Wireless electricity

Where does the idea come from?
The first person to practically demonstrate that electricity could be transmitted wirelessly was Nikola Tesla at the World's Fair in Chicago in 1893. Tesla managed to illuminate two light bulbs, which had no wires connected to them, by sending a massive electrical charge through two terminals placed 15 feet apart, with the bulbs in between them. The feat astounded those who saw it, but was the result of the discovery in 1820 by Andre-Marie Ampere that electric currents create a magnetic field.

How does it work in practice?
The principle of wireless power transmission, also known as the Tesla Effect, involves running an electrical current through a primary ‘coil’ to generate a magnetic field, with a secondary coil placed within this field so a current is induced in the second coil as well. However, the magnetic field is relatively small at a low voltage so the second coil needs to be placed very close to the first one.

How efficient is it?
One of the major shortcomings with Tesla’s original apparatus was that it required a high voltage to light the bulbs and the greater the distance between the two coils, the more inefficient the process is. Tesla decided that to send electricity over greater distances, it needed to be sent using radio waves. Though he didn’t ever successfully prove that it was possible to do so, various experiments in the second half of the 20th Century showed that by using a piece of equipment called a ‘rectenna’, microwaves can be used to send electricity wirelessly and directionally, with an efficiency of up to 95 per cent.

How has the idea developed?
Once it was proved that electricity could be sent using microwaves efficiently, it was suggested that the principles of the idea could be used by spacecraft and satellites, though this theory was soon dismissed. Sending power from earth to a spacecraft or vice versa would have required monstrous transmitters, more than a kilometre in diameter, and this just wasn’t practical. But it wasn’t until recently that wireless electricity was considered for the average consumer.

In 2006 the Massachusetts Institute of Technology (MIT, http://web.mit.edu) demonstrated that it was practically possible to harness wireless electricity for use in household devices. By extending the range of the electric field generated by a current running through a coil, it was possible to send electricity wirelessly over a reasonable distance efficiently and without using microwaves. It didn’t require a clear line-of-sight between the two coils to work and the name ‘WiTricity’ was quickly assigned to this particular method of sending electricity wirelessly.

Could we see it in our homes soon?
Some products demonstrated at the Consumer Electronics Show (CES) in Las Vegas earlier this year used the principles of wireless electricity and will be available later this year, so we could all be able to see this technology in practice very soon. One of these devices, the Powermat (www.pwrmat.com) – made by the company of the same name – lets you charge up devices such as mobile phones, iPods, handheld gaming consoles and satnavs without wires. The mats themselves do need to be plugged in to the mains, but the device can simply be laid on the mat to charge up.

PowerBeam (www.powerbeaminc.com), another company in attendance at CES, showed off a number of prototypes at the exhibition, including lamps, surround-sound speakers and digital photo frames all powered by wireless electricity. PowerBeam’s method of sending power wirelessly is unique inasmuch as it uses laser beams, which could be sent over a distance of 10 metres, to charge solar energy cells attached to the wireless device. This, however, does require a direct line-of-sight between the charger and the device, so may prove to be limited in terms of practicality.

However, what you possibly don’t realise is that you may already have devices in your home that use wireless power transmission, albeit on a miniscule scale – some electric toothbrushes use the principle, as it is safer than using a direct coupling to transfer electricity in an environment such as a bathroom, where there could be water.

Are there any associated health risks?
Though there is little research in this area, there is no strong evidence to suggest that it poses a risk to health. However, there is growing concern in some quarters about ‘electrosensitivity’, with many individuals claiming that they suffer headaches and other symptoms when in close proximity to electrical devices and Wi-Fi routers, for example. Dr Ben Goldacre has written extensively about electrosensitivity in his Bad Science blog (http://tinyurl.com/instantexpert208) and gives plenty of examples of how researchers and the media have blown several cases relating to the health risks posed by electric fields, Wi-Fi networks and microwaves out of proportion.