Scientific pocket calculator

The HP-35 killed the slide rule. Hewlett-Packard's 1972 scientific pocket calculator compressed logarithmic tables and trigonometric functions into a device that fit in a shirt pocket—making three centuries of slide-rule expertise obsolete within a decade.

The adjacent possible had been opened by HP's own 1968 desktop calculator, the 9100A. Bill Hewlett challenged his engineers in 1970: take that desk-sized machine and shrink it to fit his shirt pocket. Market research was discouraging—the most optimistic forecast suggested 1,000 units per month. Hewlett ignored the studies. If the engineer at the next bench would want it, he reasoned, it was worth building.

France Rodé led a team of twenty engineers who spent two years and approximately one million dollars on development. The result, measuring 5.8 inches long and 3.2 inches wide, was specifically sized to fit Hewlett's pocket. Hewlett suggested naming it the HP-35 simply because it had 35 keys.

The HP-35 was revolutionary precisely because of what it could compute. While pocket calculators of 1971 handled only addition, subtraction, multiplication, and division, the HP-35 performed logarithms, exponentials, trigonometric functions, and their inverses—the exact operations that had kept slide rules in every engineer's pocket for generations. It used Reverse Polish Notation (RPN), eliminating the need for parentheses and equals keys, a design choice that became standard in HP calculators for decades.

Introduced at $395 (equivalent to about $3,000 in 2024 dollars), the HP-35 was expensive but irresistible to engineers. Instead of the projected 1,000 units monthly, sales immediately exceeded 10,000. HP had made its riskiest and bravest business decision—and won.

The cascade was swift and brutal. The slide rule, faithful companion to engineers since the seventeenth century, was rendered obsolete within a decade. Schools stopped teaching slide-rule use. Manufacturers like K&E and Pickett watched their markets collapse. The HP-35 flew in space in 1973, carried aboard Skylab 3 and Skylab 4—the first scientific calculator to orbit Earth.

Path dependence locked in HP's design choices. The RPN notation, efficient but requiring learning, created a devoted user base that refused to switch. Forbes ASAP named the HP-35 one of the twenty products that changed the modern world. HP calculators would sell over 20 million units—the most popular products in the company's history—all tracing lineage to Hewlett's insistence on building what engineers needed rather than what market research predicted they would buy.