Wireless charging is any of several methods of charging batteries without the use of cables or device-specific AC adaptors. Wireless charging can be used for a wide variety of devices including cell phones, laptop computers and MP3 players as well as larger objects, such as robots and electric cars. There are three methods of wireless charging: inductive charging, radio charging and resonance charging. Inductive charging is used for charging mid-sized items such as cell phones, MP3 players and PDAs. In inductive charging, an adapter equipped with contact points is attached to the device's back plate. When the device requires a charge, it is placed on a conductive charging pad, which is plugged into a socket. Radio charging is used for charging items with small batteries and low power requirements, such as watches, hearing aids, medical implants, cell phones, MP3 players and wireless keyboard and mice. Radio waves are already in use to transmit and receive cellular telephone, television, radio and Wi-Fi signals. Wireless radio charging works similarly. A transmitter, plugged into a socket, generates radio waves. When the receiver attached to the device is set to the same frequency as the transmitter, it will charge the device's battery. Resonance charging is used for items that require large amounts of power, such as an electric car, robot, vacuum cleaner or laptop computer. In resonance charging, a copper coil attached to a power source is the sending unit. Another coil, attached to the device to be charged, is the receiver. Both coils are tuned to the same electromagnetic frequency, which makes it possible for energy to be transferred from one to the other. The method works over short distances (3-5 meters). The idea of wireless power transmission is not new. In 1899, Nikola Tesla wirelessly transmitted 100 million volts of electricity 26 miles to light 200 bulbs and run an electric motor. However, at that time direct current (DC, which is the wired method) and alternating current (AC) were competing technologies. DC, backed strenuously by Thomas Edison, emerged the winner