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Direct current also did its work well in sending low voltages over short distances, but it had limitations. First, after a half mile or so, the current quickly diminished and could not be bolstered without costly outlay for thick copper wiring. Second, although direct current could adequately serve electric light customers by delivering a steady 110 volts, more powerful currents to run motors couldn't travel over the same lines. In addition to these intrinsic problems, negotiations for central stations were often complex, since many parties with differing interests would have to come to an agreement. For all of the initial success of Pearl Street, by the end of 1884 Edison had built only eighteen central stations (compared with hundreds of isolated systems that electrified individual homes and businesses).

The true threat to Edison's system proved to be alternating current stations, which sent high-voltage current over wires to transformers that stepped down the power to a lower voltage before delivering it to individual homes and businesses. Alternating current could accommodate different voltages, so the system could power both lights and motors, and the stations could send, via thin copper wires, a steady, strong power supply farther than the half-mile radius of direct current systems. Alternating current systems could expand outward as growth warranted.

Perhaps no one understood the advantages of alternating current more than George Westinghouse, who formed Westinghouse Electric Company in Pittsburgh in 1886 and subsequently contracted with inventor Nikola Tesla to help develop alternating current systems for his company. Tesla: tall and lean, possessing intense blue eyes, hypersensitive to the sun and to the experience of passing under a bridge, which caused pressure on his skull. "I would get a fever from looking at a peach, and if a piece of camphor was anywhere in the house, it caused me the keenest discomfort," he once said. "When a word was spoken to me, the image of the object it designated would present itself vividly to my vision and sometimes I was quite unable to distinguish whether what I saw was tangible or not." He seemed to live in a fever and quieted his mind by counting his own footsteps during his frequent walks. However much this fever might have been a burden, it was also essential to his creations. He could build machines entirely in his head, down to the smallest details, understand how they worked, and know how they needed to be improved. He could revise them without ever setting them down on paper or making a model.

After immigrating to the United States in his twenties, Tesla worked briefly for Edison, who never seemed to truly acknowledge his genius and refused to pay him a promised bonus, after which Tesla left Edison's employ. But even before their final falling-out, Tesla felt hampered by Edison's fidelity to direct current. When he brought up the subject of alternating current, Edison snapped, "Spare me that nonsense. It's dangerous. We're set up for direct current in America. People like it, and it's all I'll ever fool with."

Edison publicly condemned alternating current. "It will never be free from danger," he declared, while also claiming it was unreliable and unsuitable for central station systems. He took to calling alternating current "the executioner's current," and after promoting a series of high-profile electrocutions of animals—a dog, a calf, and finally an elephant—to prove its fatal power, he publicly supported its use for the first electric chair. The competition between Westinghouse's alternating current and Edison's direct current played out publicly and bitterly in what came to be known as the War of the Currents.

Edison at first seemed to have public anxiety on his side after a series of events in New York City reinforced the dangers of high-voltage wires. First, in the winter of 1888, a blizzard crippled the city: "The wind at times seemed to make the entire circuit of the compass,