The discovery came by accident. The team at MassachusettsA state in the northeast USA. University were simply working on a sensor to measure humidity in the air. "For whatever reason," says Professor Jun Yao, "the student who was working on that forgot to plug in the power."

But then, to everyone's amazement, the device started working without it.

Inside the device was a thin layer of material containing tiny tubes. Each of these tubes, known as nanowires, was one thousandth of the diameter of a human hair.

These allowed air and water molecules from the atmosphere to pass from an upper chamber to a lower chamber.

Because the tubes were so small, the water molecules bumped into the edges, creating a charge as they went through.

And as the frequency of the bumps increased, one end of the tubes became more charged than the other, creating the kind of imbalance found in thunderclouds.

A cloud, Professor Yao explains, "is nothing more than a mass of water droplets. Each of those droplets contains a charge, and when conditions are right, the cloud can produce a lightning bolt... What we've done is to create a human-built, small-scale cloud."

The team have called their invention the Air-gen - short for "air generator".

A Ukrainian family based in Lisbon have been working on the same idea. Svitlana Lyubchyk and her twin sons Andriy and Sergiy - all professors - have come up with a disc-shaped device 4cm wide.

A stack the size of a washing machine could produce 10 kilowattsA kilowatt is 1,000 watts of electrical power. a day - as much power as an average British household uses.

Making hygroelectricityProducing electric power from air. widely available could help the fight against global warming enormously. At present 84% of the world's energy comes from fossil fuels.

HydropowerAlso known as hydroelectricity or water power. When the force of fast moving water is used to create electricity. and nuclear power account for 10.7% , with  just 2.2% coming from wind power and 1.1% from solar.¹

Yes: A Professor Yao says, "Air humidity is a vast, sustainable reservoir of energy that, unlike solar and wind, is continuously available." It could revolutionise the search for clean, renewable energy.

No: Creating electricity is one thing; distributing it is another, and would need an enormous amount of infrastructure. You cannot deliver washing-machine-sized generators to every family in the world.

Or... It could, but there is a long way to go. A large amount of investment is needed, and an enormous number of factories would have to be built which would themselves use up a lot of power.