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The Hemmi 255 was manufactured in Japan by Sun Hemmi Slide Rule MFG, Co, LTD, since the early 1950's until the early 1970"s. This rule was fabricated from a large-diameter variety of bamboo grown in Kagoshima Prefecture on the island of Kyushu, coated on all sides (except the ends) with white celluloid, and held together with metal posts. The glass indicator has a metal frame screwed to plastic ends. The 255 model was fabricated in two sizes, the 12.5 inches long version (10-inches long scales), and the 21.5 inches long 275 model (20-inches long scales).
The 255 model was specifically designed for advanced electric engineering applications. It is equipped with three scales for hyperbolic functions; the hyperbolic sine function is split in two scales running from 0.1 - 0.9 - 3.0, and the hyperbolic tangent function scale running from 0.1 to 3.0. This model also has three scales for regular trigonometric functions, graduated in radians and reversed (inverted mode), such that values have to be read in scale CI. The tangent function split in two scales running from 0.1 - 0.8 - 1.47 radians (5.73o – 45,83o – 84.22o), and the sine function scales running from 0.1 - pi/2 radians (5.73o – 90o). It has also two paired radian-degrees scales for angles conversions. Three Log-Log scales round the advanced features of the 255 model.
The scales on the 255 model are: Sh1, Sh2, Th, A [ B, TI2, TI1, S1, C ] D, LL3, LL2, LL1 on the front face, and L, K, DF [ CF, CIF, CI, C ] D, X, Theta on the back face. According with the code engraved on the rule, D B, and the Code table in the ISRM Hemmi page, this 12-inches long specimen was fabricated in February 1953.
The next operations can be performed with the 255 with a precision of two-three decimals :
SRPractice-3.pdfHere is a scan of the Model 255 User Manual, done by the ISRM:
M90_HemmiBambooSlideRuleDuplex_1951.pdfHere is a scan done by the ISRM of the 1958 Hemmi Slide Rules Catalog:
Hemmi_1958_Catalog.pdfClick on the next link to see professional panoramic pictures of the 255 model.
Click on the next link to see professional panoramic pictures of the 255 model.
References:
Ross, Paul: A Comprehensive Hemmi lide Rule Catalog. Journal of the Oughtred Society. Vol. 12, No.1, Fall 2003 https://osgalleries.org/journal/pdf_files/12.1/V12.1P41.pdf
Ross, Paul: Hemmi Slide Rule Catalogue Raisonné: Part 1: http://srtco.us/HemmiCatalogueRaisonne/Index.htm
Ross, Paul: Hemmi Slide Rule Catalogue Raisonné: Part 2: http://srtco.us/HemmiCat2/Index.htm
Robinson, Bill: Hyperbolic Slide Rule Archive. https://osgalleries.org/hyperbolic/index.html
Robinson, William K: LISTING OF ALL KNOWN SLIDE RULES WITH HYPERBOLIC SCALES, www.oughtred.org/jos/robinson-hyperbolics-dec06.html
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