The first device which could properly be called a slide rule was invented around 1622 by Anglican minister William Oughtred, based on John Napier's discovery of logarithms as described in his 1614 publication *Mirifici Logarithmorum Canonis Descripto*. Oughtred's slide rule consisted of two C/D-style sliding scales which he used to perform multiplication and division. This basic operation would serve as the backbone of nearly every slide rule manufactured for the next 350 years.
For the next two centuries, the capabilities of the slide rule evolved slowly, gaining square and cube scales in the early 1700s and log log scales in the early 1800s. In 1851, French artillery officer Amedee Mannheim developed the first "modern" slide rule, with multiplication/division scales marked C and D, and square/square root scales marked A and B. Mannheim's design also popularized the inclusion of a cursor, first suggested by Isaac Newton in 1675. The final major design change was the invention of the duplex slide rule by William Cox. The duplex slide rule has scales on both sides and a two-sided cursor used to align scales from opposite sides of the slide rule. The duplex slide rule was first sold by Keuffel & Esser in the late 1800s, but soon became the industry standard for professional slide rules from all manufacturers.
Despite the wide range of calculations which can be performed on a slide rule, inherent limitations typically confine result accuracy to betweeen two and four significant digits. Long calculations can also be tedious, unintuitive and time-consuming. These limitations became an extreme vulnerability in 1972, when Hewlett-Packard introduced the first handheld electronic scientific calculator, the HP-35. Early scientific calculators were much easier to use, and typically produced nearly instantaneous results with at least ten digits of accuracy. Although slide rule adoption had only accelerated up to this point, the electronic calculator would become a totally disruptive technology, wiping out the entire slide rule industry before the end of the 1970s. |