Sean asks:

> Are you saying that the resistance between ground points (rods or
> whatever
> they are) is independent of the distance between them? If so, my concept
> of
> the conductivity must be incorrect.

Strange though it may seem, it is almost true.  The biggest problem with a
buried earth is getting a high enough surface area in intimate contact
with the ground.  (Incidentally, the sphere is the worst possible shape
for this, of course).

Once you've got that sorted out, it is almost true to say that the
resistance between two such earths is independent of distance.  This is
because there are in infinite number of paths - electrically "in
parallel" - between the two points.  You need to think in 3D for this:
obviously there is a dead straight path between the two points, and it
will have a particular resistance.  But there are other paths "around"
this one, each increasingly more curved (and thus longer), spreading
further and further into the planet.  Theoretically, a tiny bit of your
current might flow via the earth's core on its way between the two earth
electrodes!

You will appreciate that all these paths are - in effect - resistors in
parallel, which greatly reduces the total resistance between the two
points.

> In addition, it seems to me that the
> resistance from ground point to ground point depends on the structure of
> the grounds at BOTH locations. If one is a tiny rod,making the other
huge
> won't make all that much difference.

Yes, that is basically true.  Just like a normal electric circuit, of
course.  If your current has to pass through a 100 ohm resistor and a 1
ohm resistor, most of the loss occurs in the 100 ohm resistor.  Reducing
the 1 ohm to half an ohm won't increase your current flow much at all.

By the way, there was a remarkable communications system used during WW1,
in which both the transmitter and receiver used two buried rods in the
ground, several feet apart.  The transmitter drove an alternating current
between its two buried rods, modulated with speech (I believe, or maybe it
was Morse tones).  The receiver could be a considerable distance away.
Its buried rods, being themselves several feet apart, "tapped" across one
of the infinite paths of resistance which the transmitter's current was
flowing through, and thus was able to detect a voltage drop across it.
Quite remarkable.  I've attached a simple diagram.

Steve

Steve Thackery
Suffolk, England.
Web Site: http://www.btinternet.com/~stevethack/



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