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The HP-35 Scientific Calculator was the world's first handheld scientific calculator and HP's first product that contained both integrated circuits and LEDs.

HP-35 Handheld Scientific Calculator, 1972

The HP-35 Scientific Calculator, so called because it had 35 keys, was introduced in 1972. It was the world's first handheld scientific calculator. In one of the most amazing displacements in the history of technology, the HP-35 Scientific Calculator electronic calculator, and others like it, quickly replaced the faithful slide rule that had been used by generations of engineers and scientists for rapid calculation and simple computation.

The HP 35 was HP's first product that contained both integrated circuits and LEDs (light-emitting diodes). Both technologies had been developed in HP Labs. Bil Hewlett, who, in 1968, had challenged HP engineers to make a desktop-size computer (the 9100A), challenged them again in 1971 to take that desktop computer and make it small enough to fit into his shirt pocket. When the tiny powerhouse reached the prototype stage, HP asked a local market research firm to do a market study. They did and determined that the HP-35 Scientific Calculator would never sell because it was too expensive. Bill said "We're going to go ahead anyway." The product was so popular that HP couldn't make them fast enough.

Bill remembered, "We figured, in the first year, if we could sell 10,000 calculators, we'd break even. We sold 100,000."

By the time the HP-35 Scientific Calculator was discontinued in 1975, just three and one-half years after its introduction, more than 300,000 had been sold.