Kyle Rankin @kyle@librem.one

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.

As you add, a bar extends from the bottom for that column and as you subtract, it retracts. To zero everything out, you push all of the protruding bars back into the case. The overall design is actually pretty nice, especially considering the small size of the calculator.

To subtract, find the correct column/number *above* the total and slide *up*. If you underflow, the total in that column turns red so you go down and around that shepherd's hook to deduct from the column left of it. Here I subtracted 50.

To add, insert a stylus in the number for the appropriate column *below* the totals, and slide downward. If the addition (I added 5) overflows, the corresponding column in the total turns red, alerting you that you need to go up and around the "shepherd's hook" to carry the one.

This is a Produx pocket calculator. It was designed by Otto Neuter and produced in Germany starting in the 1930s. I suspect this particular one was from the 1950s or maybe `60s, due to the coloring and the reference to West Germany on the back.

To zero things out, you pull on a lever at the top. Basic tallying, like scrabble scores, can be done pretty quickly on one of these after some practice. You can also do multiplication via repeated addition and left shifts like with other adding machines.

To subtract, you slide a panel from the bottom upwards, which aligns new numbers with the holes. Now you slide up instead of down, unless you need to carry a one, in which case you slide down around the hook. I'm not a fan of this part of the design--it is pretty cumbersome.

As you slide rows down you notice some numbers in a row are colored red. That signals that when you add those numbers, instead of sliding down, you slide up and around the "shepherd's hook" to carry the one. Here I added 5.