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In the "War of Currents" era (or "War of the Currents") in the late 1880s, George Westinghouse and Thomas Edison became adversaries due to Edison's promotion of direct current (DC) for electric power distribution over the alternating current (AC) advocated by Westinghouse and Nikola Tesla. Several undercurrents lay beneath this rivalry. Edison was the consummate experimenter, but he was no mathematician. AC cannot really be understood or exploited without a substantial mathematical and mathematical physics orientation, which Tesla had. Bad feeling was exacerbated because Tesla had worked for Edison, but reported that Edison had cheated him of a promised bonus.

During the initial years of electricity distribution, Edison's direct current was the standard for the United States and Edison was not disposed to lose all his patent royalties. Direct current worked well for the incandescent lamps that were the principal load of the day, as well as for motors. From his work with rotary magnetic fields, Tesla devised a system for generation, transmission, and use of AC power. He partnered with George Westinghouse to commercialize this system. Westinghouse had previously bought the rights to Tesla's polyphase system patents and other patents for AC transformers from Lucien Gaulard and John Dixon Gibbs.

Low frequency (50 - 60 Hz) AC currents can be more dangerous than similar levels of DC current since the alternating fluctuations can cause the heart to lose coordination, inducing ventricular fibrillation, which then rapidly leads to death. High voltage DC power can be more dangerous than AC, however, since it tends to cause muscles to lock in position, stopping the victim from releasing the energised conductor once grasped. However, any practical distribution system will use voltage levels quite sufficient for a dangerous amount of current to flow, whether it uses alternating or direct current. Since the precautions against electrocution are similar, ultimately, the advantages of AC power transmission outweighed this theoretical risk, and it was eventually adopted as the standard worldwide.

Contents 1 Electric power transmission 1.1 The competing systems 1.2 Limitations 1.3 Transmission loss 2 Edison's publicity campaign 3 Niagara Falls 4 Outcome 5 References 6 See also 7 Further reading 8 External links

Edison's DC distribution system consisted of generating plants feeding heavy distribution conductors, with customer loads (lighting and motors) tapped off it. The system operated at the same voltage level throughout; for example, 100 volt lamps at the customer's location would be connected to a generator supplying 110 volts, to allow for some voltage drop in the wires between the generator and load. The voltage level was chosen for convenience in lamp manufacture; and at the time it was felt that 100 volts was not likely to present a severe hazard of electrocution. To economize on the cost of copper conductors, a three-wire distribution system was used. In the Edison system, the three wires were at +110 volts, 0 volts and -110 volts relative potential. 100-volt lamps could be operated between either the +110 or -110 volt legs of the system and the 0-volt "neutral" conductor, which only carried the unbalanced current between the + and - sources. The resulting three-wire system used less copper wire for a given quantity of electric power transmitted, while still maintaining (relatively) low voltages. However, even with this innovation, the voltage drop due to the resistance of the system conductors was so high that generating plants had to be located within a mile (1.6km) or so of the load, otherwise very large expensive conductors would be needed or else very high voltage drops (and efficiency losses) would result.

In the alternating current system, a transformer was used between the (relatively) high voltage distribution system and the customer loads. Lamps and small motors could still be operated at some convenient low voltage. However, the transformer would allow power to be transmitted at much higher voltages, say, 10 times that of the loads. For a given quantity of power transmitted, the wire size would be inversely proportional to the voltage used; or to put it another way, the allowable length of a circuit, given a wire size and allowable voltage drop, would increase approximately as the square of the distribution voltage. This had the practical significance that fewer, larger, generating plants could serve the load in a given area.

Higher voltages could not so easily be used with the DC system because there was no efficient low-cost technology that would allow reduction of a high transmission voltage to a low utilization voltage.

The direct current electric power transmission system had severe limitations that were solved by the use of alternating current. High loads of direct current could rarely be transmitted for distances greater than one mile without introducing excessive voltage drops. The three-wire distribution system provided some improvement in voltage drop and conductor sizes, but did not eliminate the problem. Edison's response to the DC system limitations was to generate power close to where it was consumed (distributed generation) and install large conductors to handle the growing demand for electricity, but this solution proved to be costly, impractical and unmanageable.

Direct current could not easily be changed to higher or lower voltages. This meant that separate electrical lines had to be installed in order to supply power to appliances that used different voltages, for example, lighting and electric motors. This led to a greater number of wires to lay and maintain, wasting money and introducing unnecessary hazards. A number of deaths from the Great Blizzard of 1888 were attributed to collapsing power lines that cluttered cities running DC power grids.^{[citation needed]}

Alternating current could be transmitted over long distances at high voltages, at lower current for lower voltage drops (thus with greater transmission efficiency), and then conveniently stepped down to low voltages for use in homes and factories. When Tesla introduced a system for alternating current generators, transformers, motors, wires and lights in November and December of 1887, it became clear that AC was the future of electric power distribution, although DC distribution was used in downtown metropolitan areas for decades thereafter.

The advantage of AC for distributing power over a distance is due to the ease of changing voltages with a transformer. Power is the product of voltage × current (P = VI). For a given amount of power, a low voltage requires a higher current and a higher voltage requires a lower current. Since metal conducting wires have a certain resistance, some power will be wasted as heat in the wires. This power loss is given by P = I²R. Thus, if the overall transmitted power is the same, and given the constraints of practical conductor sizes, low-voltage, high-current transmissions will suffer a much greater power loss than high-voltage, low-current ones. This holds whether DC or AC is used. However, it was very difficult to transform DC power to a high-voltage, low-current form efficiently, whereas with AC this can be done with a simple and efficient transformer. This was the key to the success of the AC system. Modern transmission grids use AC voltages up to 765,000 volts.

(It should be noted that semiconductor technology has altered the economics of DC voltage conversion. Though AC predominates, there exist a number of high-voltage DC transmission lines throughout the world today.)

Edison went on to carry out a campaign to discourage the use of alternating current. Edison personally presided over several AC-current-driven executions of animals, primarily stray cats and dogs, to demonstrate to the press that alternating current was more dangerous than his system of direct current.^{[citation needed]} Edison's series of animal executions peaked with the electrocution of Topsy the Elephant. He also tried to popularize the term for being electrocuted as being "Westinghoused."

Edison opposed capital punishment, but his desire to disparage the system of alternating current led to the invention of the electric chair. Harold P. Brown, who was being secretly paid by Edison, constructed the first electric chair for the state of New York in order to promote the idea that alternating current was deadlier than DC. ^[1]

When the chair was first used, on August 6, 1890, the technicians on hand misjudged the voltage needed to kill the condemned prisoner, William Kemmler. The first jolt of electricity was not enough to kill Kemmler, and left him only badly injured. The procedure had to be repeated and a reporter on hand described it as "an awful spectacle, far worse than hanging." George Westinghouse commented: "They would have done better using an axe."

Experts announced proposals to harness Niagara Falls for generating electricity, even briefly considering compressed air as a power transmission medium. Against General Electric and Edison's proposal, Tesla's AC system won the international Niagara Falls Commission contract. The commission was led by Lord Kelvin and backed by entrepreneurs such as J. P. Morgan, Lord Rothschild, and John Jacob Astor IV. Work began in 1893 on the Niagara Falls generation project and Tesla's technology was applied to generate electric power from the falls. It took five years to complete the facility.

Some doubted that the system would generate enough electricity to power industry in Buffalo. Tesla was sure it would work, saying that Niagara Falls had the ability to power the entire eastern U.S. On November 16, 1896, electrical power was sent from Niagara Falls to industries in Buffalo from the hydroelectric generators at the Edward Dean Adams Station. The hydroelectric generators were built by Westinghouse Electric Corporation using Tesla's AC system patent. The nameplates on the generators bore Tesla's name.

Although Tesla also set the 60 Hertz standard for North America, the initial installation at Niagara was 25 Hz in anticipation of long-distance transmission to Toronto. The eventual conversion to 60-Hz lasted into the 1950s, costing Ontario Hydro dearly as it replaced all motor-driven and transformer-dependent appliances in the Toronto area.

AC replaced DC for central station power generation and power distribution, enormously extending the range and improving the safety and efficiency of power distribution. Edison's low-voltage distribution system using DC ultimately lost to AC devices proposed by others: primarily Tesla's polyphase systems, and also other contributors, such as Charles Proteus Steinmetz (of General Electric). Tesla's Niagara Falls system was a turning point in the acceptance of alternating current. Eventually, Edison's General Electric company converted to the AC system and began manufacture of AC machines.

Many cities retained their DC networks. For example, central Helsinki had a DC network until the late 1940s. A mercury arc valve rectifier station would convert AC current for the downtown DC network. New York City's electric utility company, Consolidated Edison, continued to supply direct current to customers who had adopted it early in the twentieth century, mainly for elevators. In January, 2005, Consolidated Edison announced that it would cut off DC service to its remaining 1600 customers (all in Manhattan) by the end of the year. New York City's Subway continues to be powered by DC.

High voltage direct current (HVDC) systems are used for bulk transmission of energy from distant generating stations or for interconnection of separate alternating-current systems. These HVDC systems use solid state devices that were unavailable during the War of the Currents era. Power is still converted to and from alternating current at each side of the modern HVDC link. The advantages of present HVDC over historic AC systems for bulk transmission include higher power ratings for a given line (important since installing new lines and even upgrading old ones is extremely expensive) and better control of power flows, especially in transient and emergency conditions that often lead to blackouts.

Direct current systems are still universally used in vehicles for engine starting, lighting, ignition, and battery charging. +12V DC is the most common standard in automobiles, though the industry has announced plans to move to +36V DC (nominally 42 volts at the bus) to reduce wire size requirements as more devices classically driven directly by the engine become all-electric, such as engine valves and air conditioning compressors, and new features such as heated windshields are added. 36 volts was chosen because it is a margin below the highest safe voltage for accidental contact by personnel. Note that even on the scale of a single vehicle, the considerations of voltage drop and conductor size impel use of a higher voltage to meet higher load demands. Prior to the 1950s, vehicles used a 6-volt system; the conversion to 12-volt was made for essentially the same reasons.

Hybrid vehicles use banks of batteries and brushless DC motors to supplement the power of an internal combustion engine, improving both the fuel consumption and emissions performance of the vehicle.

Small "off grid" isolated power installations using intermittent sources such as solar power, micro-hydro and wind turbines use DC at 12, 24 or 48 volts and store energy in battery banks. Low-voltage lamps and appliances can be directly driven at the battery voltage, while standard AC electrical appliances can be powered with inverters that convert DC to AC.

Most telephone transmission and switching installations distribute DC power internally so that local battery banks can instantly assume the loads should external power sources fail. -48V DC is the usual standard, though much cellular telephone radio equipment runs on +24V DC. This practice is followed in some Internet server and switching centers, especially those co-located with telephone equipment, though the development of the uninterruptible power supply has made it easier to use conventional AC-powered equipment in such critical applications.

Computer systems generally operate with DC power (a computer power supply converts AC to DC in most common applications). Some server farm engineers also prefer to deploy strictly DC power systems, arguing that doing so can improve heat efficiency and increase supply reliability.

Recently the British company Moixa Energy have called for this debate to be re-opened. Arguing that since modern consumer loads in this Century are increasingly more efficient requiring only low power DC for everyday consumer electronics, batteries in phones/portable devices/gadgets or computer equipment, they can all be powered in DC by a local renewable energy source such as wind/solar micropower generator a few metres away on a domestic roof. The current demand context and options for easier local production thus avoids the distance transmission problems that plagued the original Edison system.

• Electricity
• Nikola Tesla
• Thomas Edison
• George Westinghouse
• Sebastian Ziani de Ferranti

• Tom McNichol, "AC/DC: The Savage Tale of the First Standards War".
• Westinghouse Electric Corporation, "Electric power transmission patents; Tesla polyphase system". (Transmission of power; polyphase system; Tesla patents)
• "Westinghouse Electric & Manufacturing Company, "Collection of Westinghouse Electric and Manufacturing Company contracts", Pittsburgh, Pa.

• Savings for DC Electric Customers Who Switch to AC (A ConEd statement)
• Re-thinking the Power Axiom for Domestic DC from home renewable generators in this Century
• Thomas Edison Hates Cats - AC vs DC an online video mini-history.