Wireless transmission of electricity

· Wireless transmission of electricity
History
Wireless energy transfer, the transmission of electrical energy without wires, has been around since about 1856 in the form of mutual induction. Using induction it is possible to transmit and receive signals 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, power may be transmitted over a range of many meters. Another form of wireless energy transfer is electromagnetic radiation, such as in radio waves. For example, a fluorescent tube held near an active radio transmitter radiating more than a few watts (such as an amateur radio transmitter) will glow. The reason behind this phenomenon is similar to the physics involved in the aurora borealis.

Wireless energy transfer methods
At least four methods exist by which electrical energy can be transferred from a source to a load without the use of manmade conductors. These are:
· electromagnetic induction,
· electromagnetic radiation,
· evanescent wave coupling,
· electrical conduction.
The second method is radiative; the others are non-radiative.
Ø Electromagnetic induction
The electrical transformer is probably the simplest example of wireless energy transfer. The primary and secondary circuits of a transformer are electrically isolated from each other. The transfer of energy takes place by electromagnetic coupling through a process known as induction. (An added benefit is the capability to step the primary voltage either up or down.) The induction cooker is an example of how this principle can be used. In the induction cooker, electrical energy is wirelessly transferred into the cookware, where it is converted ohmically into heat for cooking. Electric toothbrush chargers work in a similar way. The main drawback to induction, however, is the short range. The receiver must be in relatively close proximity to the transmitter (or “induction unit”) in order to inductively couple with it.
Ø Electromagnetic radiation
Electromagnetic radiation in the form of either radio waves or light can also be used to transfer energy wirelessly. While systems based upon this method are used mostly for information transfer, a high degree of efficiency in power transmission is also achievable under certain circumstances.
The earliest work in the area of wireless transmission via radio waves was performed by Heinrich Rudolf Hertz in 1888. A few years later Guglielmo Marconi worked with a modified form of Hertz's transmitter.
Ø Evanescent wave coupling
Researchers at MIT believe they have rediscovered a way to wirelessly transfer power using non-radiative electromagnetic energy resonant tunneling. By sending electromagnetic waves around in a highly angular waveguide, evanescent waves are produced which carry no energy. Evanescent wave coupling is a process by which electromagnetic waves are transmitted from one medium to another by means of the evanescent (or decaying) electromagnetic field(s). This is usually accomplished by placing two or more waveguides close together so that the evanescent field does not decay much in the vicinity of the other waveguide. Assuming the receiving waveguide can support mode(s) of the appropriate frequency, the evanescent field gives rise to propagating wave mode(s), thereby connecting (or coupling) the wave from one waveguide to the next.
Ø Electrical conduction
From experiments performed between 1888 and 1907 Nikola Tesla concluded that the earth is an excellent electrical conductor, and an electric current can be made to propagate undiminished for distances of thousands of miles. It was also found that the earth’s natural electrical charge can be made to oscillate, "by impressing upon it [very low frequency] current waves of certain lengths, definitely related to its diameter
It was also discovered that the resistance of the earth is negligible due to its immense cross sectional area and relative shortness as compared to its diameter.
A [conducting] sphere of the size of a little marble offers a greater impediment to the passage of a current than the whole earth. . . . The resistance is only at the point where you get into the earth with your current. The rest is nothing