HP-35: The Great Slide Rule Killer
From the : The HP-35 was HP's first pocket calculator. It was introduced at a time when most calculators (including expensive desktop models) had only the four basic functions. The HP-35 was the first pocket calculator with transcendental functions and the first with RPN.
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. As a result, they later had to warn people to expect waiting lists in the Hewlett-Packard Journal. This was an unusual situation for HP. While many companies advertised calculators months or even years before you could buy one, HPs were normally available the day the first advertisements appeared.
