(note 1)I cried when my last (cheap plastic) circular slide rule dissolved in my pocket about the turn of the century and I couldn't replace it.
A slide rule - and especially, a circular slide rule - is a great device for comparing ratios. I used it when grocery shopping to compare unit pricing between varied sizes. Put the units (ie: ounces) on one scale opposite the price on the other. Then look at the units for the different size and see their comparative price. I could say "that size is the best value" faster than even today with a custom app on my pocket device. People point out that the store prints the unit price right next to the price. But that doesn't take into account special discounts or coupons. The advantage of the circular slide rule for ratios is that when one number goes off the scale you don't have to push the slipstick (the movable piece in the middle of a linear slide rule [the type you see above]) to get to the rest of the scale - it just wraps around.
All a slide rule does is add or subtract two numbers (imagine putting 2 yard sticks end-to-end). Depending on how large and finely calibrated it is, you may only be able to accurately read 2 or 3 digits if all you want to do is add real numbers. Its real power comes when you change the scale to one where the numbers are not equally spaced from 1 to 10. The basic logarithmic scale allows you to multiply (or divide) by adding powers of 10. Other basic scales compute squares and cubes or their roots and trigonometric functions. Specialty slide rules can help calculate anything from an airplane's crosswind correction to astronomical events. In an earlier life my industry regularly used a slide rule to calculate the enlargement or reduction of pictures to fit a printed layout. It was designed to be accessible to people who had barely any facility with numbers.
I still have some good linear slide rules including my father's precision (for the time) rule (boxwood?) from the 1950s. I also have the aluminum Pickett that I won for some obscure "best something student" award at high school graduation and got me through freshman and sophomore chemistry and physics. (search "slide rule image" for some idea of the varieties and detail) (note 2)
Long after calculators were a $3 commodity I was taking a friend to lunch and he asked if I still had a slide rule somewhere in storage. He tutored at a disadvantaged elementary school and wanted to show the kids what it was like in "prehistoric times." I reached in the glove compartment and handed it to him. I carried it to calculate my gas mileage - my first (about age 10) and last (late 1990s) regular use of a slide rule. (note 3)
When I took an advanced physics class (in 1973) that required adding and subtracting 8 decimal places, I bought a digital calculator. It cost $100, was big as a dictionary, and had to be plugged into the wall; but it could do four functions on up to 12 digits. First my classmates laughed at me for carrying that thing to class and moving to a seat by an outlet. Then they started sitting around me and borrowing it during class. (note 4)
Notes:
(1) Thanks to Bill Amend for my Sunday morning chuckle. All rights reserved to the holder. Click image for complete original.
(2) I was about to pull out my collection and camera - but the internet has everything. All rights reserved to the holder.
(3) When I started driving long trips alone with no radio or music player, I would calculate my mileage in my head. Nothing like doing long division to multiple decimal places in your head to build your concentration. (Do they even teach long division nowadays?) Find my spreadsheet at https://3500calories.info/links_blogger/MPG.xls.
(4) About 2002 I took a statistics class. While the course was taught with the graphing calculator most traditional students had used since high school; I wanted to get my results off a computer because that was the method I'd have to support at work. I brought my computer into class and performed all the exercises twice - on the calculator and on the computer.
