The Slide Rule: Mathematical Relic?

Supposedly superseded by the electronic pocket calculator back in 1974, are the days of the slide rule as a viable mathematical computational aid finally over?


By: Ringo Bones


While the electronic calculator and every similarly battery-operated model that superseded it since it was introduced back in 1974 had been decried by “old-school” math educators as the primary cause of the current “dumbing-down” of prospective math students, I sometimes wonder if those under-40-old-school-math-educators had ever experienced the joy of handling a slide rule. But is the slide rule still relevant in today’s world? Especially when prospective math students can easily install – if they chose to - scientific calculator apps in their PDAs and other hand-held computers?

A slide rule is a mechanical analog computer, a mechanical device consisting of moveable scales, arranged to slide opposite each other, by means of which certain mathematical operations may be carried out quickly. Also known colloquially as a “slip-stick”, it is used primarily for multiplication and division and also for “scientific” functions such as roots, logarithms and trigonometry. More often than not, slide rules do not generally perform addition or subtraction.

The working principle behind the slide rule was made possible when John Napier invented the concept of logarithm, his treatise of the subject being published in 1614. In 1632, William Oughtred, an English clergyman and teacher, arranged two logarithmic scales – which he invented back in 1622 – to form the first slide rule. The slide rule in its modern form – before being superseded by the modern pocket calculator – was developed by Amédée Mannheim, a French artillery officer, in 1850.

Amédée Mannheim’s version of the slide rule consists of a 10-inch rule, with three parts: the stock, or two fixed parallel rules, each with a scale on its inner edge; the slide, a single rule, moving between them, having a scale on each outer edge corresponding to the fixed scale to which it is adjacent; and the cursor, a square glass with a hair line which may be moved the length of he rule as an aid to reading it.

Before the advent of the pocket calculator, the slide rule was the most commonly used calculation tool in science and engineering. The use of slide rules continued to grow through the 1950s and the 1960s even as digital computing devices were being gradually introduced. Famed aerospace engineer Clarence “Kelly” Johnson was purported to have used the slide rule in designing the U2 and SR-71 Blackbird spy planes. High fidelity audio equipment engineered using slide rules during the Golden Age of Stereo – i.e. 1950s to the 1960s – were famed for their warm and smooth natural sound. By around 1974, the pocket-sized electronic scientific calculator introduced by Texas Instruments and other competing consumer electronics manufacturers, largely made the slide rule obsolete and most suppliers exited the business around 1976.

The only saving grace of the slide rule – if you’re fortunate enough to find one in a garage sale or antiques shop being offered at a keen price - over competing electronic calculating devices is that it doesn’t require batteries. And also, more “mature” old-school math teachers praise the slide rule for keeping math students on their toes in its use thus making their users smarter. To me, the slide rule or slip-stick’s primary advantage over the electronic pocket calculator is that the slide rule is 100% water proof and dust proof – as in desert conditions dust proof.

Given that the “modern” slide rule was developed by a French artillery officer named Amédée Mannheim, nobody has yet pitted the slide rule side-by-side against those dedicated palm-held ballistic computer. Like Knights Armament Company’s KAC Bullet Flight 2.0.0 described as a palm-held ballistic computer designed to provide quick firing solutions in the field. I wonder how this palm-held push-button firing solution fares well against an early 1960s era slide rule when providing firing solutions for a 123-grain Lapua Scenar round with a ballistic coefficient of 0.547 and a muzzle velocity of 2,600 feet-per-second.