In 1891, Nikola Tesla was one of the first people to demonstrate the concept of wireless power transmission, using high-frequency alternating current to illuminate gas-discharge lamps.

Tesla's dream was that one day, electricity could be beamed to homes, in much the same way that radio waves are.

One hundred and twenty years later, his dream has yet to become a reality, so what's the problem exactly?

While wireless charging does exist and can be found in devices such as electric toothbrushes anything that requires more than a few watts of power is still reliant on the wall socket for power delivery. There has been some progress in wireless charging for other portable electronics, too, but they systems aren't cheap and they have several drawbacks.

### How does wireless charging work?

To appreciate the practical difficulties in transmitting power without wires, it helps to know a little about how electricity works. When an electrical current flows down a conductor, it generates a magnetic field, orientated at right angles to the conductor.

By creating a coil, the magnetic field is amplified and if a second coil is placed within the magnetic field of the first, then an electric current will be generated in the second coil, a process known as induction.

However, because the size of the magnetic field is proportional to the energy of the current running through the coil, and the fact that inductance over distance is a fairly inefficient transfer method, the two coils have to be placed in close proximity.

In an electric toothbrush, for instance, the two coils are less than 10mm apart. In order to increase the distance between the coils, both the size of the coils and the amount of current flowing through them, has to be significantly increased, although because the magnetic fields radiate in all directions, efficiency decreases.

### Is increased resonance the answer?

One way to increase the efficiency and distance over which induction can occur, is to use resonance. Every object has a frequency at which it will naturally vibrate, called its resonant frequency. Researches at MIT discovered that if you enable the coils and fields to resonate at the same frequency, it increases the efficiency of the induction and were able to demonstrate this principle by using resonating coils to power a light bulb, over a distance of two meters.

With this sort of distance, the idea of being able to walk into a room and whatever gadgets you are carrying are immediately able to receive power from a transmitter buried in the wall or ceiling starts to gain some traction. Unfortunately, even though MIT demonstrated the principle nearly six years ago, the technology is still very much in the development stage.

Using larger induction coils is one way in which to increase transmission distance. In the MIT experiment, for instance, the coils were 60cm in diameter, but only about 45 per cent of the power was transmitted at two meters. With portable electronics, their size and the amount of free space within the casing is a major limiting factor.

An electric toothbrush is only used for a few minutes a day and spends the rest of the time being charged, so can have quite small coils. However, a smartphone has a very high capacity battery and using a standard charger, needs to achieve full charge in one or two hours.

### Charging up vehicles

One area where the size of the coil doesn't really matter is in vehicles. Using specially built inductive roadways, trials have been run which enable an electric car or bus to receive power as it travels along the road. Wireless charging points built into bus stops and parking bays have also been successfully used to recharge on-board batteries, but it's still less efficient than physically plugging a cable in.

WiTricity is one company that markets wireless charging solutions for the automotive market. The company has also demonstrated its inductive resonance technology wirelessly powering a television as well as a number of mobile phones, is supplying its technology to OEMs and believes the first products should be on the market this year.

There already some products on the market, such as Duracell's Powermat, which doesn't use the resonance technique, so is much shorter range. In addition, devices such as mobile phones don't yet have induction coils built in and so have to be fitted with special cases containing the necessary circuitry.

However, if there's one sign that a technology is becoming more mainstream, it's when car manufacturers start to adopt it. Chrysler has announced that in 2013, its Dodge Dart car will have the option for a wireless charging bin. As devices to be charged will require special sleeve or cases, it's not clear if this is a bespoke Powermat solution, or something else, but the option to do away with a wired cigarette lighter adapter is certainly a welcome move. The \$200 price tag may be a bit much to swallow though!

It seems that the technology still has quite a way to go, before it becomes an attractive proposition. Duracell has been refining the Powermat technology and has a vision where tables in bars or cafes have embedded wireless charging points. A lot of people leave their phone on the table when they are out socialising, so why not top up the battery while you're at it? However, so long as you need to add a case of sleeve to your device, the appeal of wireless charging is limited.

Ironically, some phones such as the Samsung S3, already contain part of the technology needed for wireless charging and it's even built into the battery. RFID, or Near Field Communication (NFC) uses very similar principles, in that a coil in the phone's battery, induces a current in the chip that you are trying to read, which then has enough power to transmit back the required information.

In June of this year, Intel demonstrated wireless charging between an all-in-one PC and Samsung mobile phone. Intel hopes to bring laptops featuring the technology to market in 2013, but for the concept to really work, phone and gadget manufactures also need to embrace the concept.

So far only a few manufactures, such as HTC, Samsung and LG are consortium members and the technology is expensive to incorporate.

So, it seems unlikely that we're going to see wireless charging becoming either affordable or more widespread for at least a year, but more likely 18-months to two years. Until then, we'll just have to carry on doing things the old-fashioned wire and put up with the tangle of wires.