Kyle Rankin @kyle@librem.one

Like with other Comptometers, you just press corresponding keys to add. Trained Comptometer operators performed calculations by feel (odd keys were concave, even were flat) and mostly one-handed so their eyes and left hand could stay on the sheet of figures. Here's 31337 + 5.

The Comptometer to the right is a Model F, made between 1919 and 1920. It is the mass-produced successor to the smaller (and rarer) Model E (1913-1914) to the left. The Model E introduced a "control-key" mechanism to prevent errors from half-presses, but Model F simplified it.

I'm not add-icted. I can subtract whenever I want. Of course to subtract I would need to add something complementary...

The Burroughs adding machines aren't my favorite to use, but they are my favorite to look at. Check out the lines on this 100-year-old Class 3. It reminds me of sedans from that era.

Adding to my collection...

As I mentioned the Model C was designed for Imperial measurements and had a dial that went up to 11 so inches could carry over into feet. Here I'm calculating 2' 9 3/4" + 1' 6 7/8". Doing math with Imperial measurements really makes you appreciate the Metric system.

You can also perform division using the repeated subtraction method but you have to keep track of the number of subtractions in your head. To clear the registers, just pull on the lever on the right side. This also ejects the stylus for you if it's in storage.

To multiply, perform repeated addition and shift left for each term in the multiplier. Here is 12 x 12.

To subtract, use the smaller digits on the outside of the dial to enter the subtrahend and rotate anti-clockwise. The mechanism also will decrement from the left side automatically. Here I'm subtracting 50 from 31341.

To add, put the stylus in the dial for the corresponding digit using the larger numbers, and turn clockwise until the dial stops. The mechanism automatically carries the one without having to move in different directions like with a "shepherd's hook" design. Here is 31337 + 4.

My Model A came with the original case, which was designed with a notch cut in the side so you can access the stylus storage and the clearing lever without removing it from the case. It also came with the original instructions!

From a distance they look similar but the Model C was designed for adding Imperial measurements and has a dial that goes up to 11 for adding inches. Both models feature a dial for fractions on the far right, where Model Bs only have decimal dials. All models have a ruler on top.

This is a pair of Addometers made by Reliable Typewriter & Adding Machine company. They were made from 1930-1960s. The top one is a Model A I think from the early 1930s due to "patent pending" text on the front. The bottom one is a Model C I think from the 1950s.

My favorite part of touring the U.S.S Midway was probably seeing the comms, sensor and radar stations.

Clearing this machine is also quirky, you can clear the counting register with a clockwise turn of the top crank, but to clear the main register you must lift the carriage up manually before you turn the top crank anti-clockwise. Later models automatically lifted the carriage.

To divide, enter dividend into register, clear counter and keyboard, enter divisor, shift so divisor is over column it's less than. Subtract until you hear underflow bell, add to hear the bell again. Shift left. Repeat. Quotient is in top register, remainder below. Here's 145/12.

To multiply, enter the multiplicand, set the repeat key so it doesn't clear, then crank clockwise repeatedly for the ones position. Then use the handle in the front to move the carriage to the right and repeat until the multiplier is in the counting register. Here's 768 x 1024.

To subtract, enter the minuend in the keyboard and crank clockwise as with addition to enter it in the register, then enter the subtrahend and turn the handle anti-clockwise over a full turn until it clicks at the *2nd* stop. Here I perform 31342 - 50.

To add two numbers, enter the first number in the keyboard and then turn the large crank on the side one full turn clockwise until it clicks at the top stop. Then enter the second number and turn the crank again one full turn. It automatically carries. Here I'm adding 31337 + 5.

This is a Monroe Model G mechanical calculator that I restored. This model was made between 1918 and 1920, and is the bigger, heavier, much older brother to my Monroe LN-160x.