Wireless power: Electricity in the air

More than a century ago, electrical genius Nikola Tesla performed pioneering research and development on many of the things we take for granted today, from X-rays and alternating current electricity to efficient motors and generators and the precursors of radio. But when he turned his vivid imagination to sending electricity over the air with radio waves to power all sorts of devices and appliances without cords, it was an .

Fast-forward to the present, where a company called Powercast is doing just what Tesla dreamed of: transmitting power via radio waves. "In a real sense, we're picking up where Tesla left off," says Harry Ostaffe, vice president for marketing and development at the Pittsburgh-based vendor. "We are sending power over the air for devices where it is expensive or inconvenient to change batteries."

Called power harvesting, the technique uses the company's book-size Powercaster transmitter to send either 1 or 3 watts of electricity into the air at the 915MHz radio frequency. At the receiving end, the power is pulled from the air by one of the company's Powerharvester chips, which convert RF energy to DC power.

At the moment, Powercast has two chips: One works best at close range and puts out up to 4.2 volts of continuous electricity for directly powering a very low-power device or charging a battery. The other can be used at longer distances from the transmitter to create an intermittent pulse of up to 5.25 volts for directly powering a low-power device.