Who invented the Slide rule? Willian Oughtred in 1622.

I think, first, I need to look at what a slide rule is because not many people under 60 will have ever used one. These days, if we want to do complex calculations, we can just plug them in to our phone and get the answer pretty much as quickly as we can press the enter key. However, the electronic calculator wasn’t invented until 1961 and the pocket calculator wasn’t around until 1967. It took the invention of the microchip before calculators could be made small enough to go in a pocket. So, before calculators, how did people do complex calculations? They could be done on paper with a pencil, but that would take ages and the chances of making a mistake are high. Or they could be done on a slide rule, which would be much faster and accurate.

There were different kinds of slide rule, but a basic slide rule has three parts. The whole thing looked similar to a modern ruler, and they came in lengths of 12 cm, 25 cm, and 50 cm. There were longer and shorter ones, but that was the standard size. They had one set of measurements written on the frame and another set of measurements written on a bar that could slide through the frame to be adjusted. Then there was a clear piece of plastic with a line on it that could slide up and down the rule to mark places. This was called the cursor and, interestingly, is why the word cursor moved over to computers.

A basic slide rule to do simple multiplication and division works like this. If you want to multiply by 2, you slide the moving bar along until the 1 on the moving bar is level with 2 on the measurements on the top of the frame. Then, when you read up, the numbers that are level with the numbers on the movable bar are multiples of 2. The same works for any number. If you slide the bar along until the 1 is level with 5, all of the numbers will be multiples of 5. However, the scale only goes up to 10, so you will need to multiply by 10, or 100, depending on what number you are multiplying. The opposite also works. If you want to divide by 3, you slide the movable bar along until the 3 is level with the one on the measurements on the frame and then whenever you read down, you get a number that is divided by 3. Again, you need to divide by 10 or 100 depending on the number you are using. Complex slide rules have multiple measurements on the top and the bottom frames and they can do incredibly complex calculations. They can work out up to 10 different types of calculation.

The slide rule was technically invented by William Oughtred in 1622, but he couldn’t have made it if it hadn’t been for the invention of logarithms by John Napier in 1614. At the time, study of space and science was taking off and mathematicians were increasingly having to do more and more complex calculations. The chance of making a mistake increased with the complexity of the calculation. Napier wanted to make things easier, and he worked out that there were patterns when doing certain calculations. If these patterns were known, it would make the relevant calculations a lot faster. Napier spent 20 years doing millions of calculations and writing down all of the results in a book. That meant, when other mathematicians wanted to do complex calculations, they could look in Napier’s book, find the relevant pattern, and it would make their calculation much faster. Napier called these logarithms. They freed up so much of the mathematician’s time, that many scientific breakthroughs that appeared after Napier’s time can be directly attributed to him.

Once Napier had produced his logarithms, it made math a lot faster, but people still needed to look up the various numbers in the book. In 1620, an English mathematician called Edmund Gunter took one of these logarithms and put it on a ruler. Using a pair of dividers, he was able to make calculations very quickly. William Oughtred, another English mathematician and clergyman, took two of Gunter’s rulers and put them together, so that one could slide past the other. This was the invention of the slide rule, although it wouldn’t become common for close to another 200 years. I have covered William Oughtred before because he invented the multiplication (x) symbol. Isaac Newton used a slide rule with three measures and he invented the cursor to keep track of where he was calculating.

As the Industrial Revolution comes and goes and electrical engineering, thermodynamics, chemistry, physics, and astronomy advance, calculations become even more complex. Slide rules with 8 different scales became commonplace. Without them, the calculations were just too difficult for people to do. The slide rules gave shortcuts that made it possible for people to make them. Slide rules became necessary, and they were taught in schools. They were common until the pocket calculator began to take over and it was no longer difficult for people to make complex calculations. And just like that, the slide rule was relegated to a museum. There is a site here where you can try different kinds of slide rules. https://www.sliderules.org/ It is fascinating, but difficult for me to understand. You’re smarter than me so I’m sure you will do better. And this is what I learned today.

Image By ArnoldReinhold – Own work (I took this picture of an artifact in my possession on February 3, 2006. The object itself is functional in nature, was created before 1970 and has no copyright notice, either on front or back.), CC BY 2.5, https://commons.wikimedia.org/w/index.php?curid=561688

Sources

https://www.oughtred.org/history.shtml

https://www.math.utah.edu/~alfeld/sliderules/

https://en.wikipedia.org/wiki/Calculator

https://www.maa.org/press/periodicals/convergence/logarithms-the-early-history-of-a-familiar-function-john-napier-introduces-logarithms

https://americanhistory.si.edu/collections/object-groups/slide-rules

https://www.etymonline.com/word/cursor

https://www.mathsisfun.com/algebra/logarithms.html

https://plus.maths.org/content/dynamic-logarithms

https://www.nms.ac.uk/explore-our-collections/stories/science-and-technology/napiers-bones/

http://scihi.org/john-napier-logarithm/

https://en.wikipedia.org/wiki/William_Oughtred

https://en.wikipedia.org/wiki/John_Napier

https://en.wikipedia.org/wiki/Slide_rule

https://www.nmt.edu/advancement/docs/sliderulehandout.pdf

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