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          Three “pencil” slide rules

    20th century Engineers just loved pens and pencils (hence the pocket protector, that iconic geek accessory of yesteryear). And of course, engineers lived by their slide rules. Small wonder, then, that enterprising inventors tried to pack the functionality of the slide rule into a pencil form factor – a thin rod you could clip into your shirt pocket. And if it could actually write, so much the better!
    I have three devices with this form factor in my collection, and two of them actually have pencil functionality in addition to calculating. Here they are:
Three pencil slide rules
Click photo to enlarge
    The first of these is the Nicolet slide rule pencil, and it is the most pencil-like: a compact retractable mechanical pencil with a fully functional slide rule built into its envelope. The outer black tube carries the engraved A and D scales; the inner sleeve, which is fixed to the top cap, bears four paper scales: B, C, CI, and K. One of these is visible at any time through the long Nicolet pencil slide rule
Click photo to enlarge
cutout on the outer tube, which is flanked by those A and D scales. You rotate the cap to choose which scale shows through, and pull it to move the inner scale relative to the outer two. Add the clear annular hairline cursor that slides along the pen and you have a tubular implementation of a regular slide rule, capable of multiplying, dividing, and handling reciprocals, squares and cubes.
    This slide rule is described in a patent applied for in 1953 by Nicholas E. Nicolet and Herman Neiges, of New York. Despite the praises in that document, it has a limited accuracy due to its very short (3 inch) scales and poor dimensional stability: the scales on my exemplar are visibly warped.
The photo below shows how the C and D scales are set to perform multiplication by 2.
Nicolet pencil slide rule
Click photo to enlarge
    The second device has a very different design. This is the Makeba Kombinator, manufactured in Bautzen, Eastern Germany, in the 50’s or 60’s. 
     This device has two 2-cycle scales and two 1-cycle scales, with one of each pair fixed to the hexagonal body of the pencil itself, and the others to an outer hexagonal metal tube that can slide out around the pencil, moving the scales relative to their twins. A metal-framed cursor slides along the entire device.
    The Kombinator is a precision product. It has engraved 5-inch scales that look good and stay accurate. The photo below shows how the C and D scales are set to perform multiplication by 2.
Makeba Kombinator slide rule pencil
Click a photo to enlarge
Makeba Kombinator slide rule pencil
Makeba Kombinator slide rule pencil
Click photo to enlarge
    The third device in this article is the most unusual – and is very rare; so much so that I’ve never seen or read about another one. This is the EngPen, made in Chicago by the EngPen company, of which I have no information whatsoever. An inscription on the cursor states that a patent has been applied for – but I haven’t found that either. My fellow collector Rodger Shepherd called my attention to a patent (uncovered by Panagiotis Venetsianos) granted in 1908 for a rather similar device invented by one William S. Harlow of Massachusetts.
    The EngPen is  composed of two concentric metal tubes, serving as the stock and slide, and a tubular cursor (which also carries a pocket clip). The device can easily slip over a pencil or pen; a modern pencil is a tad too thin to give a tight fit, but who knows what pencils were like a century ago? EngPen slide rule
Click photo to enlarge
EngPen slide rule
Click photo to enlarge
    The cursor is an interesting construct: unlike the other two slide rules shown here (and unlike the aforementioned Harlow patent) it does not use transparent celluloid to carry the hairline. Instead, it is of all-metal construction, with windows over the different scales.
    This all-metal construction makes the EngPen a markedly durable artifact; one must thus wonder why it is so hard to find today!
    The photo below shows the scales set for the calculation of 3 x 2 = 6.
EngPen slide rule cursor
EngPen slide rule
Click photo to enlarge
    There were a number of other models of pencil slide rules in those days, which raises the question: what made them worth producing? Sure, their owners would be certain never to be caught without a calculator, but in reality anyone competent to use a slide rule – an engineer, or a scientist – would use a larger rule, 12 inch long at least, which would have far greater precision. And even if one wanted a pocket-sized model, a straight six-inch slip stick is far easier to use than these combination models, which must inevitably embody a design compromise.
    But then, perhaps these nifty gadgets were intended for use as novelties... they sure would make excellent father’s day gifts for the engineer who has it all!...
Exhibit provenance:
    eBay, from three different sellers.
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