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Development of the HP35, the First Handheld Scientific Calculator, 1972
The HP35 was the first handheld calculator to perform transcendental functions (such as trigonometric, logarithmic and exponential functions). Most contemporary calculators could only perform the four basic operations – addition, subtraction, multiplication, and division. The HP35 and subsequent models have replaced the slide rule, used by generations of engineers and scientists. The HP35 performed all the functions of the slide rule to tendigit precision over a full twohundreddecade range.
Developed by HewlettPackard Company in Palo Alto, California at 1501 Pagemill Road and introduced in 1972, the HP35 was the first fullfunction, shirtpocketsized, scientific calculator. This invention revolutionized the profession by allowing the engineer to make almost instantaneous, extremely accurate scientific calculations, at his home, office or in the field. Three to five hours of continuous use could be expected from a fully charged battery pack.
The HP35 was the innovative culmination of mechanical design, stateoftheart technology, algorithm development and application; all unique at the time
After the development of the HP9100 desktop scientific calculator in the mid 1960s, Bill Hewlett, president of HewlettPackard envisioned the idea that HP could develop the same capability that would fit in his shirt pocket. Every few months he would walk into the corporate labs and ask how the team was doing? He stressed how important it was to get the calculating power of the desktop in his fingers.
Although semiconductor density was increasing yearly, bipolar technology was never going to be suitable, too power hungry and not small enough. Metal Oxide Semiconductor (MOS) promised high density and low power but was still in its infancy. However this didn’t stop Bill Hewlett from getting the Industrial Design group of HP Labs to mock up some ideas of shape, key layout, etc. The solid state laboratory was also working on LED displays with molded encapsulated lenses for magnification driven by low power bipolar driver circuits.
By 1970 a PMOS architecture looked promising as a candidate for scientific algorithms; a binary coded decimal (BCD) adder and 13 digit plus sign (56 bit) long multiple words in a serial circulating shift register (race track) arrangement that was very efficient of both chip size and power. The microcode word length was 11 bits; during final development shortened to 10 by having only an inferred conditional branch, a ten percent reduction in circuitry was significant at the time. The fourteen digits would just be sufficient for ten digit accuracy with an overflow or carry digit and two guard digits while still retaining sign information throughout the algorithmic iterations. The result could be displayed as either a signed mantissa and two signed exponent digits or variable length fixed point. The product had an arithmetic and register chip, control and timing circuit and several ROMs. A clock rate of 200 KHz was sufficiently high to calculate a transcendental function within a second.
The HP35 was truly a product which you knew would be successful because the engineer at the next bench wanted it.
The ubiquitous sliderule was obsolete within a year; the HP35 could do all the functions of the sliderule to ten digit precision and determine the decimal point or power of 10 exponent through a full two hundred decade range.
HewlettPackard Co. (Palo Alto, Calif.) received the IEEE Corporate Innovation Recognition honor in June 1989 for "the creation, development and introduction of the first fullfunction, shirtpocketsized, scientific calculator" the HP35.
Forbes ASAP magazine called the HP35 one of the twenty products that changed the modern world. The HP35 and its descendants would sell more than 20 million units for HewlettPackard, making them the most popular products in the company’s history.
From the computer history museum: in 1972 HewlettPackard announced the HP35 as "a fast, extremely accurate electronic slide rule" with a solidstate memory similar to that of a computer. The HP35 distinguished itself from its competitors by its ability to perform a broad variety of logarithmic and trigonometric functions, to store more intermediate solutions for later use, and to accept and display entries in a form similar to standard scientific notation.
Excerpt from the original manual; "The objective in developing the HP35 was to provide a high precision portable electronic slide rule; something only fictional heroes like James Bond, Walter Mitty or Dick Tracy are supposed to own.
The HP35 has far more computational power than previous pocket calculators. Its ten digit accuracy exceeds the precision to which most of the physical constants of the universe are known. It will handle numbers as small as 10^99 and up to 10^99 and automatically places the decimal point. It is the first pocket calculator to offer transcendental functions like logarithms and sines and cosines. The operational stack and the reverse "Polish" (Lukasiewicz) notation used in the HP35 are the most efficient way known to computer science for evaluating mathematical expressions.
The HP35 was designed with the user, in mind. As much time was spent on the keyboard layout, on the choice of functions, and on the styling as was on the electronics."
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 brandX machines were very failureprone and quite crude in design.
By contrast, the packaging of the HP35 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 HPdeveloped spring contact which used bent beryllium copper strips. The key bottoms were designed to be easy on the copper while still providing the right feel which is essentially unchanged in current calculators.
The HP35, like all the handheld 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.
Length: 5.8"  Width: 3.2"  Height: 1.3"  Weight: 8.7oz