Slide Rules

We took our grandsons to visit the MIT Museum on our recent vacation. It seems the museum had a display of slide rules and I took the opportunity to teach the boys about an instrument that was now obsolete.

Slide rules make use of logarithms. When working with logarithms, the log of the product of two numbers is equal to the sum of the logs of each number. When Napier invented logarithms in the 1600s this rule was used to simplify arithmetic because the multiplication of large numbers could now be reduced to simple addition. I don't think they teach logarithms any more. Slide rules are just logarithmic scales inscribed on a ruler. You multiply and divide by adding or subtracting lengths on the ruler.

As an engineering student, my slide rule was my constant companion. Indeed, it was one way to identify who was an engineering student. Mine served me well through graduate school in 1974. At that time, calculators were making their way onto the market but they were very, very expensive. For a while, there was a controversy about allowing students to use calculators as it may give them an advantage over poorer students who could not afford one. That didn't last long as soon calculators became cheap.

Lots of stuff was designed and built using slide rules. The scale forced you to think about the accuracy of your calculation as the scale would get cramped in numbers above 5 and you couldn't carry too many decimal places. The term "slide rule accuracy" was a common benchmark. Today, an engineer can calculate out to 6 decimal places but it doesn't mean that his answer is any better than the guy with the slide rule. It also forced you to use scientific notation in order to keep track of the decimal place. A common multiple guess test trick was to provide answers differing only by a factor of 10 to see how well you managed the decimal point.

Slide rules are no longer manufactured. K&E, who made most of them, retired their dies and donated them to the MIT Museum. Knowing how to use them is a useful skill.