has been shared by both iron and copper. For example,

the production of rails was a relatively simple operation;

steel rails made right at the plant could be installed

without further processing. When steel left the plant

destined for use in automobiles and other complex

machinery, however, a large amount of additional value

was generated as the raw steel was worked into progress-

ively more complex shapes. Consequently a much larger

value of total production per unit of steel was generated.

 The case for copper was much the same. As the

 proportion of copper going into simple wire decreased

 relative to the amount going into items requiring sub-

 sequent manufacture, the value of final products per

 unit of copper increased as well.

 Not only have changing markets resulted in more

value produced per unit of metal, but increasing

complexity of products within each market category has

increased value per unit of metal as well. Thus, war

equipment, automobiles, machine-tools, and aircraft, as

examples, became considerably more complex, and increased

complexity has served to make them more valuable.

 The substitution of other materials for iron,

steel, and copper also has tended to decrease the amount

of metal needed to generate any given level of output.

Aluminum and plastics have been particularly important

in that regard, not only capturing some former iron,

copper, and steel markets, but also denying markets that