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 entré into computers. The corporation recognized two opportunities:

• It might be possible to automate the instrumentation that HP was producing, and
• HP's customer base would likely 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 fully-featured calculator that not only included standard "adding machine" functions but also included powerful capabilities to handle:

• Floating point numbers
• Trigonometric functions
• Logarithms, exponentiation, and square roots

This was an excellent machine and 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 (to the point that they measured his shirt pocket!).

The result was the HP-35 calculator. This calculator provided functionality that was revolutionary for a pocket calculator at that time, and it did fit into Bill Hewlett’s shirt pocket. 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. Below are some of HP’s calculator models produced over the years, in numeric rather than chronological order:

• HP-12c – The financially centric calculator from the HP-10 series introduced in the 1980s. The longest running product in the HP calculator line, it remains in production.
• HP-35 -- The original
• HP-41 series – Three models in this series were released over its lifetime, the 41C, 41CV, and 41CX. The 41C had user configurable program steps and memory registers, alpha-numeric display, user programmable key mappings, and 4 expansion ports that could hold additional memory, an interface to HP-IL peripherals, a magnetic card reader/writer, or commercial application programs. The 41CV quadrupled the amount of base memory, and the 41CX added a clock and some additional functions and memory.
• HP-42S – a non-expandable follow-up to the HP-41 series. It included a two line display (dot addressable) and featured built-in matrix and complex number math.
• HP-48 series
• HP-49 series
• HP-67 and HP-97

External links

• HPMuseum.org Museum of slide rules and significant HP calculators
• HPCalc.org Information about and software for HP programmable calculators

The HP museum has a wealth of historical information about the different models, and may be a good starting point for anyone considering buying a second hand calculator. There is a lively market in old HP calculators on Internet auction sites. The HPCalc.org site provides a place for calculator programmers to share programs, and is a good source of calculator emulation programs for the PC.