An old-school tool revisited

Walk into Deanna Matthews’ office, and you can’t miss the seven-foot-long, wooden slide rule hanging above her sofa. With far more panache than a framed print, the artifact harkens back to a time when engineers used these mechanical analog computers, which look like tricked-out rulers, to tease out calculations that sent humans to the moon.

Matthews proudly explains that she got the coveted object from Emeritus Professor Paul Fischbeck when he was cleaning out his office. “He is a collector of CMU things and dumpster-diver extraordinaire in a previous life. He is also the person who would see the value in holding on to something like this,” says Matthews, teaching professor and associate department head for undergraduate affairs in the engineering and public policy.

While the sentimental value exceeds its value on eBay, the giant object served an important purpose in its day. Matthews believes that it was used in lectures as a visual aid to teach mathematical concepts, and engineering students could follow along, using their own slide rules. Students typically carried 10-inch-scale slide rules in belt holsters while a 5-inch-scale ruler was pocket perfect.  

Before the advent of the handheld calculator, engineering students used slide rules throughout their education and careers. “My dad was an engineer, and I have his slide rule,” says Matthews.

As innovation marched on, the calculator took hold, and the slide rule grew largely obsolete. Not everyone was cheering. Matthews says that some professors were concerned that students would lose their intuition about math concepts and the ability to do quick, in-their-head estimations in powers of 10.

Time proved that the calculator did not addle students’ brains, which is good because engineers are still dependent to some extent on their ability to estimate.

“One of the things that we do in my Intro to Engineering and Public Policy course is talk about estimation and making the math easy,” said Matthews. “We do things that make students think in powers of 10. Sometimes we don’t need a precise number, an estimate will work. But then there are other times when we need precision, and calculators can do that.”

Today, except for niche uses, slide rules are antiquated. However, there is something inherently ingenious about the construction of these quaint devices that operate on the principles of algorithms, which are the backbone of AI. The slide rule indirectly supported the rise of AI by broadening the use of early computing (albeit analog) and spurring the creation of digital calculators. The progression of tools, among other factors, injected confidence into an emerging culture that passionately pursued advanced computational thinking that ultimately led to the modern computer. Matthews’ slide rule highlights a particular point in the evolution of engineering education, and this inspired her and Conrad Zapanta, associate dean for undergraduate studies in the College of Engineering and teaching professor in biomedical engineering, to show students how slide rules work.  

During Carnegie Mellon’s Engineering Week 2025, the bigger-than-life slide rule was hauled to Tech Spark in ANSYS Hall where it was carefully laid flat over two tables so that Matthews and Zapanta could give students a hands-on demonstration. They also brought a few typical-sized slide rules that students could tinker with.

“The students were intrigued. They were coming over and asking, ‘What is this?’ Some of them had never heard of a slide rule,” says Zapanta, who happens to keep a slide rule in his desk. To give students context, he asked them to, “think about the logarithmic tables they had to use in high school. We explained that the slide rule is a physical version of how those work.”

After students performed a few basic calculations on the devices, the instructors overheard comments like, “I am happy I have my calculator.”

Ironically, the handheld calculator is heading in the same direction as the slide rule. “A lot of students don’t use stand-alone calculators. They tend to use their computers, tablets, or phones. I don’t see that many people walking around with a Casio or with a TI,” says Zapanta.

As our technology advances, so does the engineer’s toolbox. Along with the abacus, the sun has set on printed function tables and slide rules, and eventually the handheld calculator will join this list. Yet these tools and their impact is worthy of appreciation. They tell a long and celebratory story about how engineers apply mathematical constructs to improve our world.