## Tuesday, August 30, 2016

### Slide Rule Giggles

I've always loved slide rules. As soon as I learned in 8th grade about logarithms and how they formed the basis for a slide rule, I was hooked. Just like 2 rulers can be used to do addition and subtraction, so can 2 rulers, logarithmically scaled, be used for multiplication and division. That you have to keep your wits about you for locating the decimal is what separates the experts from the wannabes. 13 x 5.4 looks the same as 135,000 * 54. The significant figures were somewhat limited to 2-and-a-half or so, particularly as the value of the leading digit increased from 1 to 9. (The span between 1 and 2 takes up about 30% of the slide rule, while the distance between 8 and 9 is just 5%.)
Even with this, they were (and still can be) powerful calculation devices. Keep in mind that the atomic bomb was developed using just slide rules and that Apollo astronauts had slide rules in their capsule during their flights to the moon. No worries about power outages or dead batteries, they were consistent and reliable.

My wife and I both have slide rules that were passed on to us (mine from my grandfather, hers from her father) and they will never be tossed in the trash until you pry them from our cold, dead hands. They are not super fancy or collector's grade, but they are a way to still be in touch with these people, knowing that their fingers handled (and their brains engaged) the same instruments in the same way.

A recent alumni newsletter from Minnesota had a article on slide rules and it included anecdotes from various engineers who used them to get through engineering classes and even the early years of their jobs until electronic calculators came along. My favorite was this from a Hungarian emigrant:
"During his time in graduate school, [Erwin] Kelen served as a teaching assistant and decided he would have a little slide rule fun with his class.

'I was at the blackboard in front of my class, solving a problem, and I was reading the results off my slide rule. Four decimals first, then squinting, two more' he said. 'Imagine, six digit accuracy from a pocket slide rule!'

After class, all the students crowded around him, wanting to know how he could read this from a tool that basically had two decimal capability. At first, he was coy in telling them that it was a special secret that enabled him to calculate with such accuracy. Surprised, they insisted he tell them his secret.

'I 'fessed up that the last four decimals of the six were purely invention on my part and we all had a good laugh' Kelen said"
The fun of this is that the students themselves would have been limited to calculating just 2 or maybe 3 significant digits, and would have no way to prove that the last 3 (or 4) digits were wrong. I imagine having a Hungarian accent helped to give a further illusion of authority.

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