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The earth, however, does have some resistance. That's why we can't use the earth ground to reduce our wiring needs if we're playing around with 1.5-volt D cells and flashlight bulbs. The earth simply has too much resistance for low-voltage batteries.

You'll notice that the previous two diagrams include a battery with the negative terminal connected to the ground:

I'm not going to draw this battery connected to the ground anymore. Instead, I'm going to use the capital letter V, which stands for voltage. The one-way lightbulb telegraph now looks like this:

The V stands for voltage, but it could also stand for vacuum. Think of the V as an electron vacuum and think of the ground as an ocean of electrons. The electron vacuum pulls the electrons from the earth through the circuit, doing work along the way (such as lighting a lightbulb).

The ground is sometimes also known as the point of zero potential. This means that no voltage is present. A voltage—as I explained earlier—is a potential for doing work, much as a brick suspended in the air is a potential source of energy. Zero potential is like a brick sitting on the ground—there's no place left for it to fall.

In Chapter 4, one of the first things we noticed was that circuits were circles. Our new circuit doesn't look like a circle at all. It still is one, however. You could replace the V with a battery with the negative terminal connected to ground, and then you could draw a wire connecting all the places you see a ground symbol. You'd end up with the same diagram that we started with in this chapter.

So with the help of a couple of copper poles (or cold-water pipes), we can construct a two-way Morse code system with just two wires crossing the fence between your house and your friend's:

This circuit is functionally the same as the configuration shown previously, in which three wires crossed the fence between the houses.

In this chapter, we've taken an important step in the evolution of communications. Previously we had been able to communicate with Morse code but only in a straight line of sight and only as far as the beam from a flash-light would travel.

By using wires, not only have we constructed a system to communicate around corners beyond the line of sight, but we've freed ourselves of the limitation of distance. We can communicate over hundreds and thousands of miles just by stringing longer and longer wires.

Well, not exactly. Although copper is a very good conductor of electricity, it's not perfect. The longer the wires, the more resistance they have. The more resistance, the less current that flows. The less current, the dimmer the lightbulbs.

So how long exactly can we make the wires? That depends. Let's suppose you're using the original four-wire, bidirectional hookup without grounds and commons, and you're using flashlight batteries and lightbulbs. To keep your costs down, you may have initially purchased some 20-gauge speaker wire from Radio Shack at $9.99 per 100 feet. Speaker wire is normally used to connect your speakers to your stereo system. It has two conductors, so it's also a good choice for our telegraph system. If your bedroom and your friend's bedroom are less than 50 feet apart, this one roll of wire is all you need.

The thickness of wire is measured in American Wire Gauge, or AWG. The smaller the AWG number, the thicker the wire and also the less resistance it has. The 20-gauge speaker wire you bought has a diameter of about 0.032 inches and a resistance of about 10 ohms per 1000 feet, or 1 ohm for the 100-foot round-trip distance between the bedrooms.

That's not bad at all, but what if we strung the wire out for a mile? The total resistance of the wire would be more than 100 ohms. Recall from the last chapter that our lightbulb was only 4 ohms. From Ohm's Law, we can easily calculate that the current through the circuit will no longer be 0.75 amp (3 volts divided by 4 ohms) as before, but will now be less than 0.03 amp (3 volts divided by more than 100 ohms). Almost