The protractor and the Bunsen burner. Playing the recorder in music class. Drawing arcs and circles with a compass in geometry. These tools of the education trade become part of our lives for a semester or two and then we move on.

Today, NPR Ed begins a new series examining these icons of the classroom. We start off with a device that once was essential to higher-level math, in school and in the workplace, but now has all but disappeared:

The slide rule.

Slide rules are “divided into thirds, the top and bottom are fixed in place, but the middle section slides back and forth. Each section has scales — numbers and line marks for calculations,” NPR explains for the many readers who’ve never seen one.

In its simplest form, the slide rule adds and subtracts lengths in order to calculate a total distance. But slide rules can also handle multiplication and division, find square roots, and do other sophisticated calculations.

For generations of engineers, technicians and scientists, the slide rule was an essential part of their daily lives. Until, all of a sudden, it wasn’t.

I played the recorder, did experiments with a Bunsen burner, poked holes in paper with my protractor and bought — but never learned to use properly — a slide rule. I guess it was required for trig, a subject I took in 1969.

I’m celebrating my 40th college reunion this weekend. So, yes, I am old. But spry.

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