Alternating current

• 1. Introduction

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  Alternating current powers almost every home and business on Earth, yet most people have never paused to ask why. Right now, the electricity running through your walls is not flowing steadily in one direction. It is reversing, thousands of times every minute, oscillating back and forth in a sine wave that engineers first tamed in the nineteenth century. That invisible rhythm drives your refrigerator, your television, the lights above your head. So why does power arrive this way rather than as a steady stream? The answer involves a Hungarian factory, a feud between two of history's most famous inventors, a waterfall in New York, and a set of physical laws that make alternating current far better suited to long-distance travel than its rival. This is the story of the current you never see.

• 2. The Problem with Distance

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  Electrical energy is distributed as alternating current because AC voltage can be raised or lowered with a transformer, and that single capability changes everything. When power travels long distances through a wire, the wire resists the flow, and that resistance turns electricity into heat, wasting energy. The losses are proportional to the square of the current: halving the current reduces losses by a factor of four. Raising the voltage achieves exactly that, because when voltage doubles, the current needed to transmit the same power is cut in half. Power is often sent at hundreds of kilovolts along high-voltage pylons, stepped down to tens of kilovolts for regional lines, and finally reduced to somewhere between 100 and 240 volts for use inside homes. Direct current could not do this in the early days of electric power. There was then no economically workable way to step DC voltage up or down, which left direct current systems trapped: whatever voltage a generator produced was essentially the voltage delivered to the end user, with significant losses on any lengthy run.

• 3. The Hungarian Breakthrough

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  In the autumn of 1884, three engineers at the Ganz Works in Budapest changed the course of electrical history. Károly Zipernowsky, Ottó Bláthy, and Miksa Déri had been studying the open-core transformers then in use and reached a harsh verdict: those devices were incapable of reliably regulating voltage. Bláthy proposed closed magnetic cores; Zipernowsky suggested connecting loads in parallel rather than in series; Déri ran the experiments. Their joint 1885 patent applications described copper windings wrapped around a ring core of iron wires, with magnetic flux traveling almost entirely within that iron core rather than through air. The new transformers were 3.4 times more efficient than the open-core devices they replaced. The Ganz factory had already shipped what are recognized as the world's first five high-efficiency AC transformers in 1884, with specifications for the first unit recorded as 1,400 watts, 40 Hz, at a voltage ratio of 120:72 volts. In May 1885, at the Hungarian National Exhibition in Budapest, the three engineers staged a large-scale demonstration of what is widely regarded as the prototype of modern AC lighting systems, using 75 transformers in parallel to supply 1,067 incandescent Edison lamps from a generator providing 1,350 volts. Bláthy also invented the first AC electricity meter, giving utilities a way to charge for the power they sent.

• 4. The War of the Currents

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  In March 1886, Westinghouse engineer William Stanley demonstrated a lighting system in Great Barrington, Massachusetts. A Siemens generator running at 500 volts was stepped up to 3,000 volts, then stepped back down to 500 volts by six Westinghouse transformers, and the resulting power ran thirty 100-volt incandescent bulbs in twenty shops along the main street of the town. Based on that success, Westinghouse went on to build alternating current infrastructure across the United States. That expansion alarmed Thomas Edison, a committed advocate of direct current, who launched what he called the "war of the currents" in late 1887, publicly arguing that alternating current was too dangerous for widespread use. Edison's campaign did not stop AC's advance. In 1888, the induction motor arrived, filling the one gap AC systems had left open: a practical motor that ran on alternating current. Galileo Ferraris and Nikola Tesla invented it independently, with Tesla's design licensed by Westinghouse in the United States. The modern three-phase form was then developed separately by Mikhail Dolivo-Dobrovolsky and Charles Eugene Lancelot Brown in Germany, and by Jonas Wenström in Sweden. By 1893, Westinghouse built an alternating current system for the Chicago World Exposition, a showcase visible to millions of visitors.

• 5. Power Across Continents

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  The original Niagara Falls Adams Power Plant began operation in August 1895 with three two-phase generators, though it was connected to the remote transmission system only in 1896. Two days after Niagara's generators started, on the 28th of August 1895, the Jaruga Hydroelectric Power Plant in Croatia was set in operation; its generator, rated at 42 Hz and 240 kilowatts, was manufactured and installed by the Hungarian company Ganz, and the transmission line to the city of Sibenik ran 11.5 kilometers. Earlier milestones had already mapped the path: a single-phase hydroelectric plant at Willamette Falls in Oregon sent power fourteen miles downriver to Portland for street lighting in 1890; a transmission system followed in Telluride, Colorado, in 1891; and the first three-phase system was established in Frankfurt, Germany, also in 1891. Almarian William Decker completed the San Antonio Canyon Generator on the 31st of December 1892, providing power to Pomona, California, 14 miles away. Decker's 1893 design for the Mill Creek No. 1 Hydroelectric Plant near Redlands, California, incorporated 10-kilovolt three-phase transmission and established the standards for generation, transmission, and motors that the United States uses today. Meanwhile in Sweden, engineers explored transmitting power from a 45-meter waterfall at Hallsjon, Smedjebackens kommun; in 1893, a three-phase system transferred 400 horsepower across a distance of 15 kilometers, the first commercial application of that kind. The theoretical tools to analyze these systems grew in parallel: Charles Steinmetz, Oliver Heaviside, and others built the mathematical framework for AC circuit calculations, and Charles LeGeyt Fortescue published his symmetrical components method for handling unbalanced three-phase systems in 1918.

• 6. Frequency and Its Consequences

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  Most electric power today is generated at either 50 or 60 hertz, but that choice was not obvious from the start. Japan still has a mixture of both frequencies in different regions. Lower frequencies ease the design of electric motors built for demanding tasks like hoisting, crushing, and rolling, and they reduce transmission losses, which are proportional to frequency. The original Niagara Falls generators were built to produce 25 Hz power, chosen as a compromise between the needs of traction and heavy induction motors on one side and the requirement that incandescent lighting remain usable on the other, even though that frequency caused noticeable flicker. Most residential and commercial customers supplied by Niagara Falls power were converted to 60 Hz by the late 1950s, though some industrial customers at 25 Hz persisted into the early twenty-first century. At the other end of the dial, 16.7 Hz power is still used in some European rail systems, including those in Austria, Germany, Norway, Sweden, and Switzerland. Aircraft, spacecraft, military, offshore, and textile industry applications sometimes use 400 Hz, which allows lighter apparatus and higher motor speeds. Computer mainframe systems were often powered at 400 Hz or 415 Hz to reduce ripple while using smaller internal conversion units.

• 7. How Current Behaves at High Frequencies

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  Direct current flows uniformly through the entire cross-section of a wire. Alternating current does not. Because alternating current results from the acceleration of electric charge, it produces electromagnetic waves, and conductors resist electromagnetic waves, pushing the current toward the wire's outer surface. This is called the skin effect. At very high frequencies, current flows effectively only within a thin layer near the surface. The skin depth is defined as the thickness at which current density drops by 63 percent. Even at 60 Hz, the skin depth of a copper conductor is approximately 8.57 millimeters, which is why high-current conductors are typically made hollow, reducing mass and cost without sacrificing much conducting area. Because the effective cross-section shrinks, AC resistance rises above DC resistance, and more energy is lost to Ohmic heating. Engineers counter this at low to medium frequencies using Litz wire, which divides a conductor into individually insulated strands whose positions within the bundle are arranged to force more equal current distribution across the total cross-section. Litz wire is used in high-Q inductors, in windings for switch-mode power supplies, and in radio frequency transformers operating up to hundreds of kilohertz. At frequencies above roughly 5 gigahertz, even coaxial cables lose too much energy, and waveguides take over; beyond 200 gigahertz, waveguide dimensions become impractically small and fiber optics, themselves a form of dielectric waveguide, are used instead.

• 8. Origins in the Nineteenth Century

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  The first alternator to produce alternating current was built by the French instrument maker Hippolyte Pixii in 1832, based on principles established by Michael Faraday. Pixii later added a commutator to produce direct current, then the more widely favored form. The earliest recorded practical application of alternating current came from Guillaume Duchenne, inventor and developer of electrotherapy, who announced in 1855 that AC was superior to direct current for triggering muscle contractions. Alternating current technology was developed further by the Hungarian Ganz Works company during the 1870s, and then in the 1880s by Sebastian Ziani de Ferranti, Lucien Gaulard, and Galileo Ferraris. In 1876, Russian engineer Pavel Yablochkov invented a lighting system in which sets of induction coils were installed along a high-voltage AC line, connecting secondary windings to arc lamps of his own design so that the failure of one lamp would not shut down the rest. In 1878, the Ganz factory in Budapest began manufacturing equipment for electric lighting, and by 1883 had installed over fifty systems across Austria-Hungary. A bipolar open-core power transformer developed by Lucien Gaulard and John Dixon Gibbs was demonstrated in London in 1881, attracting the attention of Westinghouse; the same pair exhibited an AC system powering arc and incandescent lights along five stations of the Metropolitan Railway in London, and a single-phase multi-user distribution system in Turin, in 1884. Sebastian de Ferranti, who had been developing AC generators and transformers in London since 1882, redesigned the AC system at the Grosvenor Gallery power station in 1886, and in 1890 designed the power station at Deptford while converting Grosvenor Gallery into an electrical substation, showing how older plants could be absorbed into a unified AC supply.