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Bohn Contex Model 10 Mechanical Calculator

Updated 4/15/2000

The Bohn Contex Model 10 is a miniaturized and simplified version of the large electro-mechanical calculators (such as the Friden STW) that were the mainstay for calculating in the '50's and '60's. The Contex 10 is an entirely manual calculator, operating by 'people power' only.

The Contex 10 is the first of a line of calculators of this same basic design, with production of the Model 10 beginning in 1957. In 1961, a motorized version of the Contex 10 was introduced, called the Contex 20. This machine simply replaced 'people power' with an electric motor that drove the mechanicals of the machine. In 1965, the Model 30 was announced, automating some of the manual manipulations required to do multiplication on the earlier Contex calculators. Then, in 1968, the epitome of Contex calculator technology was introduced, the Contex 55. The 55 automated the rather tedious aspects of performing division on the earlier Contex machines. The 55 marked the first fully-automatic Contex calculator, however by the time the 55 was introduced, electronic calculators were beginning to eat away at the market share of mechanical and electro-mechanical calculators. The Contex line of machines appears to have been sold through the early part of the 1970's, by which time electronic calculators had firmly pushed electro-mechanical calculating technology aside. It is interesting to note that apparently the design of the machine caught the attention of the Soviets, enough that Soviet 'clones' of the Contex machines were manufactured and sold in the USSR under the brand name Bystritsa.

The machine exhibited here has a label on the back indicating it was "Made in Denmark". However, it appears while it was manufactured there, it was imported into the United States by Bohn Duplicator Corp., New York, NY. The label also indicates "Patents Pending". This machine is serial number 512714, as indicated on the label on the back by 'punched' 5x4 dot-matrix digits. I can not find any dates anywhere on the device, so I'm not really sure when it was made, but based on the fact that the machine does not have any Model 10 designation, my guess is that the machine exhibited here is an early example of the Contex design, perhaps from the 1957 to 1959 time-frame.

The Contex 10 is rather straightforward to use for adding and subtracting, however, it gets significantly more complex when doing multiplies and divides, which involve keeping track of multiplier or quotient 'in your head' or on a piece of paper. The machine is rather noisy in operation, not one to be using when others around you are trying to concentrate!

The insides of the Contex

Mechanically, the Contex 10 seems a wonder that it works at all. The machines are wonders of low-cost, mass-produced mechanical technology. The metal work of the machines is all low-cost stamped sheet metal -- very few machined parts are used. The gears of the accumulator and other components of the machine are made of molded plastic or nylon. It appears the machine was designed with high tolerance for variances in part dimensions. Even though these machines were very much mass-produced devices, they seem to have been designed well enough that they still tend to operate quite nicely after many years of service.

The Contex 10 calculator uses a '0-9' keypad for entry of numbers, however, the entry is 'blind', with the only indication of entry being that a red pointer advances one place to the left with the entry of each digit. Once a number has been entered, pressing down on the cycle bar on the right of the machine, then releasing it, will add the entered number to the display register. Pressing the '-' key before pressing the cycle bar will cause the entered number to be subtracted from the display register, which is interesting, because to subtract a number, the whole display register shifts to the left such that the number wheels can be turned 'backwards' to perform the subtraction. Multiply and divide are involve shifted repetitive addition (for multiply) and subtraction (for divide).

The Contex display window

The machine can calculate results to a total of 11 digits. The 12th digit at the far right of the display is a 'cycle counter' that counts depressions of the cycle bar (up to 9). This cycle counter is used for keeping track of the number of cycle bar presses during multiply and divide operations.

Another inside view

A view of the accumulator gears (plastic!)

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