The International Annealed Copper Standard
The International Annealed Copper Standard (IACS) establishes a standard for the conductivity of commercially pure annealed copper. The standard was established in 1913 by the International Electrotechnical Commission. The Commission established that, at 20°C,commercially pure, annealed copper has a resistivity of 1.7241x10^{8}ohmmeter or 5.8001x10^{7}Siemens/meter when expressed in terms of conductivity.
For convenience, conductivity is frequently expressed in terms of percent IACS. A conductivity of 5.8001x10^{7}S/m may be expressed as 100% IACS at 20°C. All other conductivity values are related back to this standard value of conductivity for annealed copper. Therefore, iron with a conductivity value of 1.04 x 10^{7} S/m, has a conductivity of approximately 18% of that of annealed copper and this is reported as 18% IACS. An interesting side note is that commercially pure copper products now often have IACS conductivity values greater than 100% IACS because processing techniques have improved since the adoption of the standard in 1913 and more impurities can now be removed from the metal. Wire of high purity has been produced having a conductivity of slightly over 103% IACS, which is very near the value expected for copper without any impurity.
A Brief History on the IACS
Prior to the establishment of the IASC standard in 1913, there was a lack of uniformity in the values for annealed copper adopted in various countries. These conditions led the Standards Committee of the American Institute of Electrical Engineers to request the U.S. Bureau of Standards to make an investigation of the subject. This was done and resulted in the establishment of standard values based on measurements of a large number of representative samples of copper from 14 important copper refiners. Resistivity measurements were made on samples of copper wire one meter in length and one square millimeter in crosssection. This investigation led to the adoption of the IACS by the International Electrotechnical Commission. The IACS standard was documented in Publication 28 of the International Electrotechnical Commission in March of 1914. Publication 28 which is titled, International Standard of Resistance for Copper, states:
The following shall be taken as normal values for standard annealed copper:

At a temperature of 20°C the resistance of a wire of standard annealed copper one meter in length and of a uniform section of one square millimeter is 1/58 ohm, which in decimal terms is 0.017241ohm (meter,mm
^{2}).

At a temperature of 20°C the density of standard annealed copper is 8.89 grams per cubic centimeter.

At a temperature of 20°C the constant mass temperature coefficient of resistance of standard annealed copper, measured between two potential points rigidly fixed to the wire, is 0.00393 per degree centigrade.

As a consequence, it follows from (1) and (2) that at a temperature of 20°C the resistance of a wire of standard annealed copper of uniform section one meter in length and weighing one gram is 1/58 x 8.89, which equals 0.15328 ohm (meter, gram).
Please note that the unit for resistivity was expressed as 0.017241ohm (meter,mm^{2}) reflecting the units of the length and crosssection of the wire standard. To reduce confusion, the crosssection unit of millimeters is converted to the length unit of meters and the resistivity of the IACS is now typically expressed as 1.7241x10^{8}ohmmeter. Also note that the units of volume resistivity and mass resistivity are interrelated through the density, which was taken as 8.89 grams per cm^{3} at 20° C, by the International Electrotechnical Commission.
For additional information see, the National Bureau of Standards Circular No. 31 (1914 Edition), which is available online at openlibrary.org. Note: Publication No. 28 of the International Electrotechnical Commission, "International Standard of Resistance for Copper (March 1914) is included in Appendix V of this document.