In 1971, Bill Hewlett, cofounder of Hewlett-Packard, took a good long look at the HP 9100A, a 40-pound electronic calculator that his company had introduced just three years before. Then he asked his engineers a question: Why can’t we make it fit in my shirt pocket?

The marketing people said that it wouldn’t sell, because slide rules—which could also calculate logarithms and other math functions—were much cheaper than this gadget would ever be. Hewlett ignored them, and a year later the HP 35 appeared at an initial cost of US $395, or nearly $2000 in today’s dollars. Engineers and scientists lined up for it, and some 100 000 units were sold in its first year, making it one of the company’s most successful products ever. The slide rule soon became landfill.

As a new Ph.D. graduate in physics, I lined up, too, but for a cheaper, four-function calculator from Texas Instruments that appeared soon afterward. I treasured my calculator deeply and briefly: someone stole it from my office a few weeks after it arrived.

The HP 35, named for its 35 keys, was discontinued after three years, replaced by more advanced models. Now, 35 years after its introduction, it’s back in a commemorative edition called the HP 35s. The color and case design are reminiscent of the original, although this one has eight more keys and is a bit thinner and lighter.

The HP 35s is well made, the two-line LCD is clear, with adjustable contrast, and the keys have an inviting feel. It comes with a well-produced 359-page user’s guide, supplemented by extensive online training modules.

Although the company’s marketing pitch takes a loving backward glance, it also portrays the retro model as the ”ultimate scientific calculator,” crammed with far more power and complexity than the original. Its more than 100 math and programming functions include numerical integration, two-variable statistics, and regression. There are 800 storage locations, 42 built-in physical constants, unit conversions, and lots of other goodies. The user can choose between Reverse Polish Notation (RPN), an efficient way to manage calculations that HP has long favored, or algebraic entry logic, used by nearly all the other calculators.

In a clever twist, a user can enter an equation and solve it numerically for any of its variables without rearranging the equation. However, the calculator lacks some features often found in high-end calculators, such as graphics and symbolic math capabilities, removable storage, and the ability to interface with other computers.

**But who needs** all these features? Like other high-end scientific calculators, the HP 35s suffers from a bad case of feature creep. That comes from the irresistible urge of designers to stuff in ever more features as a product evolves, pushed by the need to stay ahead of the competition or to entice users to ”upgrade” to new versions. This proliferation of features can introduce unexpected failure modes. In at least one HP ”all in one” printer/scanner/fax machine, for example, the user cannot scan a page if the printer cartridge has run dry.

Feature creep can also lead to overly complex but unimaginative products—and the HP 35s is a case in point. How many users would need to integrate an equation on a handheld calculator? Or know whether the true result of a calculation is slightly above or slightly below the value indicated on the display when the calculator is set to display fractions instead of decimal numbers? Or need to choose between RPN and algebraic-entry systems, with their very different approaches to doing calculations? Perhaps very advanced users might benefit from these functions, but I suspect that they would have abandoned the calculator for a computer long before they reached that point.

Even my old Pickett slide rule suffered from feature creep. It had 34 scales, only a few of which I ever bothered to learn to use.

Granted, many engineers keep a handheld calculator around for occasional use, and others rely on software versions. I myself have a few scientific calculators loaded onto my PalmPilot. But would any engineer go out and buy an HP 35s for the sake of its large grab bag of features? I doubt it. Maybe that’s why HP is also plucking the heartstrings of nostalgia.

Nowadays, the real market for handheld scientific calculators is in education, from middle school through college. The gadgets are commodity items sold in college bookstores, discount department stores, and office-supply centers. My own university bookstore has a rack filled with them. In its present display the HP 35s would occupy the high end of the Hewlett-Packard section, at a price of $59.99. Still, that is only about half as much as the top-of-the-line models from Texas Instruments, which are even more bloated with features. However, many middle school and high school teachers require students to buy graphics calculators, which would rule out the HP 35s.

Nevertheless, the HP 35s is a highly competent product with capabilities that would have been unimaginable 35 years ago, and the price is right. I can’t wait to see what Microsoft will do for the 35th anniversary of Microsoft Windows.

## About the Author

KENNETH R. FOSTER, an IEEE Fellow, is a professor in the department of bioengineering at the University of Pennsylvania, in Philadelphia. His e-mail address is kfoster@seas.upenn.edu.

## To Probe Further

See the virtual HP calculator museum at http://www.hpmuseum.org.

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