Using earth as a conductor

Question

"To use the earth as a conductor, you can't merely stick a little wire into the ground next to the tomato plants. You have to use something that maintains substantial contact with the earth, and by that, I mean a conductor with a large surface area. One good solution is a copper pole at least 8 feet long and ½ inch in diameter. That provides 150 square inches of contact with the Earth. You can bury the pole into the ground with a sledgehammer and then connect a wire to it. Or, if the cold-water pipes in your home are made of copper and originate in the ground outside the house, you can connect a wire to the pipe." [Charles Petzold, Code - The Hidden Language of Computer Hardware and Software]

I did not understand why we need a "substantial contact" to make the circuit work. If Earth is not a good conductor does it matter if we use a big contact or not? How does it change anything?

Answer 1

This is a passage from my book. I did not understand why we need a "substantial contact" to make the circuit work. If Earth is not a good conductor does it matter if we use a big contact or not? How does it change anything?

A lower resistance 'earthing rod' causes the 'ground voltage' to be less of a function of the current flowing to ground. Although the ground itself will still have resistance, the grounding circuit will be closer to the ideal.

Answer 2

In general electrical circuits involve some different parts, such as wires, voltage source, load (e.g. light bulb, motor, greenhouse heater), switch and so on. Where one part, such as a wire, connects to another part, such as load or simply another wire, you have a contact.

In a printed circuit these contacts are often made using solder. Solder is highly conducting and bonds with e.g. copper or gold making a very good contact where there is almost no resistance. Another way to make a good contact is to screw a screw down tightly against a bunch of copper threads in a wire, pushing them against another piece of metal. This is often done in household circuits. But in principle no such contact is completely perfect; they all offer a bit of electrical resistance.

In the case of contact with the soil you don't have the highly conducting bond as you get with solder, nor the high pressure forcing surfaces together as with a screw. You only have pressure of the surrounding soil pushing the soil against your metal wire or pin, and the moisture. Although water (with impurities) conducts (at low frequency) reasonably well, it is nowhere near as good a conductor as copper. So to compensate for this you want a large cross-sectional area of soil conducting your current, which brings the resistance down. To allow there to be a large cross-sectional area right from your metal pin or wire, you should make the wire or pin itself have a high surface area in contact with the soil. That's what the book is recommending.