Slide rule (thing)

The following Everythingians own slide rules.
• Eponymous ("Betsy", plastic Sterling, with orig. case). I keep in hanging on my wall next to the Tower of Power just to remind my computers that they are replaceble, if need be.
• Anotherone just carries one to annoy math teachers
• Ariels (I assume without concrete proof)
• Zerth
• Mr Option doesn't, but his uncle was once beaten up for owning a slide rule
• Big Alba does, but it's "old and sucks"
• Stavr0 owns one branded Northern Electric
• ouroboros is reported to be an old-fashioned noder as well
• fellowearthite has confessed, and on this very page!
• BlueDragon calls hers "a little stick of magic"
• Mr. Guilt says he owns "several, having one at the office, one in my home office (both see use), and, strangely, an old wooden one in my beadroom."
• Rubyflame is the proud owner of a K&E 4095-3 slide rule, and planning to get a log-log rule.
• xdjio has a Hughes-Owens and Mannheim 10-incher (whip out that big Ten inch!), bamboo with white plastic celluloid faces, and an engraved scale, vintage about 1960, and wouldn't mind learning how to use it.
• sirnonya owns three slide rules: "a small pocket slide rule that I can't find, an Acu-Math ~10" slide rule, (both given to me by my grandpa), and a Teledyne Post Versalog II 25cm slide rule. This baby is sweet! Twenty-four scales front and back (duplication of C and D). My other grandpa picked it up at a garage sale really cheap 30 years ago, never used it, was going to frame it on his wall (he was an engineer), and gave it to me instead. A slide rule makes math a lot easier to understand."
• dann says, "I've got one and actually use it! It's a simplex ACU-Math 400B, mannheim type (K, A/B, CI, C/D, L, S, and T)."
• wertperch owns a pocket rule, which he occasionally takes out to amaze and astound people with. "Sometimes even by performing calculations."

Most good slide rules include a slide with a hairline, making it much easier to line up the numbers on the logarithmic scales. The actual way the rule works is simple.

Every number has a logarithm.
If you add two logarithms together they produce a third.
The number associated with that logarithm is the product of the first two numbers.

Any mathematical operation that can be reduced to addition and subtraction can be performed on a slide rule. That, really, is all they do. Logarithm tables you don't need to memorize. For basic operations, use the top two scales (often labeled A and B).

When civilization crumbles, after the EMP fries the computers and all the batteries wear out, the person who can use a slide rule will be king. OK, scientific advisor to the king.