Calculators I’ve Known and Loved

Here is a stereotypical, nerdy engineering reflection on calculators.  Actually, the purpose is to illustrate how much technology changes in a single generation, and maybe somebody will find it interesting. So, here is a brief description of calculators that I have used in university and beyond.

Slide Rule

  Acumath slide ruleYes, I actually used this in high school and for the first couple of weeks of university in my chemical engineering program (for reasons you’ll see below).  Once you were proficient, it could be faster than a calculator for some calculations.  The downside is that you can’t add or subtract with it, only multiply, divide, do logarithms and trig functions.  Addition and subtraction you have to do in your head, or on paper.  It’s interesting to think that most engineering design done prior to the early 1970s was based on slide rule calculations.

If there was one good thing about slide rules, it was that you quickly learned the importance of significant figures (you can only get 3 or 4).  These days, students tend to write down all 12+ decimal places in their answers, which is annoying when you know that most of those digits are meaningless.

For those who would like to try using a slide rule, I found this online emulator:  or you can still buy old slide rules on eBay.

Hewlett-Packard 21 (HP 21)

My first electronic calculator!  It cost about $125 in 1976, which in today’s dollars is equivalent to about $495 according to the Bank of Canada inflation calculator.  And that’s why I had to wait until I got my Ontario Scholar cheque ($100) before I could afford to buy it (sorry, Ontario Scholars no longer get cheques).

Like all calculators at that time, it had a power-draining LED display, so you had to be sure it was fully charged before an exam.  The main thing I loved about this calculator was its use of the Reverse Polish Notation (RPN) method for performing operations.  Looking closely at the picture, you’ll note the lack of an “=” button.  With RPN, instead of pressing “3 + 4 =”, you would press “3 Enter 4 +”.  Using RPN and the memory stack seemed more efficient and elegant to me, and could minimize keystrokes in more complex calculations.  Also, people in the class who used Texas Instruments calculators (the main rival calculator) didn’t know how RPN worked, so they wouldn’t ask to borrow it when their batteries died.

If you want to try out RPN, here is an HP 35 emulator (a model that pre-dated the HP 21).  In fact, there is a whole online “museum” dedicated to HP calculators!

Hewlett-Packard 32E

Eventually my HP 21 got a bit worn out, and newer models were rapidly appearing with more functions.  So around 3rd year I bought this HP 32E (or it might have been a 31E; I forget).  According to the museum website, these retailed for $60 (about half what I paid for the HP21!). You can see in the picture that it had more buttons/functions, but it still employed RPN.  Unfortunately, the keyboard wore out by the time I graduated, so I had to dispose of it.  The company I worked for provided a Texas Instruments TI-59 programmable calculator (with magnetic card reader), so I used that for a few years.  I missed the RPN functionality though.

Sharp Pocket Computer

Sharp Pocket ComputerWhen I returned to graduate school I needed a new calculator, so I got this Sharp model (PC-1450).  It was fully programmable in BASIC, which came in handy for some courses where we were using Newton-Rhapson root finding and other minimization algorithms.  The downside was that the display was only single line (LCD), so editing and debugging programs was not pleasant.  Desktop PCs were becoming more common, so I eventually drifted away from using this calculator.  I still have it in my desk, but the batteries are dead.


Calculators are so cheap and powerful, they are no longer interesting.  I’ve got a couple of different solar-powered ones I use for quick calculations, but most heavy-duty work gets done on a PC.  Excel spreadsheets are good for many things.  More complex stuff, or design calculations needing a lot of unit conversions (feet to meters, liters to gallons, etc), I like to use MathCAD because it will do all the conversions for me automatically.  MathCAD is also nice because the output looks similar to how you would write it on paper, so that’s useful for making assignment solutions for students in my courses.  For really complex stuff, like solving 75 differential equations simultaneously to simulate complex photochemical reactions, I use Matlab.

2 thoughts on “Calculators I’ve Known and Loved

  1. I recall my HP 15C was the “calculator” du jour in the 80’s. It wasn’t cheap either, but then again PC’s were only just beginning. I do miss RPN.

    There’s a fully functional free 15C emulator app for the iPhone out there..


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