Slide Rules, Calculators and Reverse Polish: How HP and TI foiled my Math Exam

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When I tried to use my friend, Marion’s, calculator I quickly realized I had made an assumption that all TI-30calculators worked the same. Alas, I fell victim to a common assumption with computing machines–that they all work the same. At home my Mom had an HP calculator; Marion, my classmate, possessed a TI calculator. While they both performed addition, subtraction and even a square root, you did not use the them the same. So I skipped using a calculator, (which Mrs. Marcou had described as a “nice to have,”) and I proceeded, using manual techniques to pass the math exam on limits.

When I started Algebra II Trigonometry, we soon reached the topics of logarithms and trigonometry. Translating the functions into numbers could be done a number of ways: tables in the back of the textbook, calculators (new,) slide rules (old school.) Using the tables could sometimes involve interpolation and extrapolation. Calculators had started to appear on the scene; however, the high school curriculum did not yet require them. After showing me how to use it, Dad lent me his slide rule to take to high school.

Slide rule and case
Slide rule and case like my Dad’s

In the classroom, I dutifully took it out to calculate a value. Upon seeing my actions, Mrs. Marcou suggested that I leave it at home. I vaguely recall a remark that it was old-school and that I needed to update my technology.

Alas, while I could use Mom’s HP calculator at home for math homework, I could not bring it to school.  Dad gave Mom the calculator because she loved mathematics. As a substitute teacher for Montgomery County School system, she was in high demand. She could walk into a math class “cold” and teach the subject. I never heard the words “Math is too hard for girls.” In fact, I suspect that Mom out-calculated Dad when it came to higher mathematics; and Dad worked as an engineer.

Now back to that math exam: when Mrs. Marcou stated that we could use a calculator for the exam on limits, I asked Marion to borrow hers. Little did I know that the TI calculator used XX algebraic and the HP calculator used Reverse Polish. The subtle difference can be illustrated with 2+2:

TI: 2 + 2 =;  4 appears

HP: 2 2 +;  4 appears

Why would calculating require a different order the numbers and operands? My 10th-grade mind had not a clue. And in the midst of exam stress I had no time to decipher the working of the TI machine. So I went back to the techniques that Mrs. Marcou had presented in class. Turns out I really didn’t need that calculator to pass the exam.

Have a productive day,


Anne Meixner


Dear Reader, what memory does this piece spark in you? Do you have an equipment/operator failure story? Please share your comments or stories. You, too, can write for the Engineers’ Daughter–see “Contribute” for more Information.

Additional Reading

There happens to be a Slide Rule Museum.

Watch this video for log and trig calculations on a Slide Rule.

A little bit on Reverse Polish

Article on why HP engineers used Reverse Polish.

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