ANYONE whose mobile phone has ever run out of juice—which means, these days, more than half the world's population—will like the idea of getting electrical power out of the air. The notion is far from new. A little over a century ago, the inventor Nikola Tesla drew up ambitious plans to transmit electrical power without wires. He carried out a series of experiments in which electric lights were illuminated via electrostatic induction, by connecting them to metal sheets suspended in a strong electric field produced by a distant transmitter. In 1898 he proposed a “world system” of giant towers that would form both a global wireless communications network and a means of delivering electricity over large areas without wires.

The construction of the first such tower, the Wardenclyffe Tower, on Long Island, began in 1901. Tesla's backers included the financier J.P. Morgan, who invested $150,000. But before the tower was completed, Morgan and the other backers pulled out. They worried that the delivery of electricity through the air could not be metered, and there would be nothing to stop people from helping themselves.

But has Tesla had the last laugh after all? Today several firms—including Fulton Innovation, eCoupled, WiTricity and Powercast—are pursuing various technologies that deliver electrical power without wires (though over shorter distances than Tesla had in mind). WiTricity has demonstrated the ability to send enough energy across a room to run a flat-screen television using its approach, called “resonant magnetic coupling”. This is different from Tesla's approach, but the firm's founders have acknowledged his pioneering work.

In the long run, however, it may be Morgan who is vindicated, as researchers find ways to pull power out of the air without paying for it—a technique known as “energy scavenging” or “energy harvesting”. It is already possible to power small electronic devices, such as wireless sensors installed in buildings and industrial machinery, using a dedicated microwave transmitter nearby. The sensors pick up the microwaves with an antenna and convert the signal into electrical energy. But as power requirements drop and energy-scavenging technology improves, it will become increasingly practical to power these and other devices using just “ambient” energy—the sea of existing radio waves produced by television, radio and mobile-phone transmitters.

It sounds too good to be true. “There is something magical about it,” says Joshua Smith, a principal engineer at Intel's research centre in Seattle. But the science is sound, he says. Last year Dr Smith and Alanson Sample, a researcher at the University of Washington in Seattle, powered a small humidity and temperature sensor using nothing more than the energy gleaned from a television station 4.1km (2.5 miles) away. With their receiver tuned specifically to pick up signals from this one megawatt transmitter, they were able to generate 60 microwatts of power. It does not sound like much, but it was enough to power the device and demonstrate the principle. In recent weeks Dr Smith and Dr Sample, working with Scott Southwood, another researcher at the University of Washington, have built a weather sensor that measures temperature and light levels and sends a packet of data every five seconds by radio. It is entirely powered by ambient energy.

Ambient radio waves have largely been ignored as a potential power source until recently, because the power of a broadcast radio signal rapidly decreases with distance. That is not to say that radio waves cannot pack a punch from a distance. Advocates of “satellite solar power”, for example, dream of beaming gigawatts of solar power down to Earth from geostationary satellites more than 35,000km up. The same approach has been used in ground-based experiments to beam one kilowatt of power over a distance of several kilometres, notes Peter Fisher, a physicist at the Massachusetts Institute of Technology. But ambient radiation is much weaker.

One way to address this problem is to harvest radiation from multiple sources. Last year Nokia, the world's largest handset-maker, raised eyebrows with research showing that this approach could scavenge nearly 100 times as much energy as Dr Smith's approach. Markku Rouvala, an engineer at Nokia Research Centre in Cambridge, England, harvested as much as 5 milliwatts of power using a “wide band” receiver capable of mopping up radio signals between 500MHz and 10GHz—including radio, TV, Wi-Fi and mobile-phone signals—from nearby transmitters. It takes at least 20 milliwatts to keep a mobile phone operating in standby mode, but Nokia hopes that power scavenging might eventually deliver 50 milliwatts, enough to trickle-charge a phone.

At the Consumer Electronics Show in January, RCA showed off a gadget designed to harvest energy from nearby Wi-Fi transmitters, which can then be used to recharge a mobile phone. RCA says it plans to launch the device, dubbed Airnergy, later this year.

The first devices to be powered entirely by ambient energy are likely to be sensors, calculators and clocks. But the hope is that music-players, e-readers and mobile phones will eventually follow, says Dr Smith. There are other means of harvesting ambient energy, from vibrations, movement or heat. But the attraction of radio waves is that they are pretty much everywhere. It's like recycling energy, says Dr Fisher. “It's energy that's around, and is not doing anything else,” he says.