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.

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.

I don't dispute that jails are hard to break into, but they are even harder to break out of. How many of us would choose to live in a prison in real life? Instead we make risk assessments that balance personal freedom and security, and the digital world should be no different.

The security industry has largely failed to build security measures without reducing a user's freedom more than attackers. It's not only harder to build security solutions that give users control, it reduces dependence on the vendor. This is why vendors just build jails.

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.

Even though I have more "advanced" mechanical calculators like my Monroes, the more I use this Comptometer the more impressed I am at the design. It's actually faster to use than my Monroes, at least for addition and multiplication.

Experts would only use the bottom rows so their hand was stationary. To enter larger numbers they'd enter two smaller numbers so to enter an 8 they'd enter 4 two times. This and later models had features to catch "fat finger" errors and partially-depressed rows.

Division was possible via repeated subtraction starting from the left and shifting to the right. There were no underflow bells, you'd just have to know when to stop subtracting a row. This is hard to do one-handed so I didn't film a video of that.