AC vs. DC - the Battle of the Currents

Plug in an appliance at home and the device is energized by alternating current (AC). Yet for the first eighty years of the electrical age, the electrical current of choice was Volta's direct current (DC). The story of how we came to make the choice of one current over the other tells of greed and political intrigue and jealousy rather than sound scientific principles. The lead characters in this drama are icons of American industry. But let's begin at the beginning.

Volta's "pile" of zinc and silver plates separated by slices of leather soaked in brine was the first source of a steady current moving from one metal to the other and caused considerable interest because the charges could carry energy and because the charges could be made to move over a longer period of time rather than be discharged all at once. Scientists began to experiment with this new current just to see what it could do. Humphrey Davy soon found that DC current could breakdown substances that had heretofore been thought to be elements. It was Davy who first identified oxygen and hydrogen as the component elements.


It was mentioned earlier in the section on electrodynamics that a generator is a device that converts mechanical energy into electrical energy when a coil of wire is rotated in a magnetic field. The same device can produce an alternating current in a circuit, or it can produce a pulsating direct current by collecting the current with a special device called a split ring commutator. Any mechanical source of energy can be used--wind, water, and steam pressure come to mind. (The word "power" used in the context of this discussion is defined to be the amount of energy delivered to, or drawn from, a generator per unit time. Power can be measured in Watts or horsepower, among other units.) When power is supplied to the generator, the device converts the power into its electrical form, losing some along the way due to system inefficiencies. When the energy is in its electrical form, power manifests itself as the product of voltage (energy per unit charge) times current (charge per unit time). P = VI. The operational voltage output of a generator is determined by the size of the coil of wire and the speed at which the coil is rotated. Thus, there is a practical upper limit for the output voltage of a generator due to the centrifugal effects of any object spinning at high speeds. A source of mechanical energy supplies power at a rate, P. The generator coil rotates at a particular rate, causing the output voltage of the generator to be V. Thus, by setting the values of P and V, the value of the output current I is determined.
Nothing in life is ever simple, it seems. As a current passes through a wire, some of the electrical energy carried by the charges will be lost to heat energy. We find empirically that the heat lost by a wire follows this relationship: heat loss = I^2R. Thus, a generator producing current at a low voltage and high current is likely to have much of its output energy lost to heat as it is carried along the wire. This may be acceptable if the wire is a filament in a lamp. It is unacceptable if the wire is a transmission line linking the generator to the consumer. In addition to high energy losses, the wires connecting generator to consumer would have to be very thick and would cost a lot to install.
Edison staked the future of his empire on the distribution of electricity by direct current. He recognized that the heating loss that manifested themselves during the delivery of energy was a part of doing business. He envisioned an America electrified by DC power wherein every neighborhood would have its own power station and every building would draw its energy from the central plant. This plan worked satisfactorily in a densely populated New York City and in other metropolitan areas. In smaller towns, it was clear that only those buildings situated near a power station could be electrified.
Alternating current provides a different picture for the American landscape. Faraday had shown in the 1830’s that if two coils of wire were in close proximity to one another, and one wire carried and alternating current, a current could be established in the second coil, even though there was not electrical connection between the coils. Faraday also noted that the ratio of the voltage in the second coil (called appropriately enough, the secondary) to that in the first coil (the primary) was the same as the ratio of the number of turns of wire in the secondary to that in the primary. This device is called a transformer and was developed for this purpose during the decades following its discovery. Thus, if power coming from a generator is sent to a so-called step-up transformer, say with a winding ratio of 10:1, the voltage coming from the secondary coil is increased by a factor of 10. But P = VI suggests that if V goes up by a factor of 10, I goes down by a factor of 10. HL = I^2R suggests that if I is reduced by a factor of 10, heating losses are reduced by a factor of 10^2. But power cannot be used safely at these high voltages. If a second transformer is used at the consumer end of the circuit, this time with primary and secondary coils reversed, the voltage can be stepped down to safer values. In this scenario, the long wire runs through remote areas. Here, the wires a placed on high poles away from trees and buildings. Then, at a regional substation in a town, the voltage is stepped down to more nearly usable values before being delivered to the customers, the voltages are kept low near the generator and consumer so that thick insulation for the wire is not needed.

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Westinghouse learned of Tesla’s defection from the Edison camp and offered him employment. He had already acquired the American patent rights to alternating current distribution from Gaulard and Gibbs, a pair of European inventors. In time, he bought from Tesla the patent rights to the polyphase motor to complete the package of apparatus that he needed to compete with Edison. Now he needed a place to test the system. He installed his first system in Great Barrington, Massachusetts and installed another in Greensburg, Pennsylvania. While successful, these were relatively small projects. Westinghouse needed a home run.
The opportunity he needed came in 1887 when the city officials in Buffalo announced a project to build a power station at Niagara Falls. They offered a competition open to inventors of the world, with a prize of $100,000, for a successful plan to bring electricity to the city. While some twenty proposals were received, Edison and Westinghouse were the two leading contenders. Westinghouse proposed his alternating current system with high voltage lines stretching more than twenty miles from the city. Edison envisioned a canal more than two miles long along which power stations would be built to supply individual factories constructed there. The battle between these two industrial titans was fierce. Edison hammered at the idea that the high voltages of AC power were lethal. He established a traveling road show where his employees went from town to town with a large metal grid that was to be connected to an AC generator. They would routinely empty the local dog pound and tether the animals to the grid. Zap! Edison even went so far as to electrocute a circus elephant. The Edison camp was bolstered by the news that in August 1890, in Auburn State Prison, a convicted murderer, William Kemmler, became the first person to be executed by electrocution. The deed was not done which much finesse and eye-witness accounts told of how the man suffered.
All Westinghouse could do was point to his smaller American demonstration projects and show that there had been no fatalities to date. The event that tipped the scales in favor of alternating current came in 1891 at the International Electrical Exhibition in Frankfurt, Germany. There, a demonstration project caused AC electricity to be sent a distance of 110 miles at 77% efficiency and with no compromise in citizen safety. In the end, the decision to electrify Buffalo went to the Westinghouse group. In 1893, Westinghouse won the rights to light the Colombian Exposition in Chicago. Again, the project was carried off safely and efficiently.
To a smaller company than Edison Electric Company, losing such a pitched battle might have spelled financial ruin. Indeed, the Edison group found itself on the outside looking in, having actively shunned AC technology for so long. Westinghouse held some very important patents in the market place and Edison would be at a disadvantage until those patents expired. His salvation was a device called a rotary converter, a DC generator operated by an AC motor. This device, using alternating current from the neighborhood power line, could provide direct current in a building to run Edison-manufactured appliances. There were other inventions yet to come in the Edison repertoire, inventions of the phonograph and motion picture projector and improvements to the telephone. But his company suffered as the electrical industry raced away in a whole new direction than the "Wizard of Menlo Park" had anticipated.
Science and technology, and the men and women who do science and technology, are often labeled as logical, analytical and otherwise devoid of passion. The story presented in these few pages belies those images. Science is carried out by real people who try to balance high intellectual ideals with fundamental human frailties. In this regard, science is every bit as interesting to study as are history or music or literature.

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page last edited 12/24/05