It is a portable calculator able to add, subtract, multiply and divide, using as principle a ruler with numbers which is moved up and down depending of the calculation you want to do.

This model was produced in Brazil in the 50’s, but there are similar models produced that are old, even ones produced in the 20’s in other places of the world.

Even with the manual, I don’t if it is because the way people used to write was much different than I expected, or if it was a bad manual (or I’m stupid), it was not so simple to understand how it worked. However, playing around here and there I could figure out how to use it.

I made a video showing how to add and subtract. Take a look.

The video shows that the calculator has 3 areas, one at the top, other at the bottom, and one line on the middle that is used to register values. All the areas have columns with numbers and each number has a hole where the moving stick can fit in. Using the moving stick, once inside the hole of a number, you can slide the column and then registering the number in the middle area. The bottom area is used to register positive values, the top area for negative. So, to make a calculation first you register the value you need, at the video I registered 3, then, to add you select a number at the bottom area, at the video I selected number 2, and slide it. To subtract the process is the same, but using the top area.

And now, what about multiply and divide? Well, the answer is: it is not possible, you were fooled, yeah!....

Yes, in my opinion I think it is a little exaggeration to say this calculator can multiply and divide, the manual even say that to do this kind of operation you need paper and something to write on it. What the calculator really does is to use some mathematical tricks to multiply and divide using only sum and subtraction. To give one example, to calculate 3 x 7, you would have to sum 7 + 7, then to the result you would have to add 7 again, making three sevens.

If it was a multiplication with more numbers than you would have to do it one by one, but instead of me trying to explain with words, take a look at the image below.

To multiply with 3 numbers the only change is that the third number would have to be moved two columns, the second number would still be moved one column and the first number would be the same. To multiply with 4 numbers, the fourth number would have to be moved 3 columns, the third number two columns, the second number one column and the first one stays the same. And that is the idea if the number was even bigger.

The last one, to calculate a division, it is a little more complex, but the basic idea is to calculate successive subtractions and then count how many where needed. The image below can help to understand.

Observing now this 4 operations we can say that the calculator can do it all, yes, ok, but the way it does is demands much, much, much, much, much more time, right? Well, that was when I became intrigued with this calculator.

If it makes the job more complicated and slower, why does it exist?

I think the answer is that people are stupid and eventually will commit some mistake while doing some calculation for whatever reasons, like lack of sleeping, worries with something, an ingrown nail, stress, I don’t know. The mistake, depending of the situation can cause serious problems. Using the calculator the result does not rely on someone thinking, making associations of things to “remember the result”, this way, since the result is mechanic it will always be correct, no matter what the user is feeling that day.

Not only that, but the way the calculator is used allows that a person that doesn’t know how to multiply or divide to do this kind of calculation and considering this we can also imagine why this kind of portable calculator disappeared during the years. Maybe it was because, firstly, the level of instructions that a regular person could have has grown considerably, more people went to school and learned better mathematical basics. Secondly, and this reason is more obvious, make calculations that are always correct and doing them very fast became something easy and cheap during the years with digital calculators and computers.

Whatever is the truth, if my explanations have made you cry, or if my theories are completely wrong, it is always fun to observe this technologies of the past and understand why someone did that, why that was useful and the most interesting of all, why that was forgotten in a way that we, today, need to stop and think to understand why it ever existed and why it disappeared.

We don’t like to think about it, but one day will be some people from the future, looking to us and to the things we use today, asking themselves the same questions. And before I stop to write, just a curious interesting fact that connects everything, did you know that this logic to multiply and divide is the same behind the operation of the first digital calculators? Yeah, to them, even with a few more calculations that could even look as a bad decision to make calculations, the answer is much faster than ours.