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Tesla's RF Ground

In re-reading the first fifty pages of The Colorado Springs Notes
(CSN), I have retraced again the construction and measurement of
the grounds that Tesla used there. In discussing this material
will quote Tesla's actual notes whenever possible.

On the first day that Tesla began coiling inside the building
at Colorado Springs (June 15, 1899), he had two metal to earth
connections available, the water pipe and the lightning arrestor
ground. Tesla notes: 

  "Sparks went over the lightning arrestors instead of going to
ground. This made it necessary to change the connection to the
ground, separating that of the secondary of the oscillator from
the ground of the arrestors. By connecting the secondary to a
water pipe, and leaving the ground of the arrestor as before, the
sparks ceased. This indicates a bad ground on the arrestors. THE
LATTER WORKED EXCEEDINGLY WELL. The ground connection was made by
driving in a gas pipe about 12 feet deep and gammoning coke
around it. This is the usual way as here practised."

First note that arcing was occuring from the earth connection
over the arrestors. This shows that the lightning arrestor
ground, an iron gas pipe driven twelve feet into the earth, was
insufficient for even a low power test. Tesla clearly recognizes
the differences between the two grounds he has available. The
capitalized sentence above was in italics in the original. His
setup is brand new. His tune is rough. 

The following day, June 16, 1899, he had workmen going full tilt
on a dedicated RF ground for his experiments.

  "A new ground connection was made by digging a hole 12 feet
deep and placing a plate of copper 20" x 20" on the bottom and
spreading coke over it again, as customary. Water was kept
constantly flowing upon the ground to moisten it and improve the
connection but in spite of this the connection was still bad and
to a remarkable degree. It is plain that the rocky formation and
dryness is responsible and I think that the many cases of damage
done by lightning here are partially to be attributed to poor
earth connections. By keeping the water constantly running the
resistance was finally reduced to 14 ohms between the earth plate
and the water main."

Tesla clearly notes the indivuality of the water pipe and the
earth plate, just as earlier he noted the difference between the 
arrestor ground and the water pipe. I will call the earth plate
the "dedicated RF system ground" or simply "system ground".
In text following the quote above, Tesla next refers to using a
"sensitive device" to determine the presence of a ground current
around the lab. I have used resonate pickup coils with a small
neon indicator bulb on the air terminal, or even a low pressure
gas tube to detect ground currents.

Apparently the water was turned off that night, for on the
following day, June 17, 1899:

 "Measurements of resistance between ground wire and water main
showed the surprising fact that it was 2960 ohms, and even after
half an hour watering it still was 2400 ohms, but then by
continued watering it began to fall rapidly. Evidently the soil
lets the water run through easily and being extremely dry as a
rule it is very difficult to make a good connection. This may
prove troublesome. The water will have to be kept flowing con-
tinuously. The high resistance explains the difficulty, from a
few days before, of getting the proper vibration of the second-
ary. The first good ground was evidently at the point where the
water main feeding the laboratory connected to the big main
underground and this was several hundreds of feet away. This
introduced additional length in the secondary wire which became
thus too long for the quarter of the wave as calculated. The
nearest connection to earth was as measured about 260 feet away
and even this one was doubtful."

OK, Tesla has said a mouthful. First his measure of resistance
when the water is off overnight skyrocketed. Though the water
expense was unbudgeted, it ran 24-7. (The bill was finally paid
when the wood used in construction was sold after the building
was dismanteled.). This shows that Tesla was determined that no
expense was to be spared in obtaining the lowest resistance
ground connection possible. 

Tesla then notes that the first true electrical ground point
occurs on the water main at the junction to the laboratory supply
pipe. He notes at the end of the quote that even this ground
point is doubtful, possibly because his equipment is powerful
enough to push the center of the "true electrical ground point"
further up the main. 

Tesla also notes that the ground path leading to the true
electrical ground point must be considered as a parasitic
conductor length in all secondary calculations. This distance
between the base of the secondary coil and the "true ground"
affected his ability to determine the resonate frequency of the
grounded coil and kept him from establishing a sharp tune in the
system. Add the fact that the location of the true electrical
ground point on the pipe may not be stable, and would possibly
move farther away with increasing power levels, meant that the
water pipe would be completely unsatisfactory for use as the
system RF ground.

To jump ahead to page 125, the section notes indicate that
stationary standing waves were observed on the water pipe, and
the exact electrical distance from the ground plate to the
electrical node on the pipe was determined to be 550 feet. This  
would be an unpleasant amount of uncoupled conductor to add to
any secondary coil.

In conclusion, Tesla recognized the need for a dedicated RF
ground in his coil systems. His specifications were such that the
true electrical ground point for the system ground had to lay as
close as possible to the base of the secondary coil. He
recognized the need for a highly conductive pathway adapted for 
low frequency high-voltage RF.

Reference:

> THE COLORADO SPRINGS NOTES, 1899-1900
By Nikola Tesla... Hardcover, 440pp, Published by NOLIT, Beograd,
Yugoslavia, 1978. Prefaced and annotated by Aleksandar Marincic,
Assoc. Prof. of EE Beograd Univ. and advisor to the Nikola Tesla
Museum, Yugoslavia

Richard Quick 

... If all else fails... Throw another megavolt across it!
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