Concise 28 & 28N Circular Rules
The Concise 28 slide rules family was manufactured by the Japanese company Concise Ltd, based in Tokyo, Japan, to be commercialized in the United States under the Concise brand itself, or under other companies' brand as Scientific Instruments, Lafayette, Sama & Etani, and also as special orders of giveaways or promotional items for other companies. Concise Ltd. began the production of circular slider rules by 1949, and it seems that continued its production in until the 2010's. I have found very little information indicating the period of time the model 28 was produced, but according with an entry in the Smithsonian National Museum of American History, this model was commercialized in the USA from 1959 until 1966.
Consisting of two concentric circles, fabricated all white plastic where scales are engraved, screwed at their centers, with a plastic movable indicator with a hairline. Most of the circular rules produced by Concise Ltd. were no more than 4 inches diameter, the Model 28 has a 3.25 inches long diameter, very small to be a shirt pocket calculator, or a cute promotional. As a calculating device, the Model 28 has 5 scales in the front face: D ( C, CI, A, K ), where the scales in parenthesis are on the movable circle. On the back face there a units conversion table.
No serial number was engraved or printed on this little rules, so it is not possible to know the exact production year, only can be assumed they were fabricated in the 1960's:
The next operations can be solved with a precision of two-three decimals with the Concise 28:
SRPractice-2-NoTrig.pdfHere is the International Slide Rule Museum scan of the User Manual for the Models 28 and 28N:
M251_Concise_28N_Instructions_DonatedByMichaelMaloney.pdf
