While scientists at the turn of the 20th century were experimenting with the wireless transmission of information, like radio, Croatian-born inventor Nikola Tesla had a grander vision (Fig. 1). He imagined the wireless transmission of power—to supply "light, heat, or motive power anywhere—on sea, or land, or high in the air," he told The New York Times in 1904.
With the help of the October 2006 film The Prestige, where a fictionalized version of the physicist helps an aspiring illusionist develop a magic trick, it's rumored that Tesla was able to light a field of 200 incandescent bulbs from a power source 25 miles away during early experiments at his Colorado Springs laboratory.
No one's sure that happened. Yet Tesla did hold several patents relating to the wireless transmission of energy, part of his wireless "World System," which would send both information and electricity around the globe.
Over time, Tesla's ideas were met with some skepticism as he became reclusive and made bizarre claims like being able to communicate with Mars. Even though the press came to rely on him solely for scientific prophecy, much of Tesla's vision for wireless power transfer is coming into fruition today, through startup companies and research institutions.
The most widespread use of wireless power transfer, albeit on a very small scale, is in passive RFID tags. The tag has no power source, so it relies on the electrical current of an incoming radio frequency signal to power up its CMOS chip and transmit a response.
Bigger wireless-power initiatives debuted at the 2007 Consumer Electronics Show with companies like Powercast and Fulton Innovation's eCoupled technology. A Powercast transmitter can send juice up to 10 feet over RF signals. A receiver that's either built in or attached with an adapter converts the waves into electricity to charge small devices like phones or cameras.
Rather than radio frequency, eCoupled coils transfer energy through inductive coupling, which relies on a shared electromagnetic field (see "Charge Your Gadgets Without Plugging Them In."). Visteon has already used eCoupled induction coils to create wireless cell-phone and MP3 player chargers, which are expected to be released later this year.
While these applications are practical, researchers at the Massachusetts Institute of Technology have come much closer to Tesla's grander vision. At the same time that The Prestige was released last fall, the researchers—led by professor Marin Stoljacic, a great admirer of Tesla's—outlined a way to juice bigger devices without wires. By June, the team lit a 60-W light bulb from a power source seven feet away (Fig. 2).
MIT's "WiTricity" (for "wireless electricity") also relies on magnetic induction to transfer power, but with an added component—resonance. All objects resonate. When two objects resonate at the same frequency, they exchange energy efficiently. It's similar to how a wine glass bursts if it resonates in tune with an opera singer's voice.
The team used two copper coils. The sending coil gave off a non-radiative magnetic field oscillating at megahertz frequencies that resonated in unison with the receiving coil. The coils exchanged energy solely with each other, and any power not picked up by the receiving coil remained bound to the area of the sending coil. Power didn't radiate into the environment, and objects placed between the coils didn't hamper the energy transfer.
The researchers acknowledged that the transfer seems similar to standard magnetic induction used in transformers today, but they also said that such methods can only send power over short distances. Resonant coupling enables magnetic induction to work over greater distances.
And to those who argue Tesla already demonstrated resonant coupling in wireless power transfer, the team wrote in its paper that Tesla's incarnations, like Tesla coils, radiated energy inefficiently in all directions. Stoljacic and his team intend to demonstrate greater efficiency than the 40% that their experiment achieved.
For large-scale wireless power transfer like his "World System," Tesla never intended to use coupled resonances. His famous Wardenclyffe Tower, a sprawling 180-foot structure based in Shoreham, Long Island, was to send information and electricity globally via the earth as a giant conductor.
After he sent the first wireless telegraph from Virginia to Hawaii in 1915, Tesla told The New York Times that the "transmission through the earth with the proper apparatus is not more difficult than the sending of a message on a wire strung across a room. This wonderful property of the planet that, electrically speaking, is through its very bigness small, is of incalculable significance for the future of mankind."
But Wardenclyffe never did transmit a telegraph, let alone electricity. As brilliant as Tesla was, he couldn't balance a checkbook. He eventually had to mortgage the Wardenclyffe property to the Waldorf-Astoria Hotel for back rent payments.
The tower was sold for scrap metal in 1917, more than 15 years after its initial construction. Eventually, the accompanying building was purchased by Peerless Photo Products, which was later bought out by AGFA.
Today, AGFA and the New York State Department of Environmental Control (DEC) are conducting a cleanup of photo chemicals dumped on the site over the years. Once cleanup is complete, two non-profit groups—Friends of Science East and the Tesla Wardenclyffe Project—hope to turn the site into the Tesla Museum and Science Center at Wardenclyffe.
They want to raise awareness of Tesla's vital contributions in electrifying society, like setting up the first power generator at Niagara Falls and inventing the concept of alternating current. Outside of engineering circles, The Prestige was the first time many people heard Tesla's name. With recent advances in wireless power transfer, Tesla might finally be recognized as an electricity celebrity.