Historical Note
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Insulating Electric Cables In 1730, Stephen Gray, a Charterhouse Pensioner (a retired member of an austere monastic order in France), first transmitted electricity along a thread made of hemp some 270 meters in length. He dampened the thread to increase conductivity and suspended it by silk in order to provide insulation. Aside from Benjamin Franklin's earlier experiments with lightning discharges, this event is acknowledged as the birth of cables capable of carrying weak electrical currents, from which grew power lines. Early power cables were known as "conductors," while the term "cable" was reserved for telegraph and telephone lines. Eventually, rigid conductors were called "mains," and flexible power conductors were included as "cables." In England, 17 years after Gray's initial experiments, William Watson transmitted current over a 3.2-kilometer-long metal wire, completing the circuit by using the earth as a ground to transmit the return current. As a slightly "shocking" sidelight, Watson's assistant later demonstrated the electrifying result of that particular circuit when completing the connection by holding onto the far end of the wire with one hand and plunging a metal rod into the River Thames with the other hand—"completely vindicating] the result which had been anticipated." Yet, the chief scientist, not the assistant, is remembered. Although not fully recognized at that time, the major problem with electric cables would not be the conductor that carried the current, but with the material insulating the conductor. Simon Linguet explored the insulation of cables. In 1782, unhindered by his imprisonment in the Bastille, the French lawyer wrote of transmitting electric current along "gilded iron wires enclosed in wooden tubes filled with resin and buried underground." Don Francisci Salva further advanced the state of cable insulation in an article presented to the Academy of Science in Barcelona in 1795. Salva covered 22 wires with paper, coated them with pitch, and after tying them together, bound them
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further with additional paper. In 1798 Salva succeeded in transmitting effective signals by means of electrostatic discharges over a 42-kilometer-long cable of this design. As an alternative to stringing the wires overhead, Salva suggested twisting the wires together and laying them underground in subterranean tubes. Between the years of 1812 and 1815 Baron von Schilling developed a copper wire cable insulated by India-rubber, which was then dried and varnished. Von Schilling was heavily influenced by Robert Fulton's idea of detonating explosives by using electricity. Described by von Schilling as a "conducting cord," the cables were used to detonate mines beneath the River Neva and River Seine during the Napoleonic Wars. These can be called the first true electric power cables. Keeping moisture away from the conductor so as to not short the circuit remained a continuing problem. An early solution to this problem was found through a substance called gutta-percha, discover
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