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 (circa 1914). 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 ohm at 20 °C for a uniform cross section of copper 1 meter in length with a mass of 1 gram. The notation used for this is 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 modification being equivalent to a conductivity of exactly 58 siemens for a uniform 1 mm2 cross section of copper 1 meter in length at 20 °C. The German modification was adopted first 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 was developed so that there would be a global standard for the conductivity of commercially available copper. Prior to establishment of the standard, various values existed.
The standard is most often used as a comparative percentage standard 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 conductivity is 1.2% of the IACS standard.
The standard can be found at https://archive.org/stream/copperwiretables31unituoft#page/n0/mode/1up.
Electrical conductivity of aluminum alloys
The International Annealed Copper Standard is used to measure conductivity of aluminum alloys in order to verify proper heat treatment. The acceptance criteria for electrical conductivity of finished or semi-finished parts of wrought aluminum alloys are contained in SAE International specification AMS2658 Hardness and Conductivity Inspection of Wrought Aluminum Alloy Parts. A method for measuring electrical conductivity is described in ASTM International specification ASTM E 1004 Electromagnetic (Eddy-Current) Measurements of Electrical Conductivity.
The need to verify proper heat treatment arises from the fact that the several different heat treatments of the various wrought aluminum alloys result in the distinct properties that are required by the design engineers. For example, 7075 aluminum comes in several heat treat conditions which have differing tendencies to fracture due to stress corrosion cracking (SCC). The T-73 heat treatment was developed to be virtually immune to SCC failures. One way to verify that a 7075-T73 heat treated component will exhibit the proper resistance to SCC is to measure its conductivity. A properly heat treated part will fall in the range of 38.0 to 43.0 IACS.