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Mechanical calculators: Contex model B

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This machine was manufactured by Contex in Denmark. Model A was produced from 1946, and model B took over in 1951, with a more modern case, designed by Sigvard Bernadotte the Swedish designer that signs many FACIT machines. I bought it at an online auction site, in August 2020 (in the middle of the COVID-19 pandemic), from a vendor based in São Paulo. The machine seemed to be in excellent condition, and it was accompanied by a grey case. This machine is not very common in Brazil, and I got it mainly because of its simpler mechanism, based on the Comptometer, but with a half keyboard.

The machine has 8 digit columns, and the half keyboard includes only keys 1 to 5 in each column. Entering values 6 to 9 requires two keypresses (3+3 or 4+2 for 6, 4+3 for 7, 4+4 for 8, 5+4 for 9). This idea comes from the observation of experienced Comptometer and Burroughs machine operators, who used to save arm movements by doubling keypresses in the lower part of the full keyboard. As a result, many of those machines (including my Burroughs Class 5) show greater wear in the first five rows of keys.

The heaviest part of this calculator is its dark grey case, in metal. The lighter parts of the case are plastic, and the bottom is just a thin metal plate. The main operating parts of the mechanism are made using fiberboard. Apparently its low weight was a selling point, since most machines from that era were much heavier.

Curiously, all keys in the same column move when a key is pressed. Key arms are arranged as a fan, and when you press the 5 key, the fan closes and drags down the other 4 key arms in the same column. The reset mechanism is also curious: digits go to 9 when the reset button is pressed down, and 1 is added when it is released. The mechanism is too simple to include any kind of functional locking or safety features: unskilled operators easily make input mistakes, by not depressing a key to the very bottom, or cause carry errors when releasing the keys. As in the case of Comptometers and Burroughs, its most efficient operation requires pressing keys in multiple columns at the same time, instead of touch-typing each digit in turn. Adding columns of numbers can be done column-wise, giving the same result. Say you need to add 123 + 456 + 789. You can type in 3 - (3+3) - (4+5) on the rightmost column, then 2 - 5 - (4+4) on the second, and 1 - 4 - (4+3) on the third, much like adding manually.

Subtracting in the Class 5 is, as in the Comptometer, also troublesome. The operator is required to enter a tens-complement number, using preceding 9s and typing the number to be subtracted by looking at the small numbers on the keycaps. The preceding 9s are actually zeroes if you look at the small numbers in the keycaps. Oh, and the subtrahend must be reduced by one so the tens-complement works correctly. Whew. More on Burroughs Class 5 operation here.

There was practically nothing to do in this machine. Its three rubber feet were deteriorated, but I was able to replace them with round hydraulic seals, after making a countersinked hole in each one. After a general cleaning, I now only mean to restore the front Contex logo, which is all but invisible, as in the case of many units whose photos can be found online. The case has the markings of a lost handle, which I also plan to replace.

Have a mechanical calculator stored somewhere, and want to get rid of it? Send it to me!

c1951 Contex model B s/n 188562

As purchased

 As purchased

More about this machine

John Wolff shows a wider Class 5, with 13 columns instead of 9, including a column for Sterling currency units

Video: a demonstration of the Class 5 in action

An operations description, including an explanation on subtraction modes and their special cases

A beautifully preserved unit at the UK's Science Museum

A time-lapse video of an artist recreating the shape of a Class 5 using only a photo and a 3D design software package

Instructions for Operating the Burroughs Calculator. 36 page booklet

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contex_b.1599503945.txt.gz · Last modified: 2020/09/07 15:39 by clodoveu
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