Up until the invention of electronic valve computers after WW2, the world was largely run by analogue computers. These machines worked off the time-honoured tradition of carefully engineered interlocking moving parts that were manually triggered to give a certain intended result. Some, like the aircraft and missile trajectory machines used by ground and air defence during WW2 were a giant morass of wheels, cams and sensors that looked straight out of a Heath Robinson cartoon. But despite this, the most influential machines are often also the simplest. The humble slide-rule is the simplest of all mechanical computers ever invented yet remained the de facto tool for most businesses across many hundreds of years. It was only with the sudden emergence of cheap pocket electronic calculators in the 1970s that slide rules became largely obsolete and today most people (admittedly including myself) would have no clue about how to properly use one if it were thrust into their hands. As we all know slide rules are the kind of device that are of immense use to those specialists who need them, and yet completely useless to the rest of us.
It was the Scottish mathematician John Napier who is credited with opening up the whole genre through his promotion of the new field of Logarithms in 1614 via his book aptly titled ‘Description of the Wonderful Rule of Logarithms’. For those who have long forgotten their school-day maths, Logarithms are the scaled relationship between two numbers and were a way to impart a far more worldly approach to number crunching compared to Algebra.
But while Napier may have opened the mathematical door, he did not directly invent slide rules himself. It also required the efforts of the English Clergyman Edmund Gunter who immediately started creating useful logarithm tables and even invented a proto slide rule himself called the ‘Gunter Scale’. From Gunter it was only a short hop skip and jump before others realised they could transfer his useful paper-based logarithm tables onto a simple mechanical device for ease of use. Although often confused today, a slide rule is not just a simple ruler for drawing lines and nor is it intended for addition and subtraction (an ‘addiator’ does that) but is predominantly used for multiplication and division of variables only.
(portrait of William Oughtred)
The very first rectangular slide rule appeared in 1620 followed by the first circular slide rule in 1632. Both look to have been invented by Episcopalian minister and mathematician William Oughtred.
Industrialisation and scientific progress during the 18th and 19th Centuries led to the slide-rule becoming an even more vital piece of kit for every trade. Both Matthew Boulton and James Watt helped refine the accuracy of slide rules during the late 18th Century to assist them with the building of their upcoming Steam Engine plant in Birmingham. There were multiple slide rules created for engineers which measured the amount of material needed for building roads and buildings or for checking the forces acting upon a structure or water pipe; rules for farmers such as the ‘Measuring Tape & Cattle Gauge’ of 1892 to estimate the weight of a cow from its measurements; rules like the actinograph by Hurter & Driffield to help photographers measure exposure levels; rules for meteorologists for use alongside weather balloons to help them with their forecasting, loan calculators for lenders such as banks and of course many more beside.
By the first half of the 20th Century the dominance of the slide rule remained intact and most major inventions of the era had a slide rule somewhere behind it. Among other notable ones are the E-6B circular slide used by pilots and navigators, as well as those used for cryptography during WW2. Of course when the space program took off after the war it was good old slide rules such as the Nestler version used by NASA pioneer Wernher Von Braun which then helped us calculate our way to the moon.
Although I mentioned that slide rules are a simple form of analogue computer, it is interesting to note that the term ‘computer’ was not attributed to them until the mid 19th Century. In 1841 two Americans named Aaron Palmer and George Smith patented a circular slide rule which compared measurements such as feet, acres, chains and furlongs. It also gave the ability to work out monthly interest rates on a loan so must have been intended as the all-in-one device for those regularly measuring or working the land such as farmers. As a useful slide rule the patent was bought up by a gentleman named Fuller who added his own Telegraph scale to the back. This he referred to in its accompanying book as a ‘Telegraph Computer’. Although the title of ‘Computer’ was still being attributed to people up until WW2, this double-sided scale appears to have been the first time an inanimate tool was given the term as well.
The birth of digital computers just after the war did not in itself herald the end for traditional mechanical slide rules. Computers were still both big and expensive so the slide rule market remained basically strong during the 1950s and 60s. Instead it was the mixture of the transistor and the integrated chip in the late 1960s which suddenly allowed multi-functional calculators to become pocket sized, powerful and priced for the masses. Compared to a slide rule which had a very limited function, the versatility of calculators made them instantly appealing.
Today when you come across them, most slide rules look quaintly scientific and from a bygone era. Even so they are keenly collected by those who appreciate the precise engineering that went into these machines as well as the now forgotten expertise needed by those who originally used them.
Photo attributions: white slide rule by Dominic Alves; John Napier portrait via Wikipedia; William Oughtred Portrait from Lisby ; Fuller Calculator No.3 by Andrew Magill ; Pocket Calculator by Marcin Wichary