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Tesla's Colorado Springs Lightning Observations

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From Feed Line No. 6

by Gary Peterson

As seen from a present day perspective, the time which Nikola Tesla invested into his 1899 Colorado "expedition" appears to have been some of his most productive.  This might be because he had considered the previous ten years of research as practice for the work that was being conducted at the now well-known Experimental Station.  Or perhaps it is from the many recorded observations dating from this period and the group of important patents would appear over the next few years.  In fact, work on the applications for these fundamental radio patents began while Tesla was still in Colorado.  One major account that is related in the Colorado Springs Notes and in an article titled "The Transmission of Electrical Energy Without Wires," has to do with an unusual natural radio phenomenon that Tesla observed during and after an intense thunder storm which passed close by his Colorado lab.

The receiver that he used appears to have built around an oscillatory transformer similar to the type employed for high frequency lighting.  (See Nikola Tesla: Lecture Before the New York Academy of Sciences April 6, 1897 for details about these devices.)  Located on the primary side of the transformer were a sensitive device known as a rotating coherer and a mica condenser both connected to an elevated terminal and to ground.  The primary side of the circuit also included a battery for placing a small dc voltage across the coherer and a mechanical switch known as a "break."  During normal operation the make-and-break device would open and close about 72 times per second.  In the secondary side of the circuit was another coherer and battery plus a delicate relay which responded by closing every time a lightning discharge occurred in the general vicinity.  This type of receiver was extremely sensitive, being capable of recording effects as distant as an estimated 500 miles.

On July 4 Tesla writes that his observations began as the storm was approaching and still at a distance of 80 to 100 miles.  The relay on the elegantly simple receiver must have begun to chatter practically as soon as all the connections were made.  As the storm came closer, the relay continued to respond even as it was adjusted to its least sensitive setting.  As the storm continued on past the lab is when, in Tesla's word's, "the most interesting and valuable observation was made."  The relay was once again adjusted to be more sensitive and it continued to respond for a while and then stopped.  After an unspecified time had passed the relay again responded for a while and then once again ceased to play.  As recorded by Tesla this on-again off-again action continued to repeat itself with a period of about 30 minutes, at the very least six or seven times, on into the evening.  He writes, "most of the horizon was clear by that time."

A number of people have speculated as to what, exactly, Tesla had seen that summer long ago.  One proposal, put forth in the Proceedings of the Tesla Centennial Symposium, is that mechanical vibrations were being detected which were the result of terrestrial piezoelectric interactions associated with the lightning strikes.  Another theory has it that the varying ground currents were the result of stationary waves created by reflections off the Pikes Peak mountain range.  Tesla himself considered two other possibilities for the creation of what he believed were stationary waves anchored to their moving point of origin.  The first and, at the time, to him least probable, was that they were the result of reflections from the point of the earth's surface diametrically opposite to the storm.  More likely, in his mind, was that they resulted from reflections which took place within the storm itself, at a point very close to the origin of the initial wave packets.

In 1994 a new premise was set forth by Kenneth and James Corum in a paper presented at the biennial Tesla Symposium in Colorado Springs.  The proposed model is an adaptation of waveguide theory and draws upon a method of treating radiation sources in a waveguide that calls for the existence of "images" which mirror an assumed dipole source.  These images in combination with the source itself can be said to form what appears to be broadside array of radiating elements.  Furthermore, the radiation from these sources, being coherent, can interfere and cast side lobes that appear as maxima and nulls along the waveguide walls.

It is proposed that rather than registering the existence of stationary waves, Tesla might have been seeing a wave interference phenomenon similar to that described above.  The nodes and antinodes which passed by his point of observation might have been due to the superposition of partially coherent lightning induced VLF waves radiating from the primary source and an adjacent image and propagating down along the earth-ionosphere waveguide.  This model bears more than passing resemblance to Tesla's preferred scenario that did not call for the reflection of waves from the antipodes.  At the same time it is important to note that in the specifications for his Patent No. 787,412, Art of Transmitting Electrical Energy Through Natural Media Tesla stated that the lightning produced waves provided "unmistakable evidence that the disturbances created had been conducted from their origin to the most remote portions of the globe and had been thence reflected..."  In the ten short months between July 4, 1899 and the May 16, 1900 application date what had caused this tremendous shift of opinion to where he now believed and practically recorded as fact what before had been assigned the lesser probability?  Tesla himself gave us the answer in 1929 when he wrote:

"The chief discovery which satisfied me thoroughly as to the practicability of my plan was made in 1899 in Colorado Springs, where I carried on tests with a generator of 1500 KW capacity and ascertained that under certain conditions the current was capable of passing across the entire globe and returning from the antipodes to its origin undiminished in strength.  It was a result so unbelievable that the revelation at first almost stunned me."

The Corums' 1994 analysis provides us with a highly plausible physical model to explain Tesla's lightning related observations during the stormy summer of 1899.  It is also quite apparent that that year's ground breaking experiments with a specially designed high power electrical oscillator thoroughly convinced Tesla that he was seeing indications of electrical earth resonance.  This raises the question: while in he was in Colorado Springs did Tesla ever observe any indications of interference between the outgoing waves that were induced by his experimental VLF transmitter and incoming waves reflected from the antipode?

The Rotating Coherer
The rotating coherer and associated circuitry

Tesla's VLF receiver must have been very similar to the circuit illustrated here.  Each of the two coherers c and c' were constructed from a short section of 3/8 inch I.D. glass tubing capped with two brass plugs.  The intervening space was partially filled with course nickel chips.  The two adjacent batteries B and B' were adjusted with resistors r and r' to strain the devices to a point where they were just about to break down and become conducting.  An incoming signal would drive the potential across the device beyond its threshold causing a much stronger battery current to flow.  A clockwork drive mechanism was used to continuously rotate the small glass cylinders thus decohering the chips after each received impulse.  It is now believed that coherers are uniquely suited to detect the particular type of natural radio phenomenon that Tesla observed in 1899.

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