's 1925 book, Die Rechenmaschinen.

1902-06-14 Chronik der Stadt Zürich
1906-01-13 Chronik der Stadt Zürich
1909-05-19 Landolt Appeal
1925 Ernst Martin


Here are the patents for the Model A and Model B. I found no patents relating to the Model C.

PatentFiling datePublished dateNameDescription
CH 33,47420-04-1905Karl Karasek, Johannes AumundConto A.
See also: AT 27,162 B, FR 359,685, GB 1905/26,464.
CH 38,36211-11-1906Karl Karasek, Johannes AumundConto A improvement.
See also: US 883,719.
CH 60,80927-07-1912Carl LandoltConto B.
See also: AT 65,699 B, FR 460,796.


Rechnerlexikon has pages for the Conto A, Conto B, and Conto C. The latter includes an advertisement.
Rechenmaschinen-Illustrated has a Conto page with all three models.
Rechnen Ohne Strom is an interesting site that has a page about metal adders that contains all three Conto models.
Rechenwerkzeug.de has a Conto page.
Historisch Lexicon der Schweiz has a short biography of Carl Lindolt in German, French, and Italian.
The German-language case law site has a pdf document about Carl Landolt's appeal against Johannes Aumond's request to see his sales records.

© Copyright 2019 Jaap Scherphuis, .