The first telephone lines employed the same type of outdoor circuits as telegraph lines—namely, a single noninsulated iron or steel wire supported by wooden poles with glass insulators. Since electric signals require two wires, the second “wire” was a ground return through the earth. Unfortunately, the use of a single wire made the telephone circuit extremely susceptible to interference by other signals. This problem was addressed by the use of a two-wire, or “metallic,” circuit; the first demonstration of such a system occurred in 1881 on a telephone line between Providence, Rhode Island, and Boston. As the distances between telephone ...(100 of 8665 words)