Once a symbol of technology, slide rules are a fading memory among CSE alumni

Before smartphones and the laptop, no serious science or engineering student would be caught on the University of Minnesota campus without a slide rule. During the 1950s and 1960s, the remarkable instrument—small, light, efficient, and not requiring batteries—would most often be carried in a belt holster ready for calculating action.

Also known as a slipstick, the slide rule itself is a kind of computer—the very basic, very analog kind. Often no larger than a 12-inch ruler and marked with numbers, the powerful mechanical computing device operates by sliding in and out to show relationships between different sets of numbers.

Developed in the 17th century by Reverend William Oughtred, an English mathematician and Anglican minister, and others, the slide rule is based on the work of John Napier, a Scottish mathematician, physicist, and astronomer, who discovered the concept of logarithms.

The slide rule was universally used for nearly 400 years and was the most commonly used calculation tool in science and engineering. Perhaps its most impressive use was during the Apollo 13 crisis when engineers had to recalculate data to guide the crew safely back to Earth—and they had to do it quickly all with the aid of a slide rule.

Yes, the slide rule was a powerful tool until the early l970s, when things began to change.

On February 1, 1972, Hewlett-Packard introduced the HP-35, regarded as the first successful scientific pocket calculator. It sold for about $395. It was the first handheld calculator ever to perform logarithmic and trigonometric functions with one keystroke. In effect it was the world’s first electronic slide rule.

Seemingly overnight, the trusted, reliable, and handsomely crafted slide rules in the pockets of their owners were rendered obsolete.

Many slide rules went into the trash and were replaced by the new technology. Some were packed away in storage boxes or drawers to be revisited when their owners felt nostalgic. Still others may have ended up on display shelves as a reminder of a bygone era.

Yet, despite the calculating power now held in today’s smartphone or scientific calculator, the slide rule isn’t quite dead. It’s a remarkable precision instrument that holds fond memories and a special place in the hearts of many College of Science and Engineering alumni.

Read recollections from CSE alumni.

It was always a rule to never fasten your slide rule to your belt unless you wanted to be considered what is today called a “nerd.” No backpacks—just books, notebooks, etc.—with the slide rule on top. If you dropped your slide rule on the end, it had to be whacked a little to get it back in adjustment. Since a slide rule knows no decimal places, the approximation always had to be carried in the head. Some may not agree, but for me, it gave me a wonderful ability to approximate numbers for years to come. With a little practice it was always accurate enough to accomplish the results. Pleading a slide rule error usually did not work. 

—Gordon Lewis (ME ’51)

I still have my K&E Deci-Lon slide rule. It cost me $25 in 1962 when tuition was only $91 per quarter. My recollection of one of the problems with slide rules was that you had to keep track of the decimal place in your head. The small HP 35, which arrived in the early 1970s, was the best thing since sliced bread. It had reverse Polish notation, which the Texas Instrument calculators did not have. The big problem was the first HP unit cost about $400—a fortune in those days. It was like moving from the horse to the car when we moved from slide rulers to calculators. 

—Henry Hanson (ME ’66)

We were still using the slide rule when I was in college. In about 1972, when HP came out with their calculator, several students switched from the slide rule to the calculator. I thought the slide rule was the best thing since sliced bread. I purchased my K&E slide rule from a friend who had graduated in mechanical engineering and no longer had a need for it. It was one of the more expensive models made of wood. Every once in a while, it would become hard to slide. One of my fellow students said that the best lubricant was the oil from the pores on his face. Guess what? It worked! Of course, you had to clean the surface of your slide rule and your hands after this. As students, we could be creative, efficient, and save money when necessary. Also, the engineering bookstore had a policy that if we brought in our slide rule and it needed replacement screws, they would provide and replace them. Those were the good old days!

—H. Richard (Dick) Coleman (CivE ’73)

I still have my slide rule, which I can quickly pluck from behind the books in my office. It is an “upscale” metal one. Throughout most of college, I used one made out of bamboo and used the metal one my last year or two (of five). I actually used it at Honeywell Aero Division for at least five years. 

It was easy to learn how to use a slide rule. One never forgot to bring a slide rule to class, not to mention an important test. It was like forgetting to wear your shirt and pants to class, and stories about losing your slide rule was as rare as losing your billfold. I do remember having friendly problem-solving competitions in Old Main with my study group. These days, I rarely use my slide rule, however, I do multiply or divide with it occasionally for fun but never use log functions, for example.

I wonder if my classic slide rule is worth $20,000 yet? You know, like a ‘56 Chevy?

 —Ken Floren (ME ’60)

I thought it was difficult learning how to use the slide rule. I could use it for simple problems but I never was able to learn how to use all the trig functions.

I do remember leaving my slide rule in the library and having to return later—in a state of panic—to find that someone had turned it in to the person at the desk, so I got it back safely.

I’m proud to say in later years when everyone at work was using the company computer for estimating costs, I would sometimes use my slide rule to make a quick check of costs where a simple adjustment of one variable was made rather than wait for someone to make the change on the computer. I would then have an “estimate” to the 5th decimal point.

Around campus, I would carry my slide rule in my hand with my books. I did not want to be seen carrying it on my belt that would then hang down around the knees. I thought that looked kind of odd.

When calculators that could solve trig problems first appeared, they were really expensive and I did not buy one for a few years. It just seemed to me that they were unnecessary for most straightforward problem solving. However, I did use a cheap calculator for basic math problems.
Today my slide rule sits on a shelf in my office along with the How to use Pickett Trig Slide Rules manual. Both are right next to my Marks’ Mechanical Engineers’ Handbook—6th Edition. I have it mainly to show my “smart” grandchildren how we solved math problems when I was in college.

I can say that the slide rule was a very important tool when I was in college and I always had it with me.

—Orv Johnson (ME ’60)

I was studying first-year physics at the University’s Institute of Technology in about 1975. The new-fangled calculators were forbidden during physics tests because the professors believed the calculators (then very expensive) gave well-to-do students an unfair advantage over students who would not afford calculators, so slide rules were mandated for tests, but not for homework. The next year that prohibition was relaxed and everyone brought calculators to tests.

My first calculator was a Texas Instruments SR-50. I think I still have it. Later in graduate school, I replaced it with an SR-52, which was programmable and which I used to run calculations for semi-conductor physics class.
My first slide rule, which I purchased in high school, was made from plastic. It had only one slide. Later on, I acquired both our father’s professional grade slide rule and my uncle’s professional grade slide rule. Both were made from bamboo and both had multiple interchangeable slides. I still have all three.

I was one of those people who found a slide rule far faster to operate than an early HP calculator with reverse Polish notation. Reverse Polish notation and I never did make friends, even though my first job out of grad school was working for Hewlett-Packard. Once I became a full-time professional engineer I switched over to an HP 41C calculator, which I also still have.

—Marc Clarke (EE ’78 MS ’80)

I still have my K&E Deci-Lon slide rule. It cost me $25 in 1962 when tuition was only $91 per quarter. My recollection of one of the problems with slide rules was that you had to keep track of the decimal place in your head. The small HP 35, which arrived in the early 1970s, was the best thing since sliced bread. It had reverse Polish notation, which the Texas Instrument calculators did not have. The big problem was the first HP unit cost about $400—a fortune in those days. It was like moving from the horse to the car when we moved from slide rulers to calculators.

—Henry Hanson (ME ’66)

When I was a freshman, I bought a K&E aluminum slide rule at the engineering bookstore. I used it for about a year and then I ran into a guy who needed a couple of bucks. He sold me his—or somebody’s— beautiful Sun Hemmi bamboo slide rule. It was a No. 153 made in occupied Japan and I have on my desk to this day. I used it through undergraduate and graduate school. In the 1970s, scientific calculators started to become available. They cost more than $400. I stuck to my bamboo slide rule and still do a few calculations on it to make sure my batteries (brains) haven’t run out. It’s an elegant device and I love to hold it.

—Jim Rutzick (ME ’66)

My experience with the slide rule involved my career as a sales engineer. I carried a six-inch Post in my shirt pocket along with my pocket protector. Often my customers would joke with me about the fact I walked around with a slide rule and pocket protector. It didn’t bother me. It was a useful tool to calculate ratios in drive trains and in providing rough cost estimates for products. I also think it gave me the unique identity of being an engineer as well as a salesman. That was a long time ago and as my grandson suggests, now even the Bowmar Brain—one of the first early pocket electronic calculators—has disappeared in favor of the Smartphone. Life marches on. Thanks for the trip down memory lane.

—Don Craighead (ME ’57)

A few things I remember about the slide rule:

On tests and homework requiring numerical answers, you always had to know how many significant digits were expected. The slide rule is analogue, so you are estimating when you give a digital answer. For example, if the exact answer is 3.7539, what would be an acceptable answer? 4, 3.8, 3.75 or 3.76? Never beyond three significant numbers.

I had two slide rules, one that was 18 inches long when three significant digits were required, and one four inches long for ball park answers. My 18-inch slide rule had a case with a belt loop so you could wear your slide rule and be cool. I graduated from the Institute of Technology in 1968 before powerful calculators.

—Rick Ballintine Physics (’68 MBA’70)

I did use the slide rule during my undergraduate civil engineering studies in India. In fact, that was the only allowable calculating device we could use in exams, which were always closed book! I did bring a pocket slide rule with me when I came to Minnesota for my graduate studies.

My German made Aristo Studio slide rule was the gold standard and cost a fortune! The one shown is still like new after my use for four years followed by my cousin’s use for another five years in India. Considering the heat, humidity and dust in India, the shape of it today is amazing.

I didn’t have any qualms in switching to calculators when they first appeared. The first one, an HP 40, cost me $700 while my monthly salary was only $833! I moved on quickly to use digital computers like the CDC 6400 (developed at the University of Minnesota campus in the 1960s!) for my design/analysis simulations using the finite element method, both in my academic career and industrial work on nuclear power plants and Ford cars and trucks (for almost 30 years!)

—Swami Perumalswami (Civil E M.S.’66, Ph.D.’68)

I had a full-size Post slide rule when I was at the University. Most slide rules were made of bamboo and were light. The problem for students was that test anxiety produced prodigious sweating of the hands, and the bamboo would swell as a result. So unless the slide rule was adjusted to be “loose,” it would seize up, especially in a two- to three-hour final exam. I learned to adjust it before exams or take a small screwdriver with me for the inevitable emergency. I remember quite often that students would complain that their slide rule had seized up, but the professors showed no mercy for that affliction. As with the Boy Scouts, the engineering mantra was “Be Prepared.”

My first Post was stolen out of my dorm room, and as I recall, they cost about $35, which was a lot of loot back then. My folks had no sympathy, either. Being too into slide rules back then was the mark of a geek, so I don’t remember any engineers competing to see who could manipulate one the fastest. Similarly, carrying one on your belt was super-geeky, just like having a pocket protector for your pens and pencils. The HP-99 calculator came out about the time I started my first job, and I never used one because we had mechanical Monroe calculators in the mine office. I did, however, carry a six-inch Post slide rule in my shirt pocket for calculations out in the plant. Since I switched into financial work, my constant companion has been the HP-12C calculator. Incidentally, I still have the two slide rules.

—Jared Scofield (ME ’64, M.S. Mineral Eng ’66, MBA ’86)