doug — off the record

just a place to share some thoughts

Whatever happened to …

… the slide rule?

Well, mine is in the bookcase behind me, Aviva!  There’s my personal answer to your question that you posted in the Padlet devoted to this.

The educational year was my Grade 10.  We were required to purchase a slide rule for mathematics class.  We had two options:

  1. purchase a cheap plastic white tool or
  2. “if you’re serious about mathematics, you need to buy the upgraded yellow eye-saver metal one that comes with a finer hairline and a leather case.”  (My mathematics teacher no-so-subtle advice)

As you can see in the picture above, using my chair as a background, I went with option #2 – A Pickett N902-ES

We were only allowed to take the slide rule to mathematics class.  Apparently, it could be used as a weapon to avoid the boredom in other classes.  (or so I heard),  I remember the mathematics classrooms with a big teacher slide rule at the front of the class so that she/he could work along with us to solve problems with our slide rules.

We learned a great deal about mathematics from using that device including a new bit of language – “hair line”.  It was true; when we would compare the two slide rule options, the yellow one did have a thinner hair line.

What difference does that make?  It goes to the heart of using a slide rule.  Unlike a calculator, it didn’t necessarily give you the exact answer.  Instead, you learned the skill of estimation to get your answer.  Unless you were working with integers, the hair line would fall on a scale and you needed to estimate just what the answer would be.  I’ll tell you one thing; you sure learned about the beauty of numbers and decimal places.  As I look at it while writing this post, there was a special marker for pi.

Flipping the slide rule over, in very tiny print, there were instructions about how to multiply, divide, find logarithms, find the sine of tangent of an angle, and the importance of the NUMBER OF ZEROS.  You just knew that was important because, even back then, it was written in capital letters!  There was also a grid of important fraction conversions – i.e. 45/64 = .703125  (Yes, I just did check it with my computer calculator)

It got me through that class and others in high school.  Once I hit Statistics at university, it was incredibly clear that I needed to get with the digital age and purchased my first calculator – an HP21 How sad is that that even the calculator is in the HP Calculator Museum.

I have no idea why I still have the slide rule; it doesn’t conveniently get stored anywhere and I can’t remember when I last used it.

Oh, fact check.  Yes I do – re-read this blog post “Two nerds walk into a Tim Horton’s

OK, I can’t remember the last time I used it since then…

Does that inspire you to do some thinking/reminiscing?

  • have you ever used a slide rule?
  • when did you buy your first calculator?  Do you recall what features it had?
  • if you’re a classroom teacher, how best do you teach number lines and estimating?  Is it still an important skill?
  • I know some manipulative kits use slide rules or close cousins to it.  Have you ever used them?

Do you have an idea or thought that would be appropriate for my “Whatever happened to … ” series of blog posts like Aviva did? They can all, by the way, be revisited here.

Please visit this Padlet and add your idea. I’d love for it to be an inspiration for a post!


5 responses to “Whatever happened to …”

  1. Thanks for this post, Doug! As soon as you mentioned the “Tim Horton’s post,” I remembered reading that one too. I’ve never seen or used a slide rule before, but your point about estimation is so interesting. I wonder if estimation is a harder skill for some students to grasp now because of the lack of this tool. I never even considered using number lines to teach estimation. I’m curious if others do and the value that they may see in it. Thank you for another great Sunday read!


    Liked by 1 person

  2. […] … the slide rule? Well, mine is in the bookcase behind me, Aviva! There's my personal answer to your question that you posted in the Padlet devoted to this. The educational year was my Grade 10. We were required to purchase a slide rule for mathematics class. We had two options: purchase a cheap plastic…  […]


  3. Andrew Forgrave Avatar
    Andrew Forgrave

    Good morning Doug!

    I have a slide rule. It is buried in my box of keepsakes, a curiosity offered to each of the members of my grade 13 algebra class by our teacher, the head of the math department. The gift may have been offered on the last day of classes in our graduating year, and likely included a short lesson on how to use the device.

    We were all toting portable calculators at the time (mine was also an HP, the kind with the red LED display, later replaced by a programmable HP Polish Postfix model with an LCD display —the second and last calculator I ever purchased), and my memory of the event suggests that the gift was offered as an acknowledgement that the venerable devices had been clearly supplanted with a newer, more capable technology, but that they were too important in the history of mathematics to simply toss in the garbage. I’m sure they had originally been purchased by the school district (an early technology integration initiative, no doubt!) to support mathematics and science instruction. I, too, remember seeing one of those giant slide rules suspended above the blackboards.

    Speaking of blackboards, I will never forget the mastery with which Mr. Glass made use of those two walls in his classroom. Granted, our Grade 13 Algebra class would’ve been the smallest class I ever experienced in my high school career, with no more than 15 students, and perhaps less. But my most significant long-term memory from the class is of the daily work, either in our notebooks or at the board, while Mr. Glass circulated to view the work of the seven or eight of us at our seats as he simultaneously observed those working on the boards. We would periodically switch from our desks to the board and back again (DPA before DPA was a thing), and I have never forgotten his ability to spot a missed coefficient or exponent that failed to make it from one line to the next, nor his patient manner in letting us look for it.

    I have kept that slide rule as a reminder of that teacher and of that class, and of all of the wondrous things that mathematicians and scientists accomplished before they were calculators (or computers, or SMART boards).

    “If I have seen further, it is because I have stood on the shoulders of giants.”

    Liked by 1 person

  4. I studied software engineering in polytechnic at the beginning of millenium (always wanted to write that) and used TI-92+ as my calculator. Couple years earlier I had bought my first calculator, TI-85, but that didn’t cut at the polytechnic anymore. I never used slide rule at school, but always wanted one as I was fascinated by the idea and fact that there are so many different kinds of them for specific purposed. Finally, my wife got me a Faber-Castell 157/80 Mentor-Fix as a present (I still have that by my computer at the desk) and I learned how to use that.

    Liked by 1 person

  5. I went with the cheap white plastic one. Money was tight and that was what I could afford. I wish I had found the money for the better one.
    At my high school we used them in a lot of classes. We were and engineering magnet and a lot of classes used math. The leather cases on the better slide rules could be worn on a belt and many students did just that. Just like a sword. Yeah, geeks were the cool ones at my school. The poor football players were the low end of the cool scale not the high end.
    I still have mine though I really wonder how that happened. I haven’t used it for real since I got my first calculator in 1972.


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