AC vs. DC Power and the War of the Currents

Many of us don’t really understand how electricity works. It’s enough that it does work – you turn the switch, and the light comes on in the room. It may come as a surprise, then, to learn that there are actually two different kinds of electricity that we use to power the many devices in our lives. They are known as alternating current and direct current, or AC and DC (not the 70s-era rock band).

In the most simple terms, direct current only flows in one direction, while alternating current flows back and forth. For example, a flashlight runs on direct current, with the charge coming from the battery and powering the bulb. A ceiling light in your house, on the other hand, uses alternating current that is constantly changing polarity as it runs through your home’s electric system.

But why do we need two different types of electricity, and how were these dueling systems developed? The answer lies in a bitter rivalry between a pair of the most famous inventors in American history.

The Origins of Direct Current

Before the 1870s, people relied on gas lamps, candles or lanterns to illuminate their surroundings at night. There had been advances in rudimentary batteries and electric lighting, but nothing practical enough for everyday use. That all changed when Thomas Edison invented an incandescent lightbulb in 1879 that was far more reliable than anything that had come before.

With electric bulbs came the opportunity for powering houses and even entire cities, and Edison had his eyes set on cornering the burgeoning market. His bulbs ran on direct current, produced by power plants known as dynamos which used steam engines to generate the electricity. The inventor spearheaded the implementation of numerous direct current generating stations in New York City in the 1880s via his company Edison Electric, a predecessor to General Electric.­­­

Electric lights in homes and businesses were a revelation, but the use of DC electricity came with its downsides. Power came directly from the generation facility at 110 volts, and could only travel about a mile or so before it lost too much voltage. That meant using a lot of valuable real estate in the city to build power plants, while rural communities would be left out of the power revolution altogether.

Alternating Current’s Rise

One of Edison’s employees, a young man named Nikolai Tesla, had an idea to address some of the downsides of direct current. Tesla had invented a motor that generated alternating current. Alternating current is produced, appropriately enough, using an alternator that spins a magnet inside a wire coil, which creates electricity of constantly reversing polarity as the wire interacts with alternate sides of the magnetic field.

Beyond the novel form of electricity itself, the key to Tesla’s idea was transformers, or coils of differing sizes to modify the voltage of the electricity. Thanks to the power of transformers, alternating current became advantageous for large-scale generation and distribution, because the higher the voltage, the more efficient the transmission. High voltage lines are too dangerous to bring into a building, but through a transformer the voltage can be reduced to safer levels as it approaches its final destination of homes and offices.

The voltage of direct current could not be easily altered, so it proved far less useful for scaled-up operations, as you are left with the choice of either transmitting at a low, inefficient voltage, or sending dangerously high volt levels into people’s homes.

The War of the Currents

Despite the promise shown by Tesla’s inventions, Edison wasn’t interested in helping to develop the technology, so Tesla left to strike out on his own. The result was a number of patents, which he sold in 1888 to George Westinghouse, founder of Westinghouse Electric Company.

Westinghouse and Edison’s companies fought fiercely for the lucrative rights to electrify American cities in a competition dubbed the “War of the Currents.” Edison kicked off a lobbying campaign that hyped the dangers of alternating current in an attempt to prevent the proliferation of Tesla’s invention. To demonstrate that AC could literally be deadly, Edison’s employees invented an alternating current electric chair, which was used by the state of New York to execute condemned prisoners. Edison even made public demonstrations of electrocuting stray animals using alternating current in his attempts to sway the public away from the rival system.

The competition came to a head at the 1893 World’s Fair in Chicago, when Tesla won the contract to supply the event with electricity. The decisive blow came three years later when George Westinghouse used Niagara Falls to power an AC generator that brought electricity 26 miles to Buffalo in 1896. With that, alternating current had proven its utility, and it went on to dominate the electricity sector as lights came on in homes across the United States in the ensuring years and decades.

Alternating and Direct Current Generation Today

In more recent decades, technology to generate and transmit high voltage direct current, or HVDC, has come to the market, and in some cases performs more efficiently than alternating current, but AC is still the overwhelming victor in the electric grid.

Most types of power plants are designed based on the same basic principles as Tesla’s alternator, creating alternating current using a rotating magnetic field. Coal, gas and nuclear plants work by heating water and using the steam to spin the generator, while hydroelectric and wind generation facilities harness nature’s power to turn the turbines directly.

Solar panels, on the other hand, produce direct current. If the electricity is going onto the grid or to power a home’s electric system, it needs to be converted to AC first by an inverter. Otherwise, the most common sources of DC power are batteries. Relatedly, direct current is much easier to store, so as large scale battery storage proliferates in conjunction with renewable energy generation, DC has another opportunity to gain a greater foothold on the electric grid.

High voltage power lines typically carry AC electricity at around 345,000 volts, while local transmission lines are around 13,800 volts, which is still extremely dangerous for anyone who comes in contact. By the time it reaches your house, the voltage is stepped down via transformers to between 120 and 240 volts so you can safely power your electric devices and appliances.

What the Different Types of Current Mean for You

Both alternating and direct current play an important role in the average household. The appliances in your home, such as your refrigerator, washing machine and dishwasher use alternating current. In houses that aren’t on a natural gas line, most furnaces, hot water heaters, ovens and dryers run on AC as well.

But direct current has its uses. The alternating part of AC happens quickly – in the United States, the electrons change direction 60 times per second, also known as 60 Hertz. However, even though the alternation happens so rapidly, there are tiny losses of power every time the current changes direction. This is no problem for lightbulbs or other appliances that are built to use alternating current, but modern, sensitive electronics don’t fare so well with even immeasurably brief interruptions of power.

That’s why many newer devices, such as cell phone chargers, computers, and televisions use direct current, using power adapters to convert the alternating current coming from the wall outlets. The market for direct current is poised to continue to expand in the form of electric cars, which run on DC from their batteries.

Therefore, while the War of the Currents may have ended over 100 years ago, the competition between alternating and direct current to power our day-to-day lives endures.

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