Miscellany: Slide Rule

Last week's post on obsolescence got me thinking, and the slide rule popped into my head. Why? Because even though the slide rule is obsolete, I think it remains the quintessential engineer's tool, a lot more so than the modern pocket calculator. That may sound a bit strange coming from a computer engineer, so here's my reasoning:

Typical calculators contain arithmetic logic units that spit out results nearly instantaneously; you punch in numbers, you get a number back in a vanishingly small amount of time (propagation delay for the simplest ALUs is on the order of nanoseconds). The calculator thus gives no explanation of what the intermediate results are when you do, say, a problem involving the ideal gas law. Even complex graphing calculators that include support for computer algebra work in the same fashion.

Slide rules aren't like that. You have to constantly conduct sanity checks on the results you're getting, because there's no indication of the order of magnitude of your answer on the slide rule itself - heck, you don't even get "exact" figures at the end to play around with. Every slide rule calculation ends in an estimation.

Most engineers don't mind these back-of-the-envelope figures, since a fast estimate often works just as well as a slow "exact" answer. In calculations filled with implicit assumptions and idealizations of the way the physical world works, a slide rule neatly reflects human uncertainty - and how we soldier on anyway. After all, the airplane was built with pencil and paper, not with fancy electronics.