A More Complete Slide Rule Tutorial

A member at Instructables asked me to explain how to use a slide rule for the sake of those who came along about the time electronic calculators replaced slide rules. There are already a couple of tutorials posted at Instructables on using a slide rule. I aim to make this one more complete and easier to follow. It will cover all of the basic calculations, not just multiplication. Still, learning to use a slide rule is not a simple afternoon project. It takes time and practice over days, perhaps even weeks, for the principles to become part of the learner. You may want to download a PDF copy of this Instructable to your computer so you can learn the material in small bits. 

You do not need your own slide rule because this Instructable will make use of a virtual Pickett N600-ES slide rule available for anyone to use at this link. It is the slide rule shown in the graphics. With the mouse cursor on your computer and your left mouse button, you can work the moving parts on this virtual slide rule just as if you were holding an actual slide rule in your hands. Just click and drag the moving parts at the link above. You can also drag the entire slide rule across your screen for a better view. Clicking and dragging will not work on the graphics in this Instructable. It is highly recommended that you open a second browser window and bring up the virtual slide rule so you can practice each new thing you learn in this Instructable. 

The graphic shows the left end of the virtual slide rule with the various scales. From the top down you can see the LL1 or log-log 1 + and - scales; the A and B scales; the ST, T, and S scales; the C and D scales; the D1 scale, and the K scale. Different makers of slide rules arranged the scales slightly differently according to what each maker thought would make its rules more user friendly. If you go to this virtual slide rule on the Internet, you will see a radio button on which you can click to display the scales on the back side of this virtual slide rule. Do not worry about that for now. Just be aware that this rule has two sides with additional scales on the backside. Because of all of its scales and features, a slide rule really is a full scientific calculator for all types of multiplication and division problems.

Some, but not all graphics in this Instructable do utilize text boxes. They are not always easy to see because the N600-ES slide rule used an eye saver (ES) yellow and black color scheme. Sweep your computer's cursor over key areas of the graphics to find any text boxes if you do not see them.

(Note: In my experience, some browsers may introduce some inaccuracies when using this virtual slide rule. If a reading is not quite accurate, try a different browser and see if that solves the problem. I will explain how to check for accuracy in a later step.)

 

Step 1 A first check for accuracy

Before using a slide rule new to you there is a preliminary check for accuracy that should be made. The inscribed line for the "1" on the A scale should be directly above the matching line on the B scale while the "1" on the C scale is directly above the line for the D scale. If this is not the case, the rule will need to be adjusted. 

To adjust a slide rule, loosen the screwhead just to the left of the LL1 scale about a quarter turn. There is an identical screwhead at the right end of the rule. Loosen it a quarter turn, too. The top member of the slide rule's frame can now be moved a little to the left or right until the lines for "1" on all of the scales are directly inline with one another. Use a magnifying glass to be as accurate as possible. When all is in order, gently tighten the two screwheads again. The Pickett virtual rule shown is properly aligned.*

If you are using a plastic slide rule, it is permanently glued in place and (hopefully) is set accurately.

*Loosening these screws allows two adjustments. Already mentioned is aligning the top member of the frame so the 1 on the left side of each scale aligns with the 1 on the others. These screws also are used to adjust how much drag or tension there is on the movement of the slide rule parts when they are worked back and forth. If the slide rule is adjusted to be too loose, the center sliding portion can fall out and become damaged or lost. If the slide rule is adjusted to be too tight, using it for calculations becomes a lot of hard work. 

Comments

[Sovereignty]

Awesome. And I hope you go big, and go Christian for #200.

[Phil B]

When I began submitting Instructables I thought I would run out of ideas after 25 or so. It was a shock to reach 50. But, things keep breaking around our house, or an idea comes to me that I want to share. Thanks.

[bertus52x11]

...and to know that they have sent men to the moon with this thing!
Nice I'ble.

[Phil B]

Thank you for looking and for your comment. The moon shots did include a primitive on-board electronic computer, but a huge amount of the engineering and any in flight human calculations would have used slide rules. In my sample problem of 259 x 653 the difference between the slide rule answer and the electronic calculator answer was 127 out of 169127. That is less than 1/10 of one percent. That is like calculating something to be at a distance of 1km with an error rate less than 1m. In a previous Instructable that I linked I summarized some points from a 2006 article on the contribution of slide rules (the final step). Things designed with a slide rule were a little over-engineered, but the slide rule was very effective. A final step in any engineering project was to check all calculations for mistakes. 

[steliart]

Another great Ible from you my friend, thank you.
Also I would like to wish to you and your family a Happy Healthy and Prosperus New Year

[Phil B]

Thank you, Steli. I wish you a prosperous and healthy New Year, too.

[rimar2000]

Phil, this is very valuable information, I want to go out and buy a slide rule.

I don't read all the text (it is late night). You can calculate also X**(2/3) and X**(3/2) with a cheap slide rule.

[Phil B]

Thank you, Osvaldo. Ask around. Someone probably has a rule they would sell or give you. I thought you still had a slide rule.

[javandyck]

Think Geek sells a slide rule that is very inexpensive. Bearing in mind that you get what you pay for it might be a good introduction. I had to take mine apart and clean it to make it slide anything like usable. At first I thought it was glued in place.

[Phil B]

Thank you. I did link the ThinkGeek rule. But, because a number of reviewers had the difficulty you describe, I suggested interested people might want to check eBay for a comparable never sold plastic Pickett rule.

[Jayefuu]

WOW. This looks interesting and complete. I have put this on a mental list of things to read and understand this month.

James

[Phil B]

James, it is pretty complete, except for the material on logarithms and material related to placement of decimal points with square and cube roots, as well as material on square roots and cube roots of numbers less than 1. Had I digested and given all of that back in concise form this Instructable would have required considerably more steps. Over the years my most frequent uses of a slide rule have involved only a few squares or square roots and a very, very few cubes or cube roots. I think that could be true of many others, too. But, then my life's work does not involve a lot of mathematics. Still, I like to keep a slide rule next to where I sit while I am watching television. When a mathematical question comes into mind, I reach for that slide rule. Thank you for looking and for commenting.

[Jayefuu]

I've never used one. I think I'll get one and learn.

[Phil B]

If people know you are interested in slide rules, do not be surprised if someone comes to you with a slide rule that had belonged to someone in the family and the person who has it now would be very pleased to give it to you so it does not just gather dust. A nice Keuffel & Esser rule from the late 1950s with all of the scales shown on the virtual slide rule in this Instructable can go for $30 to $50 US on eBay. I recently conducted a funeral for a man who had been an engineer. His widow knew from Facebook that I like slide rules. She gave me his K & E Log-Log Trig slide rule. She was just happy it would be appreciated by someone. No one in the family had any interest in it. (I did find some corrosion had eaten the metal frames around the cursor and I spent about $15 US restoring it. The corrosion also left discoloration spots on the white celluloid skin of the slide rule. I am hoping sunlight will bleach that out. Anyway, it is fun to solve a problem with a slide rule while your friends are still looking around to see who has a calculator.

[Kiteman]

I just downloaded an iPod app called Cube Slide Rule Lite, just to use these instructions.

[Phil B]

Thank you. I think that is just about the highest praise anyone could give.

[Kiteman]

:-)