[Omaroo (Chris Malikoff)]

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From the Hewlett Packard Calculator Museum

HP-35

The HP-35 was HP's first pocket calculator. It was introduced at a time when most calculators (including expensive desktop models) had only the four basic functions. The HP-35 was the first pocket calculator with transcendental functions and the first with RPN.

Based on marketing studies done at the time, the HP-9100 was the "right" size and price for a scientific calculator. The studies showed little or no interest in a pocket device. However Bill Hewlett thought differently. He began the development of a "shirt pocket-sized HP-9100" on an accelerated schedule. It was a risky project involving several immature technologies. HP originally developed the HP-35 for internal use and then decided to try selling it. Based on a marketing study, it was believed that they might sell 50,000 units. It turned out that the marketing study was wrong by an order of magnitude. Within the first few months they received orders exceeding their guess as to the total market size. General Electric alone placed an order for 20,000 units. As a result, they later had to warn people to expect waiting lists in the Hewlett-Packard Journal. This was an unusual situation for HP. While many companies advertised calculators months or even years before you could buy one, HPs were normally available the day the first advertisements appeared.

The early versions said only "Hewlett Packard" on the faceplate and inside HP the product was simply called "The Calculator". Bill Hewlett suggested naming it the HP-35 because it had 35 keys. Later as HP developed more models, "35" was added to the faceplate. The HP-35 and probably the entire HP pocket calculator product line was the sole result of a visionary CEO who chose to ignore the market studies and produce what he felt that engineers would want. His view of the market was that if the engineer at the next bench liked it and thought it would make his job easier, then it was worth doing. The HP-35 was developed in two years, at a cost of approximately one million dollars with twenty engineers. At the time, three to five years was a typical HP development cycle.

The early HP's were built tough and built to be repaired. After removing about a dozen screws you can get to anything in the HP-35.

At the time, many of the companies producing calculators could build adequate circuits and firmware, but didn't have the experience or facilities to make a well designed housing, keyboard, and display. Many of these brand-X machines were very failure-prone and quite crude in design.

By contrast, the packaging of the HP-35 was of major importance. Its size, looks, keyboard, and display were all carefully thought out. The keyboard is divided into groups with different sizes, color and placement of nomenclature. Even differing amounts of contrast were used to separate groups. (The most used groups had the greatest contrast level.) The keys were made in a double mold process with the legends going all the way through the keys so they could never wear off. The keyboard panel used an HP-developed spring contact which is essentially unchanged in current calculators.

The HP-35, like all the hand-held HPs that followed, was required to remain undamaged after falling three feet onto concrete on each of its corners. The case had sculpted sides, such that the top caught the light and the bottom was in shadow making the calculator look thinner than it really was. All screws were hidden.

The HP-35 had numerical algorithms that exceeded the precision of most mainframe computers at the time. During development, Dave Cochran, who was in charge of the algorithms, tried to use a Burroughs B5500 to validate the results of the HP-35 but instead found too little precision in the former to continue. IBM mainframes also didn't measure up. This forced time-consuming manual comparisons of results to mathematical tables. A few bugs got through this process. For example: 2.02 ln ex resulted in 2 rather than 2.02. When the bug was discovered, HP had already sold 25,000 units which was a huge volume for the company. In a meeting, Dave Packard asked what they were going to do about the units already in the field and someone in the crowd said "Don't tell?" At this Packard's pencil snapped and he said: "Who said that? We're going to tell everyone and offer them, a replacement. It would be better to never make a dime of profit than to have a product out there with a problem". It turns out that less than a quarter of the units were returned. Most people preferred to keep their buggy calculator and the notice from HP offering the replacement.