common과 ground
Here's the trick: Once you have established a common part of the circuit, you don't have to use wire for it. You can replace the wire with something else. And what you can replace it with is a giant sphere approximately 7900 miles in diameter made up of metal, rock, water, and organic material, most of which is dead. The giant sphere is known to us as Earth.
When I described good conductors in the last chapter, I mentioned silver, copper, and gold, but not gravel and mulch. In truth, the earth isn't such a hot conductor, although some kinds of earth (damp soil, for example) are better than others (such as dry sand). But one thing we learned about conductors is this: The larger the better. A very thick wire conducts much better than a very thin wire. That's where the earth excels. It's really, really, really big.
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 a 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.
An electrical contact with the earth is called an earth in Great Britain and a ground in America. A bit of confusion surrounds the word ground because it's also often used to refer to a part of a circuit we've been calling the common. In this chapter, and until I indicate otherwise, a ground is a physical connection with the earth.
When people draw electrical circuits, they use this symbol to represent a ground:
