Powercast is a technology that uses a low-level radio frequency (RF) electromagnetic field to recharge small electrochemical cells and batteries. Powercast can be used with low-power electronic devices such as cellular telephone s, portable media players or hearing aids. The technology is under development by Powercast LLC, based in Pennsylvania. A Powercast transmitter, also called a Wireless Power Platform, sends out a continuous RF signal when plugged into a conventional utility outlet. The AC power contained in the EM field is converted to DC inside the device to be charged by means of a small rectifier chip called the receiver. The process is similar to the way an old-fashioned "crystal set" radio receiver intercepts EM energy with a wire antenna and produces audible signals in a headset without any other source of power. The charging effect occurs slowly but most small devices containing rechargeable batteries are ready for use after being left near the transmitter overnight