When the cable is properly connected into the ground stake, any stray current is diverted to the ground. If the cable had resistance, you could (in theory) mea

When the cable is properly connected into the ground stake, any stray current is diverted to the ground. If the cable had resistance, you could (in theory) measure voltage drop over it but a heavy copper cable has a very low resistance. You can't measure current without interrupting the circuit, so it's really hard to know which grounds are actually being used. And in power systems, the ground is the sum of the grounds throughout the system. If you look near where the power goes into your meter, there should be a 1/2" (or more) copper rod driven 8-10' into the ground and the ground for your house is connected directly to it. Disclaimer: I'm not a power engineer (optics, RF, embedded systems and software) but I understand the basic principles. Someone else can probably talk about how the earth ground is actually established (after every transformer I believe) with a lot more detail.