After 87 years of alternating exuberance and disappointment the world's first super conducting power line is about to become a reality. A superconducting line is one that will conduct huge electrical currents with far less resistance than that of a line made of metal wire, and which therefore conserves energy. The first large-capacity superconducting line, to begin operation by mid 2000 in Detroit, will be only 400 feet long. But it will use only 250 pounds of a new kind of superconducting wire to carry as much current as the 18,000 pounds of copper wire the line will replace.