Electronic Pocket Calculator HP-35

The HP-35 was the world's first scientific pocket calculator, and the first to use Reverse Polish Notation, which obviates the need for parentheses by placing the operators after the operands. RPN is very efficient for evaluating mathematical expressions. It has a 15-digit LED display and offers 10-digit accuracy (depending on the function).

Powered by 3 AA-size NiCd rechargeable batteries, or directly by its adapter from AC mains.

Early production versions of the calculator were labelled only "Hewlett Packard", whereas later models carried the label "Hewlett Packard 35" on the faceplate. The number refers to the fact that the calculator has 35 keys. Two bugs were discovered after 25,000 calculators had been sold, but fewer than 25% of the faulty units were returned for the free replacement that was offered by HP. These early calculators are now worth more to collectors.

The HP-35 was so rugged that it is said that sales people dropped it intentionally for demonstration purposes. The calculator revolutionised the engineering profession by obsoleting the slide rule, and HP sold over 300,000 before it was discontinued in 1975. In 2009, HP received an IEEE Milestone award for the HP-35.