Edison and the Electric Chair_ A Story of Light and Death - Mark Essig [21]
Some experts believed that in the future every house would have its own small electrical generator, but Edison always imagined his system as similar to that of gas lighting, in which many homes and offices were served by one central plant. Lighting gas was produced by distilling coal, then trapping the resultant gas, purifying it, and storing it in vast reservoir tanks. Big main lines carried the gas from the reservoirs under the streets, with smaller pipes branching off into homes and businesses and terminating in lighting fixtures. By turning a key on the fixture, customers opened a valve and released gas, which they then lit with a match. Similarly, Edison imagined that large electrical generators in a central station would produce current, which would then be carried under the streets through copper wires and into homes and offices.2
When first introduced in American cities in the 1840s and 1850s, gaslight marked a big improvement over the reeking whale-oil lamps and tallow candles commonly in use. When the electric lamp came along, its advantages over gaslight seemed similarly obvious: Electric lamps produced less heat and no smoke, soot, or poisonous fumes. Gas lighting, though, had habit and tradition on its side, as well as the power of wealthy gas lighting companies. Cheap coal drove gas production costs down, while the gas companies—thanks to cozy relationships with city aldermen—enjoyed government-protected monopolies. By the early 1880s more than 70 percent of the consumer price of gas represented pure profit for the corporations, which meant that they would make money even if they slashed prices to compete with electric light. Gas company executives used political clout and bribery to try to keep Edison from getting a franchise for his system; they failed only because the Edison Electric investors were equally powerful. The biggest obstacle to incandescent lighting, however, was not political manipulation but economics. Gaslight was cheap, and electricity was not. Edison closely studied the costs of gas lighting and tried to eliminate all waste from his system. "Everything must be got down to the last penny," he said.3
Edison used a high-resistance filament for his bulb because it would give more light with less current, and he built the most efficient generator to date. In 1880 he made a breakthrough in circuit design that reduced the use of copper, one of the single biggest costs of an electric lighting system. Edison initially had planned to use what was known as a tree circuit to carry electricity from the central station to the lamps. All conductors of electricity offered some resistance to the current that flowed through them, which caused a portion of the electrical energy to be converted into heat and therefore wasted. Because of this lost energy, voltage dropped at the farther reaches of the circuit, and the lamps most distant from the generator were dimmer than those closest to it. To correct for this, the conductors in a tree circuit were thicker (and therefore had a lower resistance) near the generator, then tapered like tree branches as they got farther away. The initial thickness of the conductors prevented losses due to resistance, thereby keeping the lights shining with equal brightness throughout the circuit.
This worked well on a small scale, but a large system like the one planned for New York would have required the conductors to be as thick as tree trunks near the generators, and the copper costs would have been prohibitive. To skirt this problem, Edison designed a new circuit known as the feeder-and-main (see figure 2). Rather than having one thick, tapering conductor leading from the generator to serve all of the lamps, as in the tree circuit, a number of conductors (feeders) 51 branched off from the generator and carried current to local circuits (mains) that served homes and offices. The mains, in other words, were