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Four-function calculators that could add, subtract, multiply, and divide were, without doubt, great feats of engineering, but they remained limited devices best suited to simple arithmetic. In 1972 a huge advance beyond the four-function was released: a pocket scientific calculator that could compute transcendental functions like logarithms and square roots and that could be programmed to solve complex equations. Hewlett-Packard (HP) and Texas Instruments (TI) led the way in making electronic calculators essential tools for professionals and students, and placed calculators at the vanguard of the era of home programming.

The HP-35 **(Image 1)** met the challenge that Bill Hewlett set to his engineering team: miniaturising the typewriter-sized HP 9100A, which had been released in 1968. The HP-35 performed all of the same functions as the 9100A, a triumph of miniaturization achieved through the use of microchips and the application of Reverse Polish Notation (RPN).

RPN used a postfix operator (for instance, 2 + 3 would be written as 2 3 +), which allowed more efficient use to be made of limited memory. HP fanatics would make much of the fact that there was no '=' sign on the device. One would simply press 'ENTER' at the end of a string of instructions, and this ability to store instructions made the '35' the world's first pocket programmable calculator. More than 50,000 units were sold in the first year despite the very high price of $395, proving the consultants who urged the company not to sell such an expensive calculator wrong.

However, HP did not initially market the device as a 'calculator' - seen by professionals and scientists as too approximate and expensive - but rather as an "electronic slide rule".People whose practices depended upon accurate calculation and had, for their entire lives, used a slide rule, could now compute rapidly and accurately on a device costing less than $5.

HP invested in advertising that showed calculations on the HP-35 done against a slide rule expert, showing that its machine was much faster and more accurate than the traditional device. Furthermore, the use of RPN was highlighted to indicate the HP-35's similarity to a much larger computer.

Launched in 1974, the HP-65 **(Image 2)** became HP's flagship model. Though it bore a hefty price of $795 at its launch, its quick rise in popularity showed that individuals were prepared to take on the expenses associated with owning a personalisable and powerful device.

In particular, the HP-65 became known for its innovative magnetic cards. These could store programs and specify functions for the calculator's top row of keys. HP created sets of applications for various professions, known as 'pacs', and created an iconography that bears resemblance to today's smartphone apps. One might say that these calculator program pacs were the first freestanding software to be made widely available. The calculator was intended for use in everything from chemical engineering to aviation to tax calculation, with special pacs for each task.

Just as importantly, users could program their own magnetic cards and submit their work to HP, who would publish programs authored by users in a library. Keeping the rejection rate high meant that having a program in the catalogue was a point of pride, and HP implemented a rewards system for its most skilled home programmers, awarding points redeemable for other programs and hardware.

HP also created a users' newsletter that discussed both hardware and programs, and made the community of users and programmers visible. Similar grassroots organisations helped coordinate programming to make the calculator more powerful and personal. One group in particular, PPC, helped facilitate smaller groups across multiple continents, and even had HP manufacture a large Read-Only Memory (ROM) attachment for a later model, the HP-41C **(Image 3)**.

Although it is hard to think of calculators as revolutionary devices in the mould of the first personal computers, users turned these programmable calculators into far more than number crunchers. TI's devices were also popular, although they opted for algebraic notation over RPN, which maintained a more direct relationship with the problems being computed. HP's devices were seen as tools on which one could experiment with programming strategies.

The 1980s involved, in the words of one prominent computer scientist, a 'reformation' of computing technologies:

"[I]n the 1980s, the rest of us became somewhat knowledgeable about computing because everyone had one of these personal computers on their desks...In certain ways, it was like the Protestant Reformation. We all could do it in the privacy of our homes, and we didn't need the priest."(1)

Programmable calculators familiarised users with the investment - of both time and money - in personal computing devices. Many users saw their calculators similarly to how we see our smartphones today: tools we could take anywhere in order to navigate the world with computing power.

Mikey McGovern

Mikey McGovern, 'Pocket programming', *Explore Whipple Collections*, Whipple Museum of the History of Science, University of Cambridge, [http://www.hps.cam.ac.uk/whipple/explore/calculatingdevices/handheldelectroniccalculators/pocketprogramming/]

http://www.hps.cam.ac.uk/whipple/explore/calculatingdevices/handheldelectroniccalculators/pocketprogramming/