The United Railroads of San Francisco ( an ancestor of the San Francisco Municipal Railway ) began installing lignum vitae insulators to support heavy feeder wires for their trolley system in 1904 . The reason for the adoption of lignum vitae was its ability to withstand the high stress at high temperature , a problem posed by heavy cables turning corners heated by high current overloads . Many of these insulators survived the 1906 earthquake and fires , despite temperatures high enough to soften the iron poles and melt the copper cables . Many of these lasted into the 1970s with a small number remaining in service into the first decade of the 2000s ( most of these came down when the overhead 600 V DC feeders were replaced with a new system of underground feeders , the rest coming out of service as aging crossarms supporting the remaining overhead feeders were replaced )
1906 earthquake
1906 San Francisco earthquake
Lignum vitae