Topband: "return" current - what is it?

[Bill Wichers]

Tom, it's worth adding to this that trying to make current measurements in the ground using 60hz is pretty useless for another reason: induced currents from the ac power system (especially in north america). 60hz will be present on just about anything -- you'll even see it on a scope by just touching an unconnected probe with your finger.

Commercial ground resistance meters use much higher frequencies (a few hundred hz up to a few khz if I remember correctly). I've had to design equipment for measuring fluid conductance before and I used a sine wave of around 450hz (high enough to avoid 60hz harmonics and "between" the nearest two as well), and a bandpass filter on the receiving end. Trying to just use 60hz will result in unpredictable measurement errors and worse yet, those errors will vary geographically even when moving only relatively small distances.

-Bill

******
1.) He used Sevik's method of determining ground conductivity, but that 
method is pretty much useless. A measurement of localized current at 60 Hz 
doesn't mean a thing at radio frequencies, because skin depths and 
dielectric effects are so vastly different.

Anyone using a measurement at 60 Hz to determine characteristics at 2 MHz is 
just kidding himself. One can have a skin depth of hundreds of feet, and the 
other just a few feet. They are not closely related at all. It was a 
terrible method to start with.

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