I was going to enter this into the "Party like it's 1929" contest because slide rules:
- Were used in 1929 (and earlier, but never at parties I'm afraid)
- Are an intrinsically easy to replicate technology; once you know how it works, you can trivially replicate it on any medium, such as soup cans, railway spikes, or discarded bills of hyperinflated currency.
- They work underwater, in a vacuum, in the sun, in the snow, in nuclear winter. Never runs out of batteries, and you can drop it off a building and it will still work. It is the ideal tool for the discerning scientist of the post-collapse-of-western-civilization world.
- Cannot be used to easily calculate compound interest far into the future, which no one should be depending on right now anyway, since real "wealth" gains are unpredictable in the face of uncertain inflation and prime interest rates in the short term.
Unfortunately I am both ineligible for prizes, and published this instructable way too long ago. Curses!
Slide rules are analog mechanical calculators. They were ubiquitous among engineers and scientists from their inception, to the development of cheap transistor calculators.
They allowed you to do quick calculations with an accuracy up to two or three significant figures. You can multiply, divide, perform exponential functions, square/cube roots, inverse functions, trigonometry, and others. You cannot add or subtract. (Actually you can but it's much harder than doing it in your head. Bonus points to whomever posts a method to do so as a comment.)
You do the math essentially by moving sticks around. If you learn how to use this, and actually do, expect to entertain questions from anyone under 40, and nostalgic comments from anyone 40 or over.
Now, all strangeness aside, I've found one situation where I really appreciated the slide rule: as a biologist doing field work.
In that special case, you know that nature is not so kind sometimes. You will freeze and burn, you will fall off things, you may be away from civilization for months. Storms can be pretty bad, and when they aren't, the countless insects scream for a blood meal. You will need to do math in absurd conditions.
So if you're part of that special group of people to whom this applies, this is a useful tool. In fact, you should never leave for the field without a slide rule and dice (for randomizing sampling and selection for treatment groups... "arbitrary" is just not good enough for proper statistics).
This photo is of the slide rule in its "start" position. The center plastic piece slides left and right, the plastic piece to the left (called the cursor) is also mobile. Most functions are accessed in this position... if you move the cursor so that it is centered on "2" on the bottommost scale, you will notice that it lines up with 8 on the X3 scale, and 4 on the X2 scale... which should come as no surprise.
But... it lands on 5 on the 1/X scale! This is our first lesson about slide rules. They do not indicate the decimal places. You need to remember that 1/2 is going to be less than 1, but more than 0.1... and thereby reason that the indicated 5 is actually 0.5. This sounds annoying but rapidly becomes trivial.
To perform the inverse of these functions, just look them up backwards. For instance, in this configuration, 6 on the X2 scale lines up nicely around 2.45 on the middle scale, which of course is the square root.
Next, we shall move to multiplication.
Step 1: Multiplication
The middle part of the slide rule is mobile. You move it around mainly when you want to perform multiplication/division. It works because one of the scales is linear, the other logarithmic.... and as I hope we all know:
Note that in this photo, the slide has been moved so that "1" on the center scale is lined up with "2" on the bottom scale.
Now, note that "2" on the bottom scale is lined up with "4" on the middle, 3 with 6, etc. If you had lined up 2.15... you would then get 4.3, and 6.45 lined up.
Division just works by looking at this backwards.
But what if you wanted to perform 2 by 7? It goes off the end of the scale! Just slide the rule the other way.
Step 2: Multiplication 2
Just slide the rule the other way so that "2" on the bottom scale is lined up with the "1" *to the far right* of the middle scale, unlike this photo.
7 will nicely line up with 1.4, which we in our infinite wisdom know is actually 14.
Now, complex functions.
Step 3: Complex Funtions
To be fair, these functions aren't complex at all. You can do them in your head if you teach yourself floating point math, eliminating the need for a slide rule totally. Then, you can convince people you're an idiot-savant. Not the best bar trick, I admit.
The center of the slide rule actually comes out completely. Flip it over and you will be delighted to see new scales.
They let you perform trigonometric functions and their inverse in the same fashion as you performed the functions described in step 1.
Finally, your slide rule may differ. It may offer more or less functions, or function to three decimal places instead of two. It may also be circular instead of linear. You may also trivially make your own slide rule given wood or paper, cutting implements, and preferably a measure of sobriety. It is after all only two sticks that move relative to each other!
There are also many more complex operations you can perform with a slide rule, which are beyond the scope of this text. None of these are random number generation, so don't forget to carry dice if you need to do this. Some hobbyists have some that have over 20 sides and come in bright colors, so generate substantial entropy per roll as compared to coin tosses... and are easier to find when you inevitably drop them into leaf litter while climbing a tree, while doing a field study.