Relic: Slide Rule

Going back to school means new gadgets—whether it be the latest smartphone or laptop or something that does its job. Going back to school also means new things not only in the tech world but also new beginnings for people starting the new year. A long time ago, many people were not accustomed to using calculators. Instead, people used the even more ancient slide rule. Until the pocket calculator came around in the 1970s, this was the tool of choice for anyone taking advanced math classes.

The slide rule is classified as a mechanical analog computer, and was developed by two individuals: the English mathematician and Anglican Church minister Reverend William Oughtred and Welsh clergyman and mathematician Edmund Gunter, who also did work in geometry and astronomy. The two based the slide rule off of John Napier’s work on logarithms in the 17th century. This was a major tool that was used in engineering, and was shaped much like a ruler.

The basic slide rule used two logarithmic scales to allow rapid multiplication and division, but more advanced slide rules used square roots, exponential equations, logarithms, and trigonometric functions, making it a very useful tool for students in advanced math classes. However, it was still capable of doing addition and subtraction (in a very roundabout way). This allowed math enthusiasts to quickly solve problems that may have taken a long time to do on paper.

There were even circular slide rules, which saved space not only physically, but allowed certain equations to be read in a certain way, such as with “off-scale” calculations that were eliminated because of the wrap-around nature of the circular slide rule. Circular slide rules were not very helpful in locating figures around a dish, and put less important scales closer to the center with lower precision. In addition, the linear slide rule was far more in use. However, the circular slide rule was converted into something more useful. In 1952, Swiss watch company Breitling created a circular slide rule that could be used within a wristwatch marketed to pilots called the Breitling Navitimer. It was used to calculate airspeed, rate/time of climb/descent, flight time, distance, and fuel consumption. It could also convert distances from kilometers to nautical miles and amounts of fuel from gallons into liters (and vice versa).

Cylindrical slide rules even existed. There were two types of slide rules: ones on a helical scale and ones on a bar scale. The advantage of the cylindrical slide rule were that much longer scales could fit on it, allowing for greater precision than circular or linear ones.

After Gunter and Oughtred’s work on the slide rule (Oughtred also invented the circular slide rule), many scientists and mathematicians sought to improve on it. Oughtred’s behavior was similar to that of one of his contemporaries, Isaac Newton (most famous for the theory of gravity), which meant that much of his ideas were published after his death by his students. By 1722, two and three-decade scales were invented, and inverted slide rules came across in 1755. The combination of these scales was known as a polyphase rule. Peter Mark Roget invented the log slide rule in 1815, which allowed people to take the logarithm of the logarithm, allowing direct calculations with roots and exponents.

The modern slide rule was created in 1859 by French lieutenant Amedee Mannheim. By the 19th century and the rise of the Industrial Revolution, engineering was seen as a recognized profession, necessitating the need for slide rules in Europe. However, slide rules did not cross the Atlantic Ocean until 1881, where Edwin Thatcher brought the cylindrical slide rule to the United States. Slide rules were used until the second half of the 20th century, when pocket calculators completely eliminated the use of slide rules (which were not very popular with the public, but used in education and engineering). Today, they are just an interesting piece of history and are considered collector’s items if they are in good condition, with Faber-Castell slide rules in particular selling for upwards of USD 200 on eBay.

Slide rules have been a major tool in helping engineers determine what to do before the advent of calculators and computers, and helped advance the field of engineering to complete greater feats than before. While much of this work is now done on a computer, the slide rule is essentially the grandfather of the calculator.

Also published in GADGETS MAGAZINE June 2016 Issue
Words by Jose Alvarez