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Friden Model STW-10 Electro-Mechanical Calculator

Updated 11/28/1999

A machine identical to this has a special place in my personal history. Back in the early '60's, when I was just a child living in Seattle, Washington (my birthplace), one of our neighbors owned a business which involved use of a calculator to generate dairy tank calibration charts. My parents were very good friends with these neighbors, and in fact, they were named my Godparents when I was born. The calculator that was used to grind through the interpolation process to generate these dairy tank charts was a Friden STW-10. My Godmother would take the worksheets generated by her husband in the field and use the Friden to perform the math necessary to generate a final chart giving the total number of pounds of milk in the tank based upon a 'dipstick' reading. When my Mom had errands to do, my Godmother would regularly babysit me and my little brother. Many times, during the time she was watching us, my 'Auntie', as I called her, would be hammering away at the Friden, working on generating the charts. To this day I can still vividly remember the spinning numbers and wonderful mechanical noises that machine made in operation. I credit the time spent watching her operate that machine to sparking an interest in Mathematics and numbers that set the stage for my career working with computers.

An old (Early 1950's) Friden ink-blotter advertisement, showing a Friden Model ST-W

The Friden Calculating Machine Co., Inc., was founded in the 1930's by Swedish-born Carl M. Friden. Once customers purchased and started using the machines, Friden calculators quickly gained the reputation of being the 'Cadillac' of calculators. In fact, early Friden advertising literature self-proclaimed the machines as the "Aristocrat of Calculators". The precursor to the STW-10 was the model ST. The ST could automatically perform all four math functions. Friden, the perfectionist that he was, was personally involved in the mechanical design aspects of his machines. As a result of his skills and dedication, along with other gifted mechanical engineers that he hired on at Friden, they ended up with a machine that was fast, extremely durable, reliable, and easy to use. The ST was a very successful seller, and enjoyed a long production run which lasted through World War II. After the war, a refined version of the ST was created, and re-introduced as the STW-10 in 1949. There were some feature and cosmetic changes (most notably changes in colors used to minimize user fatigue) from the ST to the STW-10, but the internal workings were virtually the same as the ST. The STW-10 was immensely popular, and because of its popularity, it remained in production through 1966. In 1965, Friden introduced the world to all-electronic, transistorized calculators with the Friden EC-130 which marked the beginning of the end of Friden's mechanical calculators.

Front Cover of the Friden STW-10 Operators Guide

Back in the days where electronic computers were in their infancy, the common method of performing large scale mathemetics was to have large rooms full of 'computers' (back in those days, a 'computer' was a person who's job it was to perform math operations), each having their own mechanical or electro-mechanical calculator. Insurance companies were one of the biggest users of these hybrid human/machine 'computers', generating the actuarial tables that determine the rate paid by policy holders for life, auto, and business insurance. Friden electromechanical calculators were a common fixture in such installations because of their rock-solid reliability.

The Carriage

There are three major parts to the STW-10. First, the carriage. The carriage is like that of a typewriter, being able to move to the left and right. On the carriage are two rows of mechanical number displays. The 'top' row is the accumulator, which can accumulate results up to 20 digits long. Just above the accumulator number windows are small dials which the operator can use to manually adjust the numbers in the accumulator. This manual adjustment feature was commonly used to do 'round-off' operations, which the calculator didn't have the brains to do itself.

A Close-up of Number Dials on the Carriage

Mechanicals Underneath the Carriage

The 'bottom' row of digits is the counter register, which serves as an item counter when doing addition and subtraction, and performs counting operations during multiplies and divides. The counter register has a capacity of 11 digits. Just below the counter register is a set of numbered pushbuttons that are used to set the decimal point position for division operations. To the right of the carriage are two 'slider' type controls which allow the accumulator and/or counter registers to be cleared manually.

The main keyboard

The next major part is the main keyboard. In contrast to calculator keyboards that people are used to nowadays, where each digit of a number is typed in sequentially, digits are entered 'in parallel' on the main keyboard. Sequential digit entry would have required much more complex mechanisms, which would have made the machine more expensive. Rather, each digit is entered in a separate column of keys ranging from 1 through 9 (zero was implied by having NO key pressed in a column). The 'blank' keys at the bottom of each column 'clear' the column of any entry (the digit keys lock down once pressed) in case of entry error, and a "K B CLEAR" key releases all of the keys in the main keyboard.

The multiplier keyboard

The last major part of the calculator is the multiplier keyboard. This zero-through-nine keyboard is serial entry like today's calculators. It is only used for entering the multiplicand in multiplication operations. A small window above the keyboard shows the multiplicand as it is entered digit at a time. Numbers up to 10 digits long can be entered into the multiplicand register.

Multiplier Display Register Workings

The STW-10 can perform the four basic functions completely automatically. Addition and subtraction are very strightforward -- enter a number on the 'big' keyboard, and press the [+] key to add to the accumulator, and the [-] key to subtract from the accumulator. Negative results, or overflow of the machine, causes a bell to ring to warn the user.

Left Side Sans-Cover

Multiplication is a little different. The multiplier is entered in the 'big' keyboard, and the multiplicand is entered digit at a time into the multiplication keyboard at the left side of the machine. Then, the [MULT] key is pressed, and the machine proceeds to carry out the multiplication, working right to left, by doing repeated adds and shifts of the carriage to result with the product in the accumulator, and the multiplicand ending up in the counter register. An extension to the multiply operation is that the product can automatically be added to the number already in the accumulator.

Right Side View

Division is a bit more complicated, but still automatic. One oddity is the appearance of two divide keys on the keyboard. Apparently this was a result of another calculator company having a patent on a "single key" divide function, which Friden's lawyers were concerned enough about to insist that the machine not have a single key for division. In fact, the left division key is what actually triggers the division to take place, but, if actuated by itself, the resultant quotient will be the 10's compliment of the expected result. The right divide key switches the mode of the counter register (where the quotient is accumulated) so that it increments instead of decrements during the repeated subtraction process that makes up division. So, in order to get the expected answer, both [÷] keys must be pressed at once.

Rear View

To perform division one must first press a small button on the carriage which indicates where the decimal point should be situated in the quotient. Then, the dividend is entered on the big keyboard, and the [ENTER DIVD] button is pressed. This shifts the carriage over to the position indicated for the decimal point, and enters the dividend into the accumulator. Then, the keyboard is cleared (if the "ADD" mode selector is on, the keyboard clears automatically, however, if it is off, the keyboard must be cleared manually with the [K B CLEAR] key), and the divisor entered into the big keyboard. Then, BOTH [÷] keys are depressed simultaneously, and the machine begins a process of shifting the carriage to the right to find a point where the divisor can start to be subtracted from the dividend. Once that point is found, then successive subtractions of the divisor occur until an overdraft (e.g., the accumulator goes negative) occurs, and each subtraction is counted in the counter register. A single addition of the divisor corrects for the overdraft, and then the carriage shifs to the left, and the process repeats until the quotient is left in the counter register, and the remainder is left in the accumulator. By the way, the overdraft bell is inhibited during division...the machine is noisy enough as it is without the bell ringing during a division.

A video clip of the STW-10 in operation is available for download. To check out the video, click HERE.

An interesting place to visit to learn more about Carl Friden, his company, and his calculators, is Frank Rauck's "Fridenites Web Page".


Many thanks to Nicholas Bodley, former Friden Employee, for helpful input and facts contained in this document
Text and images Copyright ©1997-2008, Rick Bensene.