The HP-35 was Hewlett-Packard's first pocket calculator and the world's first scientific pocket calculator (a calculator with trigonometric and exponential functions). Like some of HP's desktop calculators, it used reverse Polish notation. Introduced at US$395, the HP-35 was available from 1972 to 1975.
Market studies had shown no market for pocket sized calculators at the anticipated price. In about 1970 HP co-founder Bill Hewlett challenged his co-workers to create a "shirt-pocket sized HP-9100". About twelve HP-35 portable calculators were made as a "hack" by and for other engineers at HP. It is rumored that the development engineer got carried away and included a full suite of scientific functions to satisfy requests from his co-workers. When these prototypes proved popular, HP decided to turn the HP-35 into a commercial product. The HP-35 was the first calculator with a full suite of trigonometric and transcendental functions.
In the first months orders were exceeding HP's expectations as to the entire market size, which was 10,000 units per year. Before the HP-35, the only practical portable devices for performing trigonometric and exponential functions were slide rules. Existing pocket calculators at the time were only four-function, i.e., they could only perform addition, subtraction, multiplication, and division. It had been originally known simply as "The Calculator", but Hewlett suggested that it be called the HP-35, because it had 35 keys.
On July 12, 2007 HP announced the release of the "retro"-look HP 35s in commemoration of the original HP-35.
The HP-35 was named an IEEE Milestone in 2009.
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... The HP-35 was 5.8 inches (150 mm) long and 3.2 inches (81 mm) wide, said to have been designed to fit into one of William Hewlett's shirt pockets ... The LED display power requirement was responsible for the HP-35's short battery life between charges — about three hours ... The HP-35 calculated arithmetic, logarithmic, and trigonomic functions but the complete implementation used only 767 carefully chosen instructions (7670 bits) ...