For these reasons, scientists have tried to develop methods of wireless power transmission that could cut the clutter or lead to clean sources of electricity. While the idea may sound futuristic, it isn't particularly new. Nicola Tesla proposed theories of wireless power transmission in the late 1800s and early 1900s. One of his more spectacular displays involved remotely powering lights in the ground at his Colorado Springs experiment station.

Tesla's work was impressive, but it didn't immediately lead to widespread, practical methods for wireless power transmission. Since then, researchers have developed several techniques for moving electricity over long distances without wires. Some exist only as theories or prototypes, but others are already in use. If you have an electric toothbrush, for example, you probably take advantage of one method every day.

A toothbrush's daily exposure to water makes a traditional plug-in charger potentially dangerous. Ordinary electrical connections could also allow water to seep into the toothbrush, damaging its components. Because of this, most toothbrushes recharge through inductive coupling.

Photo courtesy Amazon.com Most electric toothbrushes recharge through inductive coupling.

Inductive coupling uses magnetic fields that are a natural part of current's movement through wire. Any time electrical current moves through a wire, it creates a circular magnetic field around the wire. Bending the wire into a coil amplifies the magnetic field. The more loops the coil makes, the bigger the field will be.

An electric toothbrush's base and handle contain coils that allow the battery to recharge.

If you place a second coil of wire in the magnetic field you've created, the field can induce a current in the wire. This is essentially how a transformer works, and it's how an electric toothbrush recharges. It takes three basic steps:

1. Current from the wall outlet flows through a coil inside the charger, creating a magnetic field. In a transformer, this coil is called the primary winding.
2. When you place your toothbrush in the charger, the magnetic field induces a current in another coil, or secondary winding, which connects to the battery.
3. This current recharges the battery.

You can use the same principle to recharge several devices at once. For example, the Splashpower recharging mat and Edison Electric's Powerdesk both use coils to create a magnetic field. Electronic devices use corresponding built-in or plug-in receivers to recharge while resting on the mat. These receivers contain compatible coils and the circuitry necessary to deliver electricity to devices' batteries.

© Copyright Splashpower 2006 A Splashpower mat uses induction to recharge multiple devices simultaneously.

A newer theory uses a similar setup to transmit electricity over longer distances. We'll look at how it works in the next section.

Resonance and Wireless Power

In November 2006, however, researchers at MIT reported that they had discovered an efficient way to transfer power between coils separated by a few meters. The team, led by Marin Soljacic, theorized that they could extend the distance between the coils by adding resonance to the equation.

Photo courtesy Stock.exchng A trumpet's size, shape and material composition determine its resonant frequency.

A good way to understand resonance is to think of it in terms of sound. An object's physical structure -- like the size and shape of a trumpet -- determines the frequency at which it naturally vibrates. This is its resonant frequency. It's easy to get objects to vibrate at their resonant frequency and difficult to get them to vibrate at other frequencies. This is why playing a trumpet can cause a nearby trumpet to begin to vibrate. Both trumpets have the same resonant frequency.

Research at MIT indicates that induction can take place a little differently if the electromagnetic fields around the coils resonate at the same frequency. The theory uses a curved coil of wire as an inductor. A capacitance plate, which can hold a charge, attaches to each end of the coil. As electricity travels through this coil, the coil begins to resonate. Its resonant frequency is a product of the inductance of the coil and the capacitance of the plates.

The MIT wireless power project uses a curved coil and capacitive plates.

As with an electric toothbrush, this system relies on two coils. Electricity, traveling along an electromagnetic wave, can tunnel from one coil to the other as long as they both have the same resonant frequency. The effect is similar to the way one vibrating trumpet can cause another to vibrate.

As long as both coils are out of range of one another, nothing will happen, since the fields around the coils aren't strong enough to affect much around them. Similarly, if the two coils resonate at different frequencies, nothing will happen. But if two resonating coils with the same frequency get within a few meters of each other, streams of energy move from the transmitting coil to the receiving coil. According to the theory, one coil can even send electricity to several receiving coils, as long as they all resonate at the same frequency. The researchers have named this non-radiative energy transfer since it involves stationary fields around the coils rather than fields that spread in all directions.

According to the theory, one coil can recharge any device that is in range, as long as the coils have the same resonant frequency.

The MIT team's preliminary work suggests that this kind of setup could power or recharge all the devices in one room. Some modifications would be necessary to send power over long distances, like the length of a building or a city. The team is making progress -- in June 2007, the MIT team published a paper detailing a successful demonstration of their prototype. They used resonating coils to power a light bulb over a distance of about seven feet (two meters) [Source: PhysOrg].

Other wireless power theories involve enormous distances -- like from space to the Earth. We'll look at those next.

Long-distance Wireless Power

In the 1980s, Canada's Communications Research Centre created a small airplane that could run off power beamed from the Earth. The unmanned plane, called the Stationary High Altitude Relay Platform (SHARP), was designed as a communications relay. Rather flying from point to point, the SHARP could fly in circles two kilometers in diameter at an altitude of about 13 miles (21 kilometers). Most importantly, the aircraft could fly for months at a time.

The Stationary High Altitude Relay Platform (SHARP) unmanned plane could run off power beamed from the Earth.

The secret to the SHARP's long flight time was a large, ground-based microwave transmitter. The SHARP's circular flight path kept it in range of this transmitter. A large, disc-shaped rectifying antenna, or rectenna, just behind the plane's wings changed the microwave energy from the transmitter into direct-current (DC) electricity. Because of the microwaves' interaction with the rectenna, the SHARP had a constant power supply as long as it was in range of a functioning microwave array.

Rectifying antennae are central to many wireless power transmission theories. They are usually made an array of dipole antennae, which have positive and negative poles. These antennae connect to semiconductor diodes. Here's what happens:

1. Microwaves, which are part of the electromagnetic spectrum, reach the dipole antennae.
2. The antennae collect the microwave energy and transmit it to the diodes.
3. The diodes act like switches that are open or closed as well as turnstiles that let electrons flow in only one direction. They direct the electrons to the rectenna's circuitry.
4. The circuitry routes the electrons to the parts and systems that need them.

Other, longer-range power transmission ideas also rely on rectennae. David Criswell of the University of Houston has proposed the use of microwaves to transmit electricity to Earth from solar power stations on the moon. Tens of thousands of receivers on Earth would capture this energy, and rectennae would convert it to electricity.

Stations on Earth can receive energy from the moon via microwaves.

Microwaves pass through the atmosphere easily, and rectennae rectify microwaves into electricity very efficiently. In addition, Earth-based rectennae could be constructed with a mesh-like framework, allowing the sun and rain to reach the ground underneath and minimizing the environmental impact. Such a setup could provide a clean source of power. However, it does have some drawbacks:

• The solar power stations on the moon would require supervision and maintenance. In other words, the project would require sustainable, manned moon bases.
• Only part of the earth has a direct line of sight to the moon at any given time. To make sure the whole planet had a steady power supply, a network of satellites would have to re-direct the microwave energy.
• Many people would resist the idea of being constantly bathed in microwaves from space, even if the risk were relatively low.

While scientists have built working prototypes of aircraft that run on wireless power, larger-scale applications, like power stations on the moon, are still theoretical. As the Earth's population continues to grow, however, the demand for electricity could outpace the ability to produce it and move it around. Eventually, wireless power may become a necessity rather than just an interesting idea.

Read on for lots more information about electricity, wireless power and related topics.

How MIT's Wireless Power Could Replace Cables and Outlets

MIT’s WiTricity will transmit power between a source coil, which turns grid-supplied electricity into a magnetic field, and receiving coils, which draw only the power needed to run attached devices. A lamp will draw more than a laptop or a cellphone.

Wireless power transfer is old news—more than a century ago, Nikola Tesla built magnetic coils that transmitted power without cables. But Tesla’s coils also tended to expel bursts of superhot plasma. This past June, MIT researchers announced their own coil-based breakthrough in wireless electricity—called WiTricity—that’s mercifully plasma-free.

WiTricity exploits a phenomenon known as magnetically coupled resonance, which allows two objects resonating at the same frequency to exchange energy efficiently. The team was able to wirelessly power a 60-watt light bulb using a pair of 20-in.-dia. copper coils. Drawing power from an electrical outlet, the source coil created a magnetic field, which was 7 ft. in diameter, transmitting at a frequency of 10 MHz. (Many cellphones, by com-parison, operate at 2 GHz.) Seven feet away, a receiving coil was also resonating at 10 MHz, pulling enough energy from the field to power an attached light bulb.

Because it transmits at such a low frequency, WiTricity’s magnetic field passes through intervening objects, and doesn’t appear to pose a threat to people or pets. Within five years, the team hopes to release a commercial product with better power efficiency, a larger field and more compact coils, capable of running or charging multiple devices.