Edison and the Electric Chair_ A Story of Light and Death - Mark Essig [48]
Similarly, to deliver a given amount of energy, engineers could either couple a thick (lower-resistance) conductor with a low-voltage current or couple a thin (higher-resistance) conductor with a high-voltage current capable of overcoming the resistance. Given the high cost of copper, the choice was clear: A high-voltage system with thin wires was much more economical.
Because domestic lighting required low pressures of about 100 volts, an ideal system would allow a company to transmit through slender wires at high voltages (over 1,000 volts), then lower the pressure to 100 volts for use in the home. Charles Brush, the arc lighting pioneer, was the first American to exploit the economies of high-voltage transmission. In 1882 he built an incandescent lighting system in which electricity was transmitted over longer distances at over 1000 volts, then fed into batteries near the site of consumption. Charged at high voltage, the batteries then discharged electricity at much lower voltages to operate lamps. Brush's battery system, though, was clumsy and inefficient, and it never caught on.24
In 1885 Stanley and Westinghouse began to work on a lighting plan that, like Brush's storage battery system, involved transmitting at high voltages, then lowering the voltage and distributing into homes and offices at low voltages. Their system, though, had one significant difference: Rejecting direct current, which flowed continuously in one direction, they chose to use alternating current, which changed direction back and forth many times a second.
It was a radical step. Rotating a coil of conducting wire between the poles of a magnet naturally produced alternating current, because the relationship between the coil and the north and south poles of the magnet reversed with each half turn (see figure 1). The earliest generators in the 1830s produced alternating current, but, because this current had no apparent uses at the time, instrument makers added switching devices that converted alternating-current generators into direct. From the 1830s through the 1870s, electroplating and electrochemistry—both of which required direct current—were the primary commercial use of electrical generators, and as a result most available dynamos were direct-current machines. When inventors took up electric lighting in the 1870s, they patterned their generators on the direct-current models common at the time. Charles Brush's arc lights used direct current of 1,000 or more volts, and early incandescent light systems, like Edison's, used direct current of 100 volts or so.25
In the early 1880s a few inventors realized that alternating current had one distinct advantage over direct, an advantage related to the principle of induction, discovered independently by Michael Faraday and the American scientist Joseph Henry in the 1830s. The principle stated that a variation—starting, stopping, or reversing—of the current in one coil of conducting wire will induce a current in a second coil placed in close proximity to (but not touching) the first. Direct current, which flowed continuously, without variation, produced no induction effect. But alternating current did, because it reversed direction many times a second. An alternating current in one coil of wire (the primary circuit) caused a current to flow in another coil (the secondary circuit) placed close by.26
Figure 3: A transformer consists of two distinct coils of wire wound around an iron core. The input current enters the primary coil, producing a magnetic field in the iron core that repeatedly switches on and off. The core transfers this field to the secondary coil, where it induces an output current. The degree of change in the voltage depends upon the ratio of the number of windings of the coils: This transformer steps the current up or down three times.
Induction