Interesting Accuracy Feature of the HP-35

(Reproduced from November 1976 issue of HP Journal. Thanks Eric Smith for reference to this article.) 

My HP Classic Family Grows!

My 1973 production HP-80 (left) now has an older brother, a 1972 production HP-35, received today from a well known online site, complete with  original leather case but no battery or accessories. For those not familiar with HP calculators, the HP-35 was Hewlett-Packard's first pocket calculator and the world's first scientific pocket calculator. It is a piece of computing history.

The seller had no battery or adapter to test the unit, so the purchase was a bit of a risk. I had a rebuilt battery pack (my HP-80 rebuild), and the unit fired up perfectly with all LED segments good and all key and functions working as they should. It's refreshing that there are still honest sellers out there!

As an added bonus, this unit has the well documented 2.02 ln e^x bug that today is sort after by collectors. (25,000 of the first units produced had the bug, less than 10% of the total production of HP-35).

This is an early Version 2 of the HP-35 as it does not have the 'red dot' next to the OFF/ON switch (a Version 1 marker), but still has the raised dot in the center of the 5 key. The "HEWLETT PACKARD" logo without the "35" model identifier marks this as a Version 2. HP started adding the 35 and 80 identifier to the logo soon after the introduction of the HP-80. All subsequent  HP calculators had their model number in the logo, so I'm quite pleased to have HP's first and second calculators with their original logos.

A final comparison worth noting is that the metal strip above the OFF/ON switch is vacuum-deposited plating on the HP-35, unlike the strip on the HP-80 which is a real piece of stainless steel only found on the early HP-80's before they too reverted back to the vacuum-deposit process. I'm fortunate that the silver trim on this HP-35 is nearly perfect, as this is the most common part of the calculator to wear over time.

Watch out for my next blog as I'll examine the HP-35 with a little more historical context.

Cheers for now...