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Type | Model No. | Maker | Country | Construction Material | Date | Scale Length | Area of Use |

Linear | Davis-D&P / Log-O-Log | John Davis & Sons | England | Celluloid over wood | 1907 | 25 cm | General Calculations |

**Notes: **"This Davis rule is called the “Log-O-Log” in Davis’ own advertising. The Log-O-Log has four log-log scales on a separate, interchangeable slide, with two scales on each side. The base value for the Davis’ scales was 2. The range of the log-log scales was 0.001 to 1,000. The Davis Log-o-Log was advertised until at least 1919, eleven years after Davis introduced their “Yokota” slide rule, with a complete set of log-log scales, in 1908. By 1927, only the Yokota was still being advertised. Both the Log-o-Log and Yokota were made for Davis by Dennert & Pape (D&P) in Germany. The Davis Log-o-Log, with its normal slide inserted, is a Mannheim slide rule with the usual S, L, and T scales on the back of the slide. To make their first log-log slide rule, all John Davis & Sons had to do was introduce the extra slide. Unlike the Yokota, which was also made by D&P specifically for John Davis & Sons, there is no mention of the Log-O-Log slide rule in Klaus Kühn and Karl Kleine’s history of D&P. Neither the Davis or D&P company name is anywhere on the slide rule. We know it is a D&P from the patent number in the well under the slide, and we know it is a John Davis & Sons from contemporary advertisements and the excellent description in the 1906 edition of Pickworth. "

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