Kyle Rankin @kyle@librem.one

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.

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.

It's pretty complicated all right...

This is a Marchant "Silent Speed" ACR8D electromechanical calculator from the 1940s. It is currently jammed up, stuck on a division problem it never finished, and is apparently the most complicated mechanical calculator out there. Wish me luck!

To close out this thread, here is correspondence between Felt himself (the inventor of the Comptometer) and Zenith Pub. Co.

Subtraction was weird. You'd subtract w/ addition by the complements method, but would first subtract one from the number, then press down that number using the small digits on each key. You also slide the correct metal tab at the bottom to stop the carry. Here's 9 - 2.

You can perform many calculations surprisingly quickly and w/ one-hand. A well-trained Comptometer user was probably as fast as you'd get until electromechanical calculators. Here I'm calculating 1024 x 768, which just requires pressing 768 a few times and shifting left.

The Comptometer had a superior design. Pressing a key immediately updated the register, no need to pull a lever (the lever here clears the register). This design was so effective it stayed essentially the same (with minor improvements on error prevention) through the `50s.

This is a Comptometer Model E mechanical calculator, made between 1913 and 1914. Comptometers were made starting around 1900 and were a direct competitor to the 1908 Burroughs adding machine I featured earlier.

This is the carry mechanism for the Monroe. The bars form a wave and the last few move, but were completely seized up. A few hours with some oil, pliers, a screwdriver, and ultimately a heat gun, and it finally loosened up. Subtraction finally underflows all the way to the left!

Yesterday I spent a few hours restoring this Monroe Model G calculator. It was only after putting everything back together that I discovered the accumulator is only underflowing half the digits when doing subtraction/division, so I get to take it apart again today and fix that.

Of course if you can add repeatedly you can multiply, so this machine has a "repeat" button in the bottom right corner. Once pressed, any number you enter stays pressed down after you pull the addition lever, so you can add over and over just by pulling the lever.