The Slide Rule Trading Co., Paul Ross

Hemmi Slide Rule Catalogue Raisonne

Hemmi Model Numbers

Models are listed in numerical sequence with slight re-arrangements to keep related rules together.  Model numbers beginning with P (e.g., “P40RK”) are all-plastic.  Other rules are celluloid over bamboo except as noted.
 
Model numbers that include “/” followed by a digit (“80/3”) indicate the number of hairlines on the cursor.  For example, when Hemmi model 80 was first introduced in the 1920s it was simply “model 80” with a single hairline.  But about 1932 Hemmi began supplying that slide rule in single- and triple-hairline versions and the rules bore glued-on labels “80/1” or “80/3.”  About 1951 a K scale was added and the single-hairline version dropped from the catalog; the surviving rule was model 80/3K--there was no single-hairline model 80/1K.  After another modification, the rule became model 80K with three hairlines and no option of a single hairline.
 
In summary, a model number like “80/3” indicates that the rule has three hairlines and tends to imply, but does not guarantee, that there was also a single-hairline “80/1” model available.  A model number like “80K” gives no information about how many hairlines are on the cursor.

The letter "A" added to an existing model number indicates a change in bookkeeping without any change in the slide rule itself.  Model 90A is the same as model 90; model 149A is the same as model 149.  Other letters are mnemonic indicators of the changes made--model 34RK is just model 34 with R and K scales added; model 259D is the same as model 259 but with the scales rearranged and a DI scale added.

Model numbers specify a unique arrangement of calculating scales and length.  Model numbers did not change when the cursors were redesigned or over-range scale extensions added or measuring scales rearranged.  (For a discussion of measuring scales on closed body Hemmi slide rules, click here.)  If you have a Hemmi slide rule with the same scale arrangement as a rule shown here but with a different cursor, scale extensions, or measuring scales, it's the same model.