Today in 1972, the HP-35 was introduced. The world’s first handheld-sized scientific calculator, ultimately made the slide rule, which had previously been used by generations of engineers and scientists, obsolete. Named for its 35 keys, it performed all the functions of the slide rule to 10-digit precision and could determine the decimal point or power-of-10 exponent through a full 200-decade range. The HP-35 was 5.8 inches (150 mm) long and 3.2 inches (81 mm) wide, and said to have been designed to fit into one of William Hewlett’s shirt pockets. The Museum of HP Calculators:

Based on marketing studies done at the time, the HP-9100 was the “right” size and price for a scientific calculator.  The studies showed little or no interest in a pocket device. However Bill Hewlett thought differently.  He began the development of a “shirt pocket-sized HP-9100” on an accelerated schedule. It was a risky project involving several immature technologies. HP originally developed the HP-35 for internal use and then decided to try selling it. Based on a marketing study, it was believed that they might sell 50,000 units. It turned out that the marketing study was wrong by an order of magnitude. Within the first few months they received orders exceeding their guess as to the total market size. General Electric alone placed an order for 20,000 units.

See also “The ‘Powerful Pocketful’: an Electronic Calculator Challenges the Slide Rule,” HP Journal, June 1972

Today, an emulation of the HP-35 is available for the Apple iPhone and iPad.