4. Veins and Arteries
A couple of pages back, I compared the flow of electricity to the flow of water. Now it's time to change to another fluid metaphor.
Blood. (It's thicker than water, you know.)
You may not remember much of your high school biology, but you probably do remember this basic cycle:
Oxygenated blood gets sent out of the heart/lung machine, full of power and at high pressure. It goes out to perform some kind of work - in organs or limbs or skin or whatever. This work depletes it of oxygen. The lower-pressure, depleted blood is sent back to the heart-lung complex through a different return path. (In the drawing, blood is going out to do work via the red arteries on the right side, and coming back in via the blue veins on the left side.)
You can think of electricity as working more or less the same way - with one important difference.
Electricity comes out of the generation machine (which for us is all parts of the system from the power plant all the way to the service panel, inclusive) full of power and pressure. It goes out and does work - powering a light or an appliance - and then comes back in through a different return path. (In the drawing, power is going out to do work via the hot (black) wires on the right side, and coming back in via the neutral (white) wires on the left side.)
The important difference is that your body never stops circulating blood, because there is no time at which it is not "doing work." Even if you're asleep and motionless, your body is still doing a certain amount of work just keeping itself alive. To the circulatory system in your body, there is no such thing as a "no-work" condition.
In power circuits, there is definitely such a thing as a no-work (AKA no-load) condition. See the outlet in my drawing? It doesn't have anything plugged into it. It is not doing any work. And power had therefore better not be flowing from the hot wire to the neutral one. The lightbulb, on the other hand, is doing work; it's making light and heat, and power is flowing from hot to neutral through it.
If you were to connect a hot wire straight to a neutral wire without load (do not do this), say if you opened up an outlet and held a hot wire in contact with a neutral one, you would first notice that the wires got hot very fast. But in a well-wired house, you would have less than a second to notice this, because the breaker or fuse for that circuit would blow - before you had a chance to catch something on fire. The electrons were racing around the circuit you made with nothing whatsoever to do, making lots and lots of heat.
A complete circuit - that is, from power source to hot to neutral back to power source again - that does no work, has no load, is a short circuit and it is a bad thing. Even in places where it isn't a fire hazard it is an enormous power drain, one that could bleed a house dry and make you wonder why your electric bill was bigger than your mortgage payment. If you have a short circuit anywhere in your house at any time, something is seriously wrong.
Where there is no load, there must be no connection
(so that a circuit is not completed).
Anywhere there is a complete circuit, there must be a load.
House circuits are wired to have many places where a load might be, but where there is no connection until there's actually a load. The circuit below is doing absolutely nothing - no connections anywhere - because none of the three outlets has anything plugged into it. If you plug something into the first outlet (and turn that thing on), that part of the circuit will have power flowing through it, but outlets two and three still will not.
Remember that the outlet itself does not close a circuit. It is a place for a circuit to continue. The thing you plug into the outlet becomes a physical continuation of the same circuit.
That drawing is a little crude, but the point of it is, the hot and neutral wiring to the outlet is continued in the cord to the lamp, through the lamp socket into the bulb, and hot does not meet neutral until the actual filament of the bulb - the actual point at which work is done.
Of course, if items like the light in the illustration above automatically completed the circuit as soon as they were plugged in, we would live sadder lives. The only way to turn off a light fixture would be to unscrew the bulb; the only way to turn off a plug-in appliance like a vacuum cleaner would be to unplug it. In practice, things often have switches.
All a switch does is make or break a physical connection between two wires*. It's more practical (and easier on the heart) than touching two wires together to make a connection and pulling them apart again to break it - but inside the switch, that's exactly what's happening.
A switch, in other words, is a controllable break in a wire - and that wire is always the hot wire. Think about it. If you want to restrict the flow of water to a sink, you don't put a valve in the drain. If you want to restrict flow of blood to an organ, you put a clamp on the artery, not the vein.
House wiring, of course, is not as tidy as these square-cornered diagrams. For one thing, no one has come up with an easy way to divide a single wire into two pieces of wire so it can branch off in two directions. For another, wires are run through walls in certain ways for convenience reasons (more on that later).
This picture is closer to what the three-outlets diagram above would actually look like in practice. At each of the first two outlets, wires need to be fastened together - one incoming wire, one wire to the outlet, and another to continue on to the rest of the circuit. This is done by a wire nut which is a little plastic doodad you screw onto the wires to connect them (the orange objects in the drawing are supposed to be wire nuts, but I'm not the world's best artist). You could do the same thing by just twisting the ends of the wires together, but the wire nut is easier and safer (since it also insulates/conceals the bare ends of the wires).
The third outlet, at the end of the circuit, doesn't need wire connections because the circuit has nowhere to continue to.
Imagine that these were normal double outlets, and that there could be many feet of wire between one outlet and the next, and you get closer to what actual wiring is like.
Here's one more example before we move on. On the left is a concept sketch; on the right, something closer to the way it would actually be wired. In each case, power arrives at this point in the circuit from the left, powers an outlet and an overhead light which has a wall switch, and then continues off to power other parts of the circuit to the right. Notice where the switch is. The wiring to the switch is such that it only controls the hot power to the light; not to the outlet and not to any parts further on in the circuit. Notice that power has a way to travel further into the circuit even if no work is being done here - i.e. even if the light is off and nothing is plugged into the outlet.
 This is pretty much all there is to the theoretical portion of house wiring. But before we get a little more hands-on, it might be useful to have a little history. Don't worry, it'll be interesting - it'll have Thomas Edison being really nasty, and why there are no utility poles in Manhattan, and what happened when we flirted with aluminum, and how to get 240 volts in a 120-volt house, and most importantly, what that third prong is for.
* There are kinds of switches that make or break connections between more than two wires, and they all have special names and special purposes. But ninety-five percent of the switches in home wiring are basic on-off switches - single pole single throw in the jargon - that are meant to make or break continuity along a single hot wire.
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