HP-35

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.

In about 1970 HP co-founder Bill Hewlett challenged his co-workers to create a "shirt-pocket sized HP-9100". At the time, slide rules were the only practical portable devices for performing trigonometric and exponential functions, as existing pocket calculators could only perform addition, subtraction, multiplication, and division. Introduced at US$395, like the HP-9100 it used RPN. The "35" in the calculator's name came from the number of keys.

The original HP-35 was available from 1972 to 1975 but in 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.

The calculator used a traditional floating decimal display for numbers that could be displayed in that format, but automatically switched to scientific notation for other numbers. The fifteen-digit LED display was capable of displaying a ten-digit mantissa plus its sign and a decimal point and a two-digit exponent plus its sign. The display used a unique form of multiplexing, illuminating a single LED segment at a time rather than a single LED digit, because HP research had shown that this method was perceived by the human eye as brighter for equivalent power. Light emitting diodes were relatively new at the time and were much dimmer than high efficiency diodes developed in subsequent decades.

The calculator used three "AA"-sized NiCd batteries assembled into a removable proprietary battery pack. Replacement battery packs are no longer available, leaving existing HP-35 calculators to rely on AC power, or their users to rebuild the battery packs themselves using available cells. An external battery charger was available and the calculator could also run from the charger, with or without batteries installed.

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