Hewlett-Packard, "HP-35 User Manual." 1972.

When William Hewlett and David Packard hiked the Colorado Rockies at the beginning of the 30's, they were far to know that one day they will be responsible for the dismiss of the most venerable calculating device ever designed, the slide rule. Since its invention in the 1600's, generations of mathematicians and engineers, used this wonderful tool to do things such as: calculating the structural components of a bridge, determining the speed of a chemical reaction, calculating the concentration of hydrogen ions in an alkaline solution, estimating the probability of success of a certain event, finding the impedance of a capacitor for an electrical circuit, optimizing the diameter of a pipe in a chemical complex, determining the free enthalpy of a thermo dynamical system, and so on.

As classmates in Stanford University, they dreamed about starting a company where they could develop and market new technological inventions. Encouraged by his professor, Frederic Terman, William Hewlett developed an audio oscillator as part of the requirements towards obtaining his degree in Electrical Engineering in 1939. They used this idea as the basis to start a company and Hewlett-Packard was born in the garage of a Palo Alto house in 1939 with an investment of $538.

Eleven years after, in 1950, with 200 employees Hewlett-Packard had grown to be one of the largest manufacturers of test and measurement electronic instruments.

By the beginning of the 60's, the company started using computers as a way to improve their products. Then, in 1966, the HP2116A real-time computer was introduced. Sold at $30,000 with 8Kb of memory, it was designed to analyze the data generated by HP measurement instruments, an activity that was handled manually with the help of adding machines and slide rules.

Later, in 1968, Hewlett-Packard introduced the HP-9100, an electronic desk-top programmable calculator aimed to compete with the WANG 300 series launched between 1965 and 1968. Sold at $4,900, the HP-9100 used a CRT display, which was a big improvement over the WANG systems based on tele-type devices. At this time, Bill Hewlett directed Hewlett-Packard engineers towards a project to develop a scientific calculator so small that would fit in a shirt pocket. Four years later, on the 1st. of July of 1972, "the one single product that forever changed the whole image of the computer industry," (1) the HP-35, was born ... and the slide-rule passed the torch.

In the design of the HP-35, Hewlett-Packard used the new chips developed by their Santa Clara neighbors at INTEL, and chips manufactured by AMI and Mostek, another young company, founded in 1969, who played an important role in the early developments of calculators and microcomputers. It combined the LSI (Large Scale Integration) IC (Integrated Circuits) technology with the LED (Light Emitting Diode) displays. It had one microprocessor chip, one I/O control chip, three ROM (Read Only Memory) chips, and one clock chip. It was powered by 3 AA battery cells, and had an AC adapter.

The HP-35 was the first handheld electronic calculator ever to perform logarithmic and trigonometric functions. Entering the data required the use of the Reversed Polish Notation (RPN), where the operator is entered after entering the number.

The original model was manufactured in the USA (later models were made in Singapore), had just the label "HEWLETT.PACKARD" on the front plate (the "35" was added after the HP-80, a business model, was introduced in February 1 1973). This model also had a small hole with an "on" red light close to the ON/OFF switch, it was removed in later models because it was obviously redundant. The labels for the special functions were printed above the key not on the key-tops. It has several bugs, for example, typing: "2.02 ln e×" yield 2.0 instead of 2.02.

Its original price was $395, which was later decreased to $295 when the HP-45A was introduced in May of 1973. The price dropped to $225 when the HP-65, the first programmable model (the "Superstar"), was introduced in January 19, 1974. Later, when the HP-55A was introduced in January 1st of 1975, the HP-35 was still available in the market and its price dropped to $195. Soon after, it was discontinued.

The market of pocket calculators exploded during 1972 and 1974. Dozens of manufacturers and hundreds of models appeared around the world during that period, most of them were standard four operation units. Texas Instruments was the main competitor in the field of scientific calculators. Soon after the introduction of the HP-35, in 1973, Texas Instruments launched the SR-50 Slide Rule Calculator for $170, well below the HP-35's price.

At the beginning, teachers didn't see with good eyes the use of these electronic devices as a replacement of the venerable slide-rule, and its usage were banned during formal tests. Nevertheless, students as well as teachers had to focus more on the problem analysis, rather than on the numerical skills involved in manipulating the figures. In 1975, Keuffel & Esser manufactured its last slide rule. Yes, the days of the slide-rule were over.

1) G. Harry Stine, "The Untold Story of the Computer Revolution - Bits, Bytes, Bauds, and Brains," Arbor House, New York.

Irene Kim, "Handheld Calculators: Functions at the Fingertips." Mechanical Engineering Magazine, Vol. 112, No. 1, Jan 1990, p. 56-62.

Palo Alto Historical Association, "History on a Sign Post: the Streets of Palo Alto." http://www.commerce.digital.com/palo-alto/historical-assoc/streets.html

Guy Ball and Bruce Flamm, "The Complete Collector's Guide to Pocket Calculators." Wilson/Barnet Publishing, CA, 1997.

David G. Hicks, "The Museum of HP Calculators."
http://www.hpmuseum.org/hp35.htm

Wlodek Mier-Jedrzejowicz, "Collecting HP Calculators."

Jeff Thieleke, "Hewlett-Packard Calculator Resource."

Aashir Shroff and Joyce Gemperlein, "An Interview with Bill Hewlett." http://www.thetech.org/revolutionaries/hewlett/i_a.html

Sheldon Edelman,, "Made in USA...finally!" THE ELECTRONIC ENGINEER, San Francisco, March 1972 - http://www.public.usit.net/rfurr/hp-35.htm

Mark W. Greenia, "Computers and Computing: A Chronology of the Men and Machines that Made Computer History," Lexikon Services Publications, 1990.

Bobbi A. Kerlin "INFORMATION and COMMUNICATION TECHNOLOGY. Timeline of Significant Events."