The Pose-Marre pen slide rule

A mysterious one-of-a-kind device

    Pencil and Pen Slide Rules are a paradox. On the one hand, they are highly inadequate for any serious engineering calculation, due to the low precision imposed by their short scales. On the other, they seem to have had an intense allure for the designers of a century ago, who have tirelessly devoted their ingenuity to the patenting and production of many different designs in this category . Which is why pencil slide rules are fairly easy to come by today.
    And yet, not always easy. The device shown here is a pen slide rule so rare that only this one exemplar is known at the moment. It is also of quite unusual design, and is exquisitely manufactured -- though by who remains unclear. I tend to call it the Pose-Marre slide rule because this is the only text on it, but what exactly this text means is not entirely clear either.
 

    As seen in the photo, this is a metal pen, 12.7 cm long (excluding the protruding plunger at the top, which screws in to extend the pen refill) and 12 mm in diameter. The metal parts are machined with great precision from solid rods of non-ferrous metal, probably a hard aluminium alloy. The body consists of two halves that are joined by a hollow pivot at the midpoint -- hollow, to allow the refill to run down its centre.
    The two logarithmic scales, printed on paper, are glued around the ends of the top and bottom halves of the pen, one above and one below the pivot. Thus, one can rotate the scales relative to each other by grasping the pen in two hands and twisting the top and bottom parts in opposite directions. A transparent plastic tube surrounds the scales, at once protecting them and carrying a longitudinal hairline on its inner surface. Each scale spans one log cycle (so, they are C and D scales), but this cycle is split into three parallel rings, covering (bottom to top): 1-2.16, 2.16- 4.65, 4.65-10. The cursor hairline is therefore essential for aligning numbers on different rings of the C and D scales with each other during a calculation.
    The pen function relies on a ballpoint ink refill pushed up by the usual spring; the mechanism for extending it relies on a plunger at the top that simply screws in when rotated, pushing the ballpoint out the hole at the other end (see photo below). To retract the point you screw the plunger out in the other direction. The refill that came in the device is quite modern looking and of a model still available today.

    I’ve purchased this pen slide rule from a German collector, but he had no information about its origin. Furthermore, no collector I’ve asked had ever seen its like, and that includes the world expert on pencil slide rules (sadly, recently deceased), Rodger Shepherd. It took the patent-hunting expertise of Panagiotis Venetsianos to uncover Gebrauchsmuster (a kind of German patent) number 7338885, awarded in 1974 to Karl Opdenhövel .The drawings therein (see below) leave no doubt that Mr. Opdenhövel had invented the Pose-Marre pen, even though that name is nowhere listed in the document.

• It does not carry any branding information -- no maker name, no model number, no patent reference.
• It was produced in a machine shop, using methods too expensive for a mass produced item.
• Its refill retraction mechanism uses a continuous screw plunger. This could seem a good idea to a mechanical engineer designing a proof of concept model, but there are numerous better ways to implement the same functionality -- notably the familiar double-click button found on countless ballpoint pens since the 1950’s.

We must conclude that this unit is a prototype, or possibly even the patent model, and that for some reason it was never developed into a commercial product. On second thought this may not be so surprising, given the patent’s grant date of Feb. 7, 1974 -- two years after the HP-35 calculator had rendered all slide rules obsolete.
    There remains the mystery of the “Pose-Marre” inscription on the pen’s barrel. A web search elicits a few traces of a now-defunct steel processing company, Pose-Marre Edelstahlwerk GmbH, operating in the last decades of the 20th century in Erkrath, Germany. I managed to find on LinkedIn an ex-employee of that company, and he told me he never saw this slide rule there (despite being a chemical lab technician). No surprise there: it is unlikely that a heavy steel manufacturer would have included a slide rule in its product line!
    I note, however, that Erkrath is located less than an hour’s drive from Altschermbeck, where Mr. Karl Opdenhövel lived (according to his patent application). It is therefore possible to conjecture that he turned to Pose-Marre to make him his prototype in their machine shop, either for love or for money, and that they chose to mark it with their name. For all we know he may have actually worked at that company himself and done the machining with his own hands. The exact truth must remain a mystery, but this does not detract from the beauty of the resulting slide rule!
 

More info:
    The Opdenhövel patent can be seen here.