Kyle Rankin @kyle@librem.one

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.

Yes, I said the 2nd stop. This older Monroe has some quirks they fixed in later models, like precise stops at around 1:00 for addition and 7:00 for subtraction. It locks the carriage between those stops so if you overshoot too far you won't be able clear the bottom register.

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.

Security is like golf. You should try to get closer to the hole (a secure system) with each stroke. You'll probably use different clubs along the way. Some people only swing for holes in one and spend most of their time no closer to the hole.

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.

I shave w/ Derby Extra blades, $10 for 100, but was shopping for something new. Discovered Derby Premium for a similar price, but a 2-pack (200 blades!) was on sale for $14. At 1/week this is a 4-year supply. At my rate I won't need blades again until my son is in college.

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.

This is a Tasco Pocket Arithometer. It was probably made in the 1940s but is based off of older Arithometer designs. It is sized to fit easily into a shirt or coat pocket. To add numbers you place a stylus in the appropriate row and drag down, which adds to the existing total.

Today's other antique find is this pair of mechanical pocket calculators. Both came with their original case but sadly neither had their stylus.

Each pocket calculator in my collection has a slightly different way of performing arithmetic, probably due to patents.

Today's antique finds: a Gillette red tip and a Slim adjustable. The red tip came with a wooden case which I don't think is original--my understanding is these originally came in a plastic case.

Well this is disappointing. While trying to loosen up stuck mechanisms a few teeth broke off of an important gear. Guess this is now a parts machine.