Dr. Dennis R. Clark of the Stanford University Medical Center (Northern California), Drug Assay Lab, at work with his HP-35 Calculator. The HP 35, introduced in 1972, was the world’s first handheld scientific calculator. In one of the most amazing displacements in the history of technology, the HP 35 electronic calculator quickly replaced the faithful ‘slip-stick’ - the slide rule - used by generations of engineers and scientists for rapid calculation and simple computation. Until the 1970s, every high school physics student was required to have his/her own 25cm slide rule. Every high school physics classroom had a large demonstration slide rule, which was used to teach students how to use their slide rules. By the 1990s, one generation after the invention of the HP 35, few high school physics students even knew what a slide rule is.

When Hewlett-Packard introduced the HP 35, a small revolution took place in shirt pockets around the world. It began in the early 1970s, when HP co-founder Bill Hewlett, impressed by the small size of an arithmetic calculator he’d seen, became convinced that HP could expand the technology into a pocket-size calculator capable of performing trigonometric, logarithmic and exponential functions. The result was the HP 35 - named for its 35 keys - a product that fundamentally changed the way engineers, scientists, mathematicians and students worked and banished the slide rule to the history books. A marketing study done in early 1971 alerted HP that there was a small market for a pocket-size calculator with scientific and mathematical functionality - it also recommended that the new calculator be the size of a typewriter or adding machine. But Bill Hewlett was convinced that engineers would prefer a calculator that would fit into a shirt pocket. Bill was right.

In the first three years after its introduction in 1972, sales of the HP 35 exceeded 300,000 units. The little ’electronic slide rule’ weighed only 250 grams. The device made it possible to make complicated calculations in the field and on the road with the speed and accuracy that far surpassed that of a slide rule and sold for $395 U.S.

Before the introduction of the HP 35, most people calculated more complex mathematical operations, such as reciprocals, trigonometric functions (sines, cosines, tangents, arcsines…) exponents (squares, cubes, square roots…) etc. with a slide rule. The placement of the decimal point was one of the tougher skills for novice slide rule users to learn, but the need for this skill simply disappeared when the calculator arrived.

Earlier calculators handled only four functions: addition, subtraction, multiplication and division. The HP 35 not only performed complex scientific and mathematical functions, but it was also the first calculator to use Reverse Polish (Lukasiewicz) Notation for programming efficiency.

Manufacture of the HP-35 ceased in 1975.