Pocket Calculator Isn’t A Brain Or Magic

If you predate the pocket calculator, you may remember slide rules. But slide rules take a a little skill to use. There was a market for other devices that were simpler or, in some cases, cheaper. One common one was the “magic brain” or Addiator which was a little metal box with some slots that could add numbers. However, using clever tricks it could also subtract and — in a fashion — multiply. [Our Own Devices] has a teardown of the device you can see in the video below. It is deceptively simple, and the description of how it works is at least as interesting as the peek inside.

We remember these on the market and, honestly, always thought they were simple tally mechanisms. It turns out they are both less and more than that. Internally, the device is a few serrated sheet metal strips in a plastic channel. The subtraction uses a complement addition similar to how you do binary subtraction using 2’s complement math. Multiplication is just repetitive addition, which is fine for simple problems.

Normally, these devices are hard to open because they are riveted. However, [Jim] drilled his out and replaced the rivets with screws. It looks odd but much nicer for opening it up.

If the video doesn’t make sense, you can always consult the original instructions. We were surprised we couldn’t find a JavaScript simulator somewhere, although this online calculator might help a little. According to the Slide Rule Museum, the original version of this was the “Baby Calculator” made by a company around Chicago, possibly as early as 1917. The magic brains were Japanese imports and there were many other variants that you can find in the museum’s document.

We couldn’t help but think of the Smarty Cat slide rule we have in our collection. Most of our slide rules are, however, more conventional.

18 thoughts on “Pocket Calculator Isn’t A Brain Or Magic

  1. I was handed down one of these when I was 6 and a Marine brat in Japan. I was familiar with the concept of adding machines, abacus still being ubiquitous at the front counter of mom & pop stores, and my grandfather owning an electro-mechanical device stateside. But, I wasn’t able to make heads or tails of the algorithm to make the Magic Brain work. It wouldn’t have helped to ask mom or dad, because I was expecting a device that (if I’d had the words) implemented the algorithm in hardware. This being the mid-60s, a Curta would have done nicely. My techno fulfillment didn’t come until 7 years later when I got my first electronic calculator for Christmas.

  2. Actually addition isn’t always the easier operation to implement in electronics. The Univac 1100 series (at least the earlier ones) implemented an electronic subtractor and used subtraction of the one’s complement to perform addition.

  3. I was given a pencil case in the 4th grade which had a slide cursor window that lined up x with that dreaded multiplication table which we had to memorize and I couldn’t and the nun scolded me for using it.

    So this one multiplied (ROM) only 2 to 9 or higher I don’t remember.

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