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I'm reading Tesla's Colorado Springs Notes and was wondering if you could clarify a couple of points.   What would allow electricity to be transmitted through the earth to great distances as Tesla proposed without loss of power?  Also, are there any pictures of his rotating coherers?

Wireless Transmission of Electrical Energy
In regards to Tesla's statements about electrical transmission by earth currents with only minimal losses, this claim is founded upon the fact that in his system the energy is carried by electrical currents flowing through conductors.  This is in contrast to transmission by radio waves in which the energy is radiated through free space in a similar fashion as light emitted by an electric lamp.  Generally speaking, only a small fraction of the energy radiated by a radio antenna is recoverable.  There are examples of microwave transmission and reception where a far greater percentage of the energy can be recovered.

Tesla called his method of wireless transmission and reception the "disturbed charge of ground and air method."  This was because a Tesla coil transmitter electrically influences both the earth and the space above it.  He made a point of describing the process as being essentially the same as sending electricity through a wire.  In his system one of the conductors is the earth itself.  Compared with, say, copper wire, one might not think that dirt, rock, etc. would make a very good electrical conductor.  Actually, the Earth's vastly greater cross-sectional area provides a remarkably low resistance path for the flow of earth currents.  The greatest losses are apt to occur at the points where the transmitting and receiving stations are connected with the ground (see "Observations on Ground Terminal Construction").  This is why Tesla stated,

"You see the underground work is one of the most expensive parts of the tower. In this system that I have invented it is necessary for the machine to get a grip of the earth, otherwise it cannot shake the earth.  It has to have a grip on the earth so that the whole of this globe can quiver, and to do that it is necessary to carry out a very expensive construction." [Nikola Tesla On His Work With Alternating Currents, p. 203, Appendix II, Foreclosure Proceedings]

In order for energy to be transmitted by conduction, an electrical circuit benefits from a second path through which current can flow.  This is called a closed circuit.  In the case of Tesla's system, the second path is established between stations' elevated terminals through the rarified upper level atmosphere.  In operation the elevated terminals are charged to a very high electrical potential--on the order of 10- to 100-million volts.  They serve as discharge terminals for the formation of capacitively coupled discharge plasma through which connections to the conducting upper level atmosphere are made.  Under certain circumstances the connection to the upper atmosphere is made by electrostatic induction.

Rotating Coherer
The coherer is the component in Tesla's communications receiver circuits that 'detects' the energy of incoming surface waves and space waves.  It is actually called a "detector."  Another early detector, invented by B.F. Miessner, is the "cat's whisker" detector used in older crystal radios.  The introduction of the Audion triode vacuum tube detector was the next major improvement in receiver design.  Tesla wrote about the application of vacuum tubes for this purpose stating, "this is undoubtedly the most delicate wireless detector known" [The True Wireless].

While I'm not aware of any photos of Tesla's rotating coherers, there are a few illustrations.  The best may be the one found  on page 99 of the Colorado Springs Notes, a cleaned-up copy of an original Tesla drawing.  Another source, with both drawings and a description, is the U.S. Patent Method of and Apparatus for Controlling Mechanism of Moving Vessels or Vehicles.  The term "coherer" relates to the manner in which such devices function.  Their operation depends upon the creation of an imperfect connection between two or more electrical conductors.  The construction can be as simple as crossing two oxidized copper wires one on top of the other.  As an increasing voltage from a battery is placed across the two conductors, a point is reached at which the imperfect, high-resistance junction abruptly changes to a low-resistance junction, the junction (or junctions) "cohere" together, and much greater current flows.  The form of coherer used for wireless communications typically consisted of a glass cylinder partially filled with small metal particles, typically oxidized nickel, with electrical terminals at both ends.  The trick to making a coherer sensitive to wireless signals is to set the battery, i.e., bias voltage at a level just below the point at which the device shifts to it's low-resistance state.  Connect a ground and an antenna, one on either side of the coherer, and an incoming signal of sufficient strength will make it conduct.  The additional current flow can be used to close a relay, indicating the presence of the signal.  The act of restoring the junction to it's high-resistance state, called decohereing, involves mechanical disturbance of the device.  This can be done by simply tapping it, or, as Tesla conceived, rotating it either continuously along it's axis, or intermittently end-for-end following each successive activation.

See Tesla's Colorado Springs Lightning Observations for a short write up on Tesla's July 4, 1899 electrical-storm observation and the coherer-type receiver that was probably used.  See also Tesla's July 28 diary entry for a description and drawing of what appears to be a preferred receiver configuration [Colorado Springs Notes].

Revised: 05/28/2004 

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