The Slide Rule Trading Co., Paul Ross

Hemmi Slide Rule Catalogue Raisonne

Body Style and Size

Body Style

We arbitrarily classify linear slide rules as having three styles of construction: Duplex, Open Body, and Closed Body.

Duplex: The most common design for advanced slide rules.  Duplex rules consist of two stators, held rigidly parallel by end brackets, with a movable slide between them.  The back of a duplex slide rule looks pretty much like the front.  There are calculating scales on both sides of both stators and both sides of the slide; the cursor covers both side of the rule.

Open Body:  Similar to duplex in that there are two stators held by end brackets and one slide which is visible for most of its full length on both sides but the cursor covers only the front of the slide rule.  There are no calculating scales on the backs of the stators of an open body slide rule.  If there are calculating scales on the back of the slide it must be taken out, turned over, and re-inserted to be used.  Many of Hemmi’s cheaper all-plastic rules used open-body construction with integral plastic end brackets.  There are no measuring scales on the edges of open body slide rules.

Closed Body: (Sometimes referred to as “Mannheim” but we use “Mannheim” to designate the set of scales A [B, C] D with or without trig and log scales.)  Closed body rules have a full-length metal or plastic plate joining the stators.  This plate obscures all but the very ends of the back of the slide.  If there are scales on the back of the slide they can be used by setting them to an index near the end of the rule or by turning the slide over in the rule.  Many closed body rules have measuring scales on the edges.  “//Closed||” indicates a closed body rule with its top edge slanted and bottom edge vertical—both suitable for measuring scales.  "||Closed||" indicates that both edges are square to the front of the rule and just “Closed” indicates a closed body slide rule whose edges are not suitable for measuring scales.

Body Dimensions

Dimensions are actual scale length x face width x thickness.

Scale length: Scale length given in this Catalogue Raisonne does not include over-range extensions; it is the distance between the scale indices only.  For the usual C and D scales it is the distance between the "1" mark on the left and the "1" mark on the right.  Over-range extensions to the left of the left index and right of the right index, if present, make the overall scale length 20 to 50 mm longer than the scale length given here.  The overall length of the rule is from 30 to 70 mm longer than the scale length and varied depending upon when the rule was made--sometimes even from batch to batch.

Scale length of Hemmi rules is always a convenient exact number of millimeters.  (Ten inches is 254 mm but the scale length of a "ten-inch" slide rule is actually 250 mm.)

Simple folded scales are usually folded at π or the square root of ten.  Consider, for example, a typical CF scale folded at π without over-range extensions:  The leftmost tick mark usually represents 3.14 (slightly less than π) and the rightmost tick represents 31.6 (slightly greater than 10π).  If the true scale length from 3.14159 to 31.4159 is 250 mm, the apparent scale length from 3.14 to 31.6 (leftmost tick to rightmost tick) will be about 250.5 mm.  The effect is very noticeable on rules like Hemmi model 200 where the scales are broken into six parts and the break points are the sixth roots of ten.

Face Width: Face width is the same as overall width in most cases but is 3 to 8 mm less than overall width on those closed body rules which have a slanted edge carrying a measuring scale.  Face width is important because closed- and open-body rules with the same face width use the same cursors, regardless of rule thickness.  (Except "thin" cursors will not fit on regular rules.)  Face width is usually give in millimeters except where it is more accurately described in inches.  (It is clear that some rules were designed to specific inch widths.  For example models P23, P24, and P36 are 25.4 mm = 1 inch wide and clearly narrower than models 34P, 34R, 34RK etc., which are 26.0 mm wide.)

Thickness: The body thickness of Hemmi closed- and open-body slide rules is not a critical dimension; it varies from batch to batch.  The dimensions give in this Catalogue are indicative of typical specimens. However, thickness is critical for duplex slide rules.  For example: One cannot interchange cursors between duplex model P270 (46 mm wide x 5 mm thick) and model P280 (46 mm wide x 4 mm thick).

Where all three dimensions are given they were measured on at least one actual specimen.  Where only length and face width are given, the face width was usually scaled from a photograph or illustration.  Where only the scale length is given, no specimen or good illustration was available.

Measuring Scales

Closed body bamboo rules usually have measuring scale on the top and bottom edges.  Sometimes there is also a 28-56 cm scale in the gutter for measuring inside dimensions.  As nearly as I can determine, all Hemmi models with two measuring scales were made both with inches on top/centimeters on bottom and with centimeters on top/inches on bottom.  Many were also produced with other combinations, such as cm/cm, cm/blank and inches/blank.  Many combinations occur within each model number; it is beyond our ability to catalog such variations.

Although there is wide variety in the measuring scales on Hemmi rules, the measuring scales on Frederick Post and Hughes-Owens rebranded rules are quite consistent.  It is possible that measuring scales were engraved to the distributor’s order.

Hemmi duplex, open body and all-plastic slide rules do not have measuring scales.