Slide Rules were the Original Personal Computers

Unless you are above a certain age, the only time you may have seen a slide rule (or a slip stick, as we sometimes called them) is in the movies. You might have missed it, but slide rules show up in Titanic, This Island Earth, and Apollo 13. If you are a fan of the original Star Trek, Mr. Spock was seen using Jeppesen CSG-1 and B-1 slide rules in several episodes. But there was a time that it was common to see an engineer with a stick hanging from his belt, instead of a calculator or a cell phone. A Pickett brand slide rule flew to the moon with the astronauts and a K&E made the atomic bomb possible.

Slide rules are a neat piece of math and history. They aren’t prone to destruction by EMP in the upcoming apocalypse (which may or may not include zombies). Like a lot of things in life, when it comes to slide rules bigger is definitely better, but before I tell you about the 5 foot slide rule in my collection, let’s talk about slide rules in general.
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Bil Herd: Computing with Analog

When I was young the first “computer” I ever owned was an analog computer built from a kit. It had a sloped plastic case which had three knobs with large numerical scales around them and a small center-null meter. To operate it I would dial in two numbers as indicated by the scales and then adjust the “answer” by rotating the third dial until the little meter centered. Underneath there was a small handful of components wired on a terminal strip including two or three transistors.

Science Fair Analog Computer
Science Fair Analog Computer

In thinking back about that relic from the early 1970’s there was a moment when I assumed they may have been using the transistors as logarithmic amplifiers meaning that it was able to multiply electronically. After a few minutes of thought I came to the conclusion that it was probably much simpler and was most likely a Wheatstone Bridge. That doesn’t mean it couldn’t multiply, it was probably the printed scales that were logarithmic, much like a slide rule.

Analog slide rule on digital calculator
Old meets new: Analog and digital computation

Did someone just ask what a slide rule was? Let me explain further for anyone under 50. If you watch the video footage or movies about the Apollo Space Program you won’t see any anyone carrying a hand calculator, they didn’t exist yet. Yet the navigation guys in the first row of Mission Control known aptly as “the trench”, could quickly calculate a position or vector to within a couple of decimal places, and they did it using sliding piece of bamboo or aluminum with numbers printed on them.

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Papercraft dial is the slide-ruler of current limiting resistors


This paper dial makes selecting current limiting resistors a snap. [Giorgos Lazaridis] came up with the tool, which he describes in detail in the Worklog tab of his writeup. If you want one of your own he also posted a PDF which you can print, cut, and tack together.

At this point we can calculate resistor values for LED circuits without looking at reference material. But it wasn’t always like that. This wheel will be a fantastic tool for those just starting out in hobby electronics who are trying to grasp the theory behind lighting up a simple project. The outer wheel references the source voltage, with the inner being a gauge of forward voltage across the LED(s). Line those two values up and you can read the optimal resistor value in the window seen to the right. But wait, there’s more! As you can see in the video after the break the opposite face of the dial also includes a window which will tell you the power dissipation so that you may choose a properly rated resistor. Slick!

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Slide rule for musical scales

For all those engineers who dabble in music [Magnetovore] has your back. Musicians simply must know their scales and he came up with a papercraft slide rule for major and minor scales.

The system is very easy to use. He’s uploaded PDF files that let you print out the mask for the top layer and bar chart and directions for the bottom layer. The top layer is laid out like a piano keyboard, with windows for each key and a couple of windows to identify the major and minor scales being displayed. Just slide the mask until each key is a solid color. The color codes show the tonic, third, and dominant for each key so you know where to start. In the video after the break you can see how it works by playing all of the non-black keys in order. But wait, if you order now you’ll get the slide rule for Cello scales at the same low-cost; free!

This is a fun quick-reference, but you really should know your Circle of Fifths. Continue reading “Slide rule for musical scales”