International Annealed Copper Standard
The International Annealed Copper Standard (IACS) is a standard established in 1914 by the United States Department of Commerce. It is an empirically derived standard value for the electrical conductivity of commercially available copper.
Sometime around 1913 several copper samples from 14 important refiners and wire manufacturers were analyzed by the U.S. Bureau of Standards. The average resistance of the samples was determined to be 0.15292 Ω for copper wires with a mass of 1 gram of uniform cross section and 1 meter in length at 20 °C. In the United States this is usually written as "0.15292 ohm (meter, gram) at 20 °C".
Germany proposed a slight modification of this value to "0.15328 ohm (meter, gram) at 20 °C", this being equivalent to a conductivity of exactly 58×106 S/m at 20 °C. The German modification was adopted by the International Electrotechnical Commission in 1913 and subsequently published by the United States Department of Commerce on October 1, 1914 as the International Annealed Copper Standard (IACS).
The standard is most often used as a comparative property in the specification of the conductivity of other metals. For example, the conductivity of a particular grade of titanium may be specified as 1.2 % IACS, meaning that its electrical conductivity is 1.2 % of the copper specified as the IACS standard.
The standard can be found at https://archive.org/stream/copperwiretables31unituoft#page/n0/mode/1up.