The Slide Rule Trading Co., Paul Ross

Hemmi Slide Rule Catalogue Raisonne

Determining Dates of Hemmi Slide Rules

Jirou Hemmi and Co. was founded April 15, 1895 but no Hemmi slide rules from before about 1913 are known.  Hemmi was granted Japanese patent 22129 in 1912 for laminated bamboo construction and success seems to have followed quickly thereafter.  Tamaya and Co. in Tokyo started selling Hemmi slide rules in 1913 as did the Hughes-Owens company of Canada in 1914. The Frederick Post Company of Chicago began selling Hemmi slide rules in 1931.

One can estimate dates of Hemmi slide rules from markings on the rules.

Before 1928 Signed  “J. Hemmi,” not “Hemmi.”
1927/32 Hemmi introduced a new model numbering system between 1927 and 1932. (I have not been able to narrow the date.)  Rules with model numbers 1-18 date from before this changeover, rules with model numbers 20 and above were made after the changeover.  EXCEPTIONS:  Hemmi continued offering models 1, 5 and 8 to its distributors until WWII or later; these rules occasionally turn up with Post or Hughes-Owens model numbers but no Hemmi model number. 
1928 -1946 Brand name is “Hemmi” (not “J. Hemmi”) and “SUN” is in quotation marks.

The company name was officially changed from "J. Hemmi and Co" to "Hemmi Seisakusho & Co." (Hemmi Engineering Works Co.) in 1928 but company was referred to as "Hemmi Seisakusho" at least as early as 1917.   Hemmi was incorporated as a public corporation 1933 with no change in name.   
1937-40? The first inch or first five centimeters of the measuring scales on ten-inch closed body rules is extra-finely divided.  Although discontinued about WWII on ten inch rules; a short section of extra fine divisions continued until the end of production on models 86K and 86/3K.
1947-49 No quotes around SUN, marked “Made in Occupied Japan.” 

Name changed to "Hemmi Keisanjaku Co."  (Hemmi Slide Rule Co.) in 1946.  Hemmi continued to operate under that name through at least 2015.

The Allied Powers occupation of Japan lasted from August 1945 until April 28, 1952 but the requirement that export goods be marked "Made in Occupied Japan" was in effect for a shorter period--from 20 February 1947 to 5 December 1949.  (Thanks to Wataru Tsuchihira who found the original orders in the Japanese National Library.)

WWII-1955 From WWII until 1955, Hemmi "Mannheim" slide rule models 30, 32, 34R, 34RK, 50 and 50W had S (sine) scales that ran from 5o44' to 90o and were keyed to the C and D scales.  Note the S scale on the upper slide in the illustration.    Before WWII and after 1955 the S scale ran from 34' to 90o and was keyed to the A and B scales.   The S scale on the lower slide in the illustration is an example.  Both slides are from Hemmi model 50W slide rules; the upper from a rule dated September 1952; the lower from a 50W dated May 1965.
1950-1952 No quotes around SUN, no date code.
1951-1975 There is a small date code in the form “YM” engraved into each slide rule but often not colored.  That can make it difficult to find.  Often it is in the lower left of the rear of  the rule but can appear almost anywhere.  It sometimes takes inspection with a magnifier in raking light to find the code.  

First letter of the date code indicates the year of manufacture with A = 1950.  Second letter indicates month with A = January.  Thus “BB” indicates February 1951.  All-plastic rules use the same date codes preceded by “^” so that the date code looks like “^YM.”

“A” date codes are rare; I know of only two rules so marked.  (Both are model 86/3K rules marked "AL" (Dec 1950)).  Most rules from 1950-52 escaped dating.  The latest date code I'm aware of is "ZB" (Feb 1975) on a Hemmi 254WN owned by Warren Salomon. 

The date codes reveal the date of most post-1950 Hemmi slide rules but it is not absolutely reliable.  Some Hemmi slide rules that clearly should have date codes, don’t.  A very few have two different date code. 

Ted Hume and I were the original decypherers of the Hemmi date code system; we published it in the Fall 2000 issue of Journal of The Oughtred Society.

1973-1975 The date coding system ran out with “Z” in 1975. Application of date codes seems to have been erratic for a few years before then; "X," "Y," or "Z" date code are often missing.  Hemmi stopped making slide rules about 1975.

Patent and Other Dates

Here is a list of dates which may be useful:  Rules which are imprinted with a patent number cannot have been made before the patent was issued but there is no limit to how many years after the patent date the rule was made.  The absence of a patent number provides no information about the date of manufacture; slide rules intended for sale in Japan never carried US or British patent numbers--no matter when they were made.
4/15/1895 "Hemmi Jirou & Co." founded.  Rules were marked "J. Hemmi" or "Tamaya" (Hemmi's first retailer).
5/11/12 Japanese Patent 22129 for laminated bamboo construction.
1917 British Patent 107562  for laminated bamboo construction.
4/5/20 Japanese Pat. 51788 for method of attaching glass to cursor.  Initially used on metal-framed cursors; later on frameless ("Type B").  (Date thanks to Clay Castleberry and Atsushi Tomozawa.)
2/3/20 US Patent 1329902 for laminated bamboo construction.
ca 1921 Japanese Patent 58115 ("Type A" cursor). 
1921 US import marking requirement changes from "Japan" to "Made in Japan"  (but not consistently enforced until ca 1925). 
1925 Pythagorean scales, P & Q introduced. 
ca 1927 Model numbering system revised.  Models 1-18 dissapear; model numbers above 20 introduced.
1928 Company renamed "Hemmi Seisakusho & Co."  (Hemmi Engineering Works & Co.) Rules are marked just "Hemmi," no longer "J. Hemmi."
1929-45 "SUN" in quotes.
1931 Gudermanian scale, Gtheta, introduced.
5/4/37 US Patent 2079464 for Gudermanian/hyperbolic scale.
1946 Company renamed "Hemmi Keisanjaku Co."  (Hemmi Slide Rule Co.)
1947-49 Rules made for export are marked "Made in Occupied Japan."
1950 Date codes introduced.
Hemmi owned many other patents, but to my knowledge, the above includes all the patent numbers which actually appeared on products.


Cursors are not reliable indicators of the age of slide rules; too many broken cursors were replaced with newer models.  That having been said, it would be foolish to ignore the cursor when estimating the age of a slide rule.  Here are scans of some Hemmi cursors with information about their ages.

Unmarked Cursors

The Hemmi cursors shown here carry no brand or maker's identification.  Tom Dilatush has compared one to several old Faber cursors and is of the opinion that the design was copied from Faber.
 Cursor A is finished in bright nickel plate. (Specular reflection makes it appear dark in scan.)  I have seen only one such cursor on a Hemmi rule--a very early Hemmi model 1.  This cursor may not be authentic Hemmi but the rule it came on is consistent with a date of about 1913.

Cursor B is unpolished soft aluminum.  The plain version (non magnifying, non decimal-indicating) appears only in the 1913 Tamaya catalog.  Magnifiying and decimal-indicating versions (but not plain versions) appear in the 1914 Hughes-Owens and the 1919 Tamaya catalogs.

Cursor C is unpolished soft aluminum and essentially identical to B except that the frame around the glass is narrower and the inside frame corners are not rounded.  The plain version first appeared in the 1914 Hughes Owens catalog and magnifying and decimal-indicating versions first appeared in the 1919 Tamaya catalog (in which some rules were still shown with magnifying and decimal-indicating versions of cursor B).    Available in standard, magnifying, decimal-indicating, and magnifying-decimal-indicating versions.

Cursor D is the "decimal-indicating" variant of C.  Hemmi's 1912 patent application shows this style cursor.

Cursors Marked "J. Hemmi"

Cursor A is polished aluminum.  I hypothesize that it was made only briefly about 1925.   (One specimen came in a case which bore the notation "My works have been established for 30 years, and the quality of the products of the works is perfect ..."  This implies a date of 1925.)   The cursor carries Japanese patent number 51788.

Cursor B is "frameless" glass, also patent 51788.  ("Made in Japan" has been removed from the specimen shown.)  Hemmi referred to it as the "Type B" cursor.  It first appeared in the 1926 Hemmi catalog and had vanished from Hemmi material by 1936. A magnifying version was available.

Cursor C is Hemmi's famous "Type A"--patent 58115--cursor.  Construction is polished aluminum.  Available in standard, magnifying, decimal-indicating, and magnifying-decimal-indicating versions.  Type A was Hemmi's top of the line cursor from 1925 to WWII.

An interesting variant of the type A cursor is the "extended magnifying lens" cursor from 1926-27.  None is known to have survived.

Cursors Marked "Hemmi"

Company name was changed from "J. Hemmi" to "Hemmi" in 1928.
Cursors A and B are three-line standard and decimal-indicating "Type A" cursors.  (Three-line cursors are indicated in catalog listings by "/3."  Cursors A and B are from Hemmi model 80/3 and 81/3 slide rules respectively.)  Magnifying and magnifying-decimal-indicating Type A cursors also exist.  "Hemmi" (not "J. Hemmi") name indicates dates from 1928 to WWII.

Illustration C shows Hemmi's Improved Type A cursor with a narrow chrome-plated frame.  This improved type A cursor was introduced about 1934 on Hemmi's less sophisticated rules and gradually supplanted the type A cursor on other rules, completely replacing it around WWII.   Dates: 1934-end of production.

Type A Cursors Marked Only "SUN"

Some Hemmi type A cursors are marked only "SUN" with neither "J. Hemmi" nor "Hemmi" as manufacturer.  In some cases, as in the cursor shown here, that may be because there was not enough room on the cursor to include the manufacturer's name but I have many full-size type A "SUN" cursors with ample room that carry no manufacturer's name.   The cursors so marked seem to span the whole lifetime of type A cursors from1925 to WWII.

Cursors Marked "Made in Occupied Japan"

The Allied Powers occupation of Japan lasted from 1945 until 1952 but Hemmi slide rules and cursors were marked "Made in Occupied Japan" only from 1947 to 1949.  All Hemmi closed-body rules used improved type A cursors during this period.

All-Plastic Cursors

About 1927-28, all-plastic models 24 and 34 carried celluloid cursors; they are the only all-plastic cursors I know of before WWII.

Hemmi re-introduced all-plastic cursors on its less sophisticated rules in the 1960s. There is a wide variety, ranging from small cursors with high magnification to elaborate, multi-line cursors.   These cursors are too varied to be described by any general principles but their slide rules usually carry date codes.

Duplex Cursors

All ten- and twenty-inch Hemmi bamboo duplex slide rules used chrome-framed glass cursors like those shown in the illustration at right.  The design on the left was used on 1-3/16-inch- and 1-5/8-inch-wide rules from their introduction c1930 until about 1955.  The design on the right was used on all 1-3/4-inch wide duplex bamboo rules of any date (c1950 until end of production) and on 1-5/8-inch-wide rules after c1955.

All Hemmi five-inch bamboo duplex rules use chrome-framed glass cursors like this.

Hemmi plastic duplex slide rules used all-plastic cursors like those shown in the illustration at right.  The design on the left was used on extra-thick (about 5mm) duplex rules.  The design on the right was used on thinner (4mm) rules.