overall: 1.8 cm x 33.1 cm x 6.1 cm; 23/32 in x 13 1/32 in x 2 13/32 in
Germany: Bavaria, Nuremberg
This ten-inch aluminum linear duplex slide rule is coated with white enamel and has aluminum endpieces. One side of the base has scales for sin/cos, tg/ctg (tangent/cotangent), DF, D, and the square root of 1 – x<sup>2</sup> (a Pythagorean or P scale). On the slide are CF, CI, and C scales. The right end of the slide is marked: ECO BRA (/) Nr. 1611. The lower right corner of the base is marked: System (/) DARMSTADT. On the other side, the base has K, A, LL3, LL2, and LL1 scales. The slide has B, lg, and C scales. The numbers on the C, LL3, LL2, and LL1 scales are green, which is unusual. The indicator is plastic with aluminum edges. The letter Q is on the hairline on one side. On the other side, Q is on the hairline and W and PS are on shorter hairlines at the top of the indicator. These hairlines are for circle conversion and peripheral horsepower conversion, respectively.
The rule fits in a cardboard box covered with maroon synthetic leather. The top edge of the box is marked No. 1611. The front is marked with the Ecobra logo, and the back is marked MADE IN GERMANY.
Alwin Walther (1898–1967) of the Technische Hochschule in Darmstadt, Germany, developed the Darmstadt system of scales in 1934. His arrangement was aimed particularly at engineers. ECOBRA or Eco Bra was a brand name of Bayerische Reisszeugfabrik, a Nuremberg maker of drawing instruments that was purchased by Joseph Dietzgen in 1909. The company began producing slide rules before World War II, and after the war, Eugene Dietzgen Company of Chicago distributed ECOBRA rules in the United States. Metal rules were more popular in the United States than they were in Europe.
Reference: Dieter von Jezierski, <i>Slide Rules: A Journey Through Three Centuries</i>, trans. Rodger Shepherd (Mendham, N.J.: Astragal Press, 2000), 34–35, 52–53.