In a recently updated post, [Codex99] has a detailed history of the HP-35 pocket calculator. Unless you are a certain age, you probably don’t think much of calculators. They are cheap and not very essential in this day of cell phones and PCs. But in the 1970s they were amazing technology and the desire of every engineer and engineering student.
The story opens in 1965 when Tom Osborne — who was not an HP employee — build a floating point calculator he called the Green Machine. Apparently, he had painted the balsa wood case green. He had been showing it around but failed to get any interest until he showed it to Bill Hewlett. Hewlett wanted it to do trig functions and offered him a six-week consulting gig to work on improvements.
HP engineer Dave Cochran helped out and also helped envision making the device keystroke-programmable. By 1968, this collaboration led to a 40-pound desktop calculator — the HP 9100 — that was the size of a typewriter. It could be yours for only $4900. Keep in mind, that same amount would buy two brand new cars in 1968.
If you are wondering how you go from a 40-pound desk calculator to an HP-35, read the post. Bill Hewlett famously told the team:
I want it to be a tenth of the volume, ten times as fast, and cost a tenth as much.
That seemed like an impossible task in those days, although like watching Titanic in the theater, you already know how the story is going to end.
The post takes an interesting side trip to talk about other period calculators including those from Friden and Wang. These devices, and the 9100, all used discrete transistors and diodes, along with exotic memory techniques and big displays like CRTs or nixies.
You can probably guess what would allow Hewlett’s tenth desires to come true: integrated circuits. HP wasn’t the first to build an IC calculator. Sharp’s EL-8 was portable and Busicom’s Handy LE could actually fit in your hand. These calculators were simple affairs, though. Generally, they had four basic functions with a few embellishments. They were not scientific calculators.
[Codex99] has a lot of detail over how the design came to be ranging from the source of the math algorithms to the industrial design of the iconic calculator’s case. There were a few details we found interesting. The HP-35’s name came from it having 35 keys. At the time, HP set prices by taking the bill of materials cost and multiplying it by pi, which seems perfect for an engineering company.
By 1972, the company held its breath, hoping to sell 10,000 units at $395 to break even. GE alone wanted a quote for 20,000 units! Demand outstripped supply by 18 months even after adding a night shift. The calculator sold until 1975, although the price dropped to $295 when the HP-45 arrived.
There’s a lot more to the story. Check out the original post. If you want more technical details and pictures of several prototypes, check out the HP Museum. If you want to try an HP-35 (remember, it uses RPN) [Neil Fraser] has an online one for you to play with.