1. Introduction

In 1825 William Sturgeon invented the electromagnet, a conducting wire wrapped around an iron core. The principle of EM induction — that a changing magnetic field can induce an electrical current in an adjacent wire — was discovered by Michael Faraday in 1831. Combining these two discoveries, Nicholas Joseph Callan was the first to demonstrate the transmission and reception of electrical energy without wires. Callan’s 1836 induction coil apparatus consisted of two insulated coils — called the primary and secondary windings — both placed around a common iron core. A battery intermittently connected to the primary would ‘induce’ a voltage in the longer secondary causing a spark to jump across its free terminals.

In an induction coil or electrical transformer, which can have either an iron core or an air core, the transmission of energy takes place by simple electromagnetic coupling through a process known as mutual induction. With this method it is possible to transmit and receive energy over a considerable distance. However, to draw significant power in that way, the two inductors must be placed fairly close together.

If resonant coupling is used, where inductors are tuned to a mutual frequency, significant power may be transmitted over a range of many meters.

In 1864 James Clark Maxwell mathematically modeled the behavior of electromagnetic radiation. Some early work in the area of wireless transmission via radio waves was done in 1888 by Heinrich Hertz who performed experiments that validated Maxwell’s mathematical model. Hertz’s apparatus for generating electromagnetic waves is generally acknowledged as the first radio transmitter. A few years later Guglielmo Marconi worked with a modified form of the Hertz-wave transmitter, the main improvement being the addition of an elevated conductor and a ground connection. Both of these elements can be traced back to the 1749 work of Benjamin Franklin and that of Mahlon Loomas in 1864.

Nikola Tesla also investigated radio transmission and reception but unlike Marconi, Tesla designed his own transmitter — one with power-processing capability some five orders-of-magnitude greater than those of its predecessors. He would use this same coupled-tuned-circuit oscillator to implement his conduction-based wireless energy transmission method as well. Both of these wireless methods employ a minimum of four tuned circuits, two at the transmitter and two at the receiver.

As wireless technologies were being developed during the early 1900s, researchers further investigated these different wireless transmission methods. The goal was simply to generate an effect locally and detect it at a distance. Around the same time, efforts began to power more significant loads than the high-resistance sensitive devices that were being used to simply detect the received energy. At the St. Louis World's Fair (1904), a prize was offered for a successful attempt to drive a 0.1 horsepower (75 W) air-ship motor by energy transmitted through space at a distance of least 100 feet (30 m).

Except for RFID tags, wireless power transmission over room-sized or community-sized distances has not been widely implemented. Rightly or not, it has been assumed by some that any system for broadcasting energy to power electrical devices will have negative health implications. With focused beams of microwave radiation there are definite health and safety risks. Considering the hazards associated with powerful radiation, the physical alignment and targeting of devices to receive the energy beam is of particular concern. However with the use of resonant coupling, wavelegnths produced are far lower making it no more dangerous than being exposed to radio waves.