Jaap's Mechanical Calculators Page

Contex Calculators

Contex B   Contex 10

My Contex B
Contex B Video
My Contex 10
Contex 10 Video
How To Use a Contex 10

The company "Brdr. Carlsen" (i.e. the Carlsen Brothers) was founded by the three brothers John, Henning, and Erling Carlsen in 1945 in Hellerup, Denmark. It was to produce the simple calculator that they had designed and patented. The Zeuthen og Aagaard office supply company became interested in distributing them, and production soon moved to a new factory in Gentofte to keep up with the demand. A few years after a new model began to be produced in 1957, they moved to yet another factory in Birkerød. The company was dissolved in 1972 when electronic calculators made the electromechanical calculators obsolete. The Contex brand name was then used for a number of electronic calculators and is still in use now as a brand of document scanner.


Contex A and B

Contex B

Contex started in 1945 with a simple half-keyboard adding machine that is similar to the Plus adder by Bell Punch, except that it had its display register above the keyboard instead of in front of it, and was much more cheaply made. Its keyboard has 8 columns of 5 keys, and a 9-digit register. There are versions with a decimal keyboard, and versions for use with English currency (pounds, shillings, pence, or pounds, shillings, pence, farthings).

The first version of the Contex, nowadays called the model A, had a dark brown Bakelite case (later white plastic), and an open keyboard a bit like that of a typewriter.
In 1951 the machine was given a new design by Sigvard Bernadotte, and this is now called the model B. It had a grey and white plastic case, and a keyboard more like that of a computer, with black and white square keys with almost no gaps between them. The internal mechanism was virtually identical to model A.

Contex 10

Contex 10

Later they created a more complicated series of models with a 10-key keyboard, still cleverly constructed so as to be as cheap to manufacture as possible.

Contex 10 was the first of these, released in 1957. Like the model B, the case was again designed by Sigvard Bernadotte. The first versions had a grey case, but later they changed that to a lighter beige colour. There is a 10-digit register at the front, with an extra 1-digit counter which is used for division. It has ten keys for the digits, a clear key, keys for multiplication, subtraction, and division, as well as keys for shifting the input to the left or right. Apart from the keys there is a large push button (almost a lever) that performs an addition or subtraction. An explanation of how to use it can be found below.

Contex 20, 30, 55

These models were electrically driven calculators, all clearly based on the design of the Contex 10. The Contex 20 was released in 1960, and is essentially the same as the Contex 10 except that there is no heavy push button for performing an addition, and instead that is replaced by a simply key that causes the work to be done electrically. The functionality of the Contex 20 is virtually identical to the Contex 10.

The Contex 30, released in 1963, improves on the Contex 20 by including automatic multiplication. You had to enter the second number of the multiplication from right to left, and it would perform the multiplication as you entered it. Division was still performed one digit at a time however. The Contex 55, released in 1967, had automatic division and had a second output register especially to store the division result.

Model Numbers

The table below shows the years of production, the approximate amounts produced, and some of the serial numbers that I have found in various other online sources. From about 1961 onwards, the serial number is preceded by a 3-digit code which specifies the exact model type, and below I have enclosed that in brackets. The exact ranges and dates of the serial numbers are unclear.

Years of
Serial Numbers
Contex A 48,0001945-1951 37338; 41230; 41337; 42084; 67332; 73053; 79192; 79390; 82354; 83167; 86860; 116263; 118768; 124773; 130161; 131698; 156596; 158589; 165565; 168410; 180380; 180922;
Contex B 189,6091951-1970 137386; 140858; 155105; 203419; 203653; 205079; 246818; 537817; 911428; 945937; 946514; (411)003870; (411)017042; (411)024875; (411)605885
Contex 101,059,3811957-1971 425901; 447531; 467745; 512714; 762204; (421)003019; (424)001829; (431)027205; (431)808360; (434)005097; (434)039458; (434)077240; (434)820405
Contex 20 392,6681960-1971 619746; 623994; 668711; (453)004781; (453)012430
Contex 30 393,0001963-1971 (473)048358; (473)080824; (473)022492; (473)086504; (473)134098; (473)148242; (473)150856; (473)169919; (473)174815
Contex 55 145,8331967-1971 (483)004617; (483)007548; (493)005189; (493)011463; (493)016818; (493)035101; (493)058403; (493)059918; (493)088787
Total 2,228,4911945-1971

My Contex B

The Contex B that I have has the serial number 537817. This model was introduced in 1951 to replace the functionally identical Contex A, and was produced in steadily declining numbers until 1970. From the serial number, I suspect mine was made in around 1959.

Its register is located behind the keyboard and has 9 digits. The keyboard has 8 columns with only five keys, numbered 1 to 5. Entering a digit larger than 5 involves two key-presses - for example to enter 9 in a column you have to press the 5 and then the 4 (or vice versa). During the downstroke of a key, the digit you entered is added to the corresponding digit in the register.

On the left of the keyboard is the clearing button. As you press it down all the digits of the register turn to 9, and when released 1 is added so that the register overflows to zero.

Contex B
Contex B, front
Contex B, underside
Contex B, label

The levers inside the machine are not made of metal but of Pertinax, a composite made of resin bonded paper that is also used for cheap printed circuit boards. While this kept the weight down of the model A, the casing of model B makes it quite heavy still.

The mechanism used in the Contex is simple, and lacks the safety features of the more expensive machines. The biggest flaw is that partially pressing a key will only enter part of the number. For example, if you push down on a 5 key but release it before fully depressing it, then any number from 1 to 4 could be added to the register. Mis-hitting a key and thereby slightly disturbing an adjacent key will almost always make the answer incorrect.
If you enter numbers using both hands to enter digits on several columns at the same time, then you must be sure to move you hands in concert, pushing down simultaneously and then releasing simultaneously. If one hand lags behind the other, it is possible for a carry to become lost, resulting in the wrong answer.
The low cost of this machine makes up for these weaknesses.

Contex B, inside
Contex B, inside
Contex B, inside
Contex B, inside
Contex B, inside
Contex B, inside

Contex B Video

Here is a video where I demonstrate my Contex B.

My Contex 10

The Contex 10 is a 10-key mechanical calculator. It has a 10-digit internal input register. Typing in a number will set this internal input register, but the only visual sign of this is the red pointer that moves to the left to indicate how many digits you have entered. The input can be added or subtracted from the 11-digit output register at the front of the machine.

My Contex 10 has serial number (434)077240. This model was produced from 1957 till 1971, but in about 1963 (when the Contex 30 came out) it was improved slightly by the addition of a key to the right of the 0 key. My machine is of the improved type, but it is hard to determine its manufacturing date more precisely as the serial numbering scheme is unclear.

The large push-button at the right is pushed to perform an actual addition or subtraction. Pushing this down takes some effort, and this provides the mechanical energy to work the mechanism. On the underside of the machine is a small switch which you can use to lock the push-button on the down position, which helps keep the machine safe and compact for transport or storage. The underside also has a large label with a short instruction manual.

Contex 10
Contex 10
Contex 10, front
Contex 10, side
Contex 10, rear
Contex 10, underside
Contex 10, serial number
Contex 10, instructions
Contex 10, inside
Contex 10, inside
Contex 10, inside

Contex 10 Video

Here is a video where I demonstrate my Contex 10.

How To Use a Contex 10

Clearing the output register: Press C. Note that this does not affect the input register.

Clearing the input register: Shift red digit pointer to the right as far as possible. If the black stopper button is in the way, move that to the right first.

Inputting a number: The input register should be clear (the red digit pointer is at 0). Simply type the number into the keypad. With every key press, the red pointer moves one step to the left. The input has a maximum of 10 digits, and any further digit keypresses are ignored. If you make a mistake, simply clear the input (as above) and try again. The manual warns against using the key to correct the last entered digit, but it does seem to work except for the first digit entered.

Addition: Input a number. Push down the big button. This adds the number to the output register at the front, and automatically clears the input for the next number you want to add, so you can repeat the process as often as you need to add a list of numbers.

Subtraction: I assume the output register already holds non-zero number, and that you want to subtract a smaller number from it. Enter the number you want to subtract. Press the - key (which stays depressed), and then push down the big button. The output register will shift to the left, and the entered number gets subtracted from it. The input is cleared automatically, and the - will pop back up. You can then add or subtract another number in the same way.

Back Transfer: The Contex 10 does not have automatic back transfer, but you can manually copy the result from the output register into the input register. To do so, clear the input register. Type in the number showing in the output register. Finally press C to clear the output while keeping the input untouched.

Negative numbers: If you subtract so much that the result would be a negative number, then the output register will underflow. For example, it will show 99999996735, or something similar with leading nines. To negate the number you can simply subtract it from zero as follows: Back transfer as much of the output register as you can, but since you can only input at most 10 digits, you will need to skip (at least) one of the leading nines. In this example you would enter 9999996735 and then press C. Subtract it by pressing -, and then the big button. The result is 90000003265. Ignore the leading nine to conclude that the number represented -3265. You can skip more leading nines during the back transfer, in which case you will have more leading nines to ignore in the end result.

Multiplication: Multiplication is done by repeated addition in much the same way as on traditional pinwheel calculators. Suppose you want to multiply some number X by 157. First input the number X you want to multiply. Press the × key (which will stay depressed). Now work your way through the second number one digit at a time, from right to left. The rightmost digit is 7, so you have to press the big button 7 times. This adds X times 7 to the output register. Note that the counter digit to the right of the output register is useful for keeping count. Then press to shift the input to the left. The next digit is 5, so press the big button 5 times. This adds X times 50 to the output register. Press once more. The last digit is 1 so press the big button just once, which adds X times 100 to the output. The output register has now had a total of X times 157 added to it, as required. To end the multiplication, clear the input, which also releases the × key. You can follow this by a back transfer if you wish to multiply the result by another number. On the Contex 30 and 55, the repeated addition and shift-left is done for you automatically, and you simply have to enter that second number from right to left.

Advanced Multiplication: Sometimes shortcuts are possible by the use of subtraction. For example, to calculate X × 98, you can do (X × 100)-(X × 2). The latter involves far fewer addition/subtraction operations. To do this calculation, enter the number X, press ×, press twice, and then the big button. This has done the first part. Now press twice to shift the input back to the units position, press - followed by the big button twice. Note that the - key remains down for this operation.

Division: Clear the output and input registers. Enter the first number. Now press , by which I mean the key to the right of the zero. This shifts the input all the way to the left. If you have an older Contex without that key, just press the normal key repeatedly until it stops working. Press the big button to add you number to the left of the output register. Enter the second number, your divisor. Again press the key to shift it all the way to the left. Press Div and the - key, and you're ready to start the division. Repeatedly press the big button, continuing until it locks up. Doing this will have repeatedly subtracted the divisor until the register underflowed, and then added it once to undo the last subtraction. The red counter digit on the right will show the number of times a subtraction was done successfully, so it is the first digit of the answer. Write that digit down. Press . This shifts the input to the right, and also clears the counter digit, pushes in the - key again and unlocks the big button. Again repeatedly press the big button till it locks, copy down the red digit from the counter as the next digit of the answer, and press . Continue until you have written down sufficient digits of the quotient.


Here are a few advertisements I found in online newspaper and magazine archives.

1946-07-12 Friesch Dagblad
1947-02-05 Leeuwarder Courant
1947-07-28 Le Confedere
1948-03-13 Algemeen Dagblad
1948-09-07 De Tijd
1948-12-04 Nieuwsblad van het Noorden
1951-02-12 Barrier Miner (NSW)
1951-06-04 News (Adelaide)
1951-08-02 The Courier-Mail (Brisbane)
1951-08-08 The Sydney Morning Herald
1951-10-03 The Sydney Morning Herald
1951-11-27 The Sydney Morning Herald
1951-12-17 The Argus (Melbourne)
1952-01-26 The Advertiser (Adelaide)
1952-02-05 The Courier-Mail (Brisbane)
1952-02-09 Morning Bulletin (Rockhampton Queensland)
1952-04-08 The Sydney Morning Herald
1952-04-28 The Courier-Mail (Brisbane)
1952-04-29 The Courier-Mail (Brisbane)
1952-05-19 The Courier-Mail (Brisbane)
1952-05-24 The Advertiser (Adelaide)
1952-06-16 The Argus (Melbourne)
1952-07-02 The Courier-Mail (Brisbane)
1952-07-07 The Courier-Mail (Brisbane)
1952-08-12 The Sydney Morning Herald
1952-09-15 The Argus (Melbourne)
1952-09-23 The Sydney Morning Herald
1952-11-06 The Courier-Mail (Brisbane)
1952-12-07 The Sydney Morning Herald
1952-12-08 The Argus (Melbourne)
1953-01-19 The Advertiser (Adelaide)
1953-01-19 The Argus (Melbourne)
1953-02-09 The Argus (Melbourne)
1953-03-05 The Sydney Morning Herald
1953-03-16 The Argus (Melbourne)
1953-03-26 The Courier-Mail (Brisbane)
1953-05-18 The Argus (Melbourne)
1953-05-22 The Sydney Morning Herald
1953-07-13 The Argus (Melbourne)
1953-07-18 The Advocate (Burnie Tasmania)
1953-07-24 The Sydney Morning Herald
1953-08-08 The Advocate (Burnie Tasmania)
1953-08-13 The Courier-Mail (Brisbane)
1953-09-03 The Advertiser (Adelaide)
1953-10-19 The Advertiser (Adelaide)
1953-11-17 The Sydney Morning Herald
1953-12-02 The Australian Women's Weekly
1953-12-14 The Argus (Melbourne)
1954-01-28 The Courier-Mail (Brisbane)
1954-02-08 The Courier-Mail (Brisbane)
1954-02-10 The Sydney Morning Herald
1954-02-11 Queensland Country Life
1954-02-13 The Argus (Melbourne)
1954-03-13 The Mail (Adelaide)
1954-03-25 The Courier-Mail (Brisbane)
1954-04-05 The Argus (Melbourne)
1954-04-12 The Sydney Morning Herald
1954-06-10 Queensland Country Life
1954-06-12 Northern Star (Lismore, New South Wales)
1954-06-14 The Argus (Melbourne)
1954-09-28 The Sydney Morning Herald
1954-10-12 The Argus (Melbourne)
1954-10-19 The Sydney Morning Herald
1954-11-15 The Courier-Mail (Brisbane)
1954-12-13 The Argus (Melbourne)
1955-04-25 The Argus (Melbourne)
1955-07-09 The Argus (Melbourne)
1955-08-11 The Central Queensland Herald (Rockhampton Queensland)
1956-04-16 The Argus (Melbourne)
1957-06-13 New Scientist
1958-03-26 Algemeen Dagblad
1959-06-03 Nieuw Suriname
1959-10-23 The Daily Collegian (Pennsylvania)
1960-02 Nations-Business
1960-04 Nations-Business
1960-05 Nations-Business
1960-12-09 Daily Illini
1960-12-19 De Telegraaf
1961-03-13 Columbia Daily Spectator
1961-10-06 Life
1962-01-20 Nieuw Suriname
1962-03-26 Aviation Week
1962-10-26 Life
1962-11 The Rotarian
1963 Sears Business Equipment and Supply Catalogue
1963-01-24 Nieuw Suriname
1965-10 Nations-Business


Here are some of the patents for the Contex A/B. Full scans of the Danish patents are not available on espacenet (yet?), but can be found at datamuseum.dk.

PatentFiling datePriority dateNameDescription
DK 67,523 C31-10-194430-08-1948John CarlsenCarry Mechanism
See also SE 131,349
DK 67,614 C31-10-194413-09-1948John CarlsenKeyboard
See also SE 122,584, US 2,472,519.
DK 68,862 C18-10-194504-04-1949John CarlsenCarry mechanism
See also US 2,503,613.
DK 69,219 C18-10-194516-05-1949John CarlsenCarry mechanism
DK 70,338 C18-10-194512-12-1949John CarlsenCarry mechanism
DK 71,678 C18-10-194506-11-1950John CarlsenCarry mechanism
See also US 2,499,946.
DK 76,248 C18-10-194517-08-1953John CarlsenNumber wheel stop

Here are some patents for the Contex 10-55.

PatentFiling datePriority dateNameDescription
DK 89,068 C21-03-195730-05-1960Erling Adolf Carlsen, Henning Gunnar CarlsenContex 10
See also: AT 208,107, DE 1,179,741, ES 240,765
DK 90,132 C21-03-195712-12-1960Erling Adolf Carlsen, Henning Gunnar CarlsenContex 10
See also: CA 622,181, CH 361,151, FR 1,203,626, GB 849,752, US 2,988,274.
DK 93,050 C27-04-196019-03-1962Erling Adolf Carlsen, Henning Gunnar CarlsenContex 20
DK 109,912 C07-02-196629-07-1968Erling Adolf Carlsen, Henning Gunnar CarlsenContex 55 back-transfer mechanism
See also GB 1,104,143, AT 264,170 B, FR 1,510,472, NL 6,701,655.


datamuseum.dk is a Danish site with excellent information about Contex and the Carlsen brothers.
John Wolff's Web Museum's Contex page shows all models.
Christian Hamann also shows all models.
Repairing a Contex 10 shows lots of photos of the internal mechanism.
Retro Calculators' Contex page shows models A, B and 10.
The Old Calculator Museum site has a Contex 10 page with some mechanism photos, as well as a deconstruction of an advert.
Vintage Calculators shows a sterling currency Contex B.

© Copyright 2017-2018 Jaap Scherphuis, .