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In the 1960s, Hewlett-Packard was becoming a diversified electronics company with product lines in electronic test equipment, scientific instrumentation, and medical electronics, and was just beginning its entry into computers. The corporation recognized two opportunities: it might be possible to automate the instrumentation that HP was producing, and HP's customer base were likely to buy a product that could replace the slide rules and adding machines that they were now using for computation.
With this in mind, HP built the HP 9100 desktop scientific calculator. This was a full-featured calculator that included not only standard "adding machine" functions but also powerful capabilities to handle floating-point numbers, trigonometric functions, logarithms, exponentiation, and square roots.
This new calculator was well received by the customer base, but William Hewlett saw additional opportunities if the desktop calculator could be made small enough to fit into his shirt pocket. He charged his engineers with this exact goal using the size of his shirt pocket as a guide. The result was the HP-35 calculator. This calculator provided functionality that was revolutionary for a pocket calculator at that time.
Through the years, HP released several calculators that varied in their mathematical capabilities, programmability, and I/O capabilities. Some of them could be used (via HP-IL) to control the instruments other Hewlett Packard divisions produced.
HP calculators are well known for their use of Reverse Polish Notation (RPN).
Programmable HP calculators allow users to create their own programs.
Below are some of HP’s handheld calculator models produced over the years, in numeric rather than chronological order: