Code_ The Hidden Language of Computer Hardware and Software - Charles Petzold [12]
Some substances are significantly better than others for carrying electricity. The ability of an element to carry electricity is related to its subatomic structure. Electrons orbit the nucleus in various levels, called shells. An atom that has just one electron in its outer shell can readily give up that electron, which is what's necessary to carry electricity. These substances are conducive to carrying electricity and thus are said to be conductors. The best conductors are copper, silver, and gold. It's no coincidence that these three elements are found in the same column of the periodic table. Copper is the most common substance for making wires.
The opposite of conductance is resistance. Some substances are more resistant to the passage of electricity than others, and these are known as resistors. If a substance has a very high resistance—meaning that it doesn't conduct electricity much at all—it's known as an insulator. Rubber and plastic are good insulators, which is why these substances are often used to coat wires. Cloth and wood are also good insulators as is dry air. Just about anything will conduct electricity, however, if the voltage is high enough.
Copper has a very low resistance, but it still has some resistance. The longer a wire, the higher the resistance it has. If you tried wiring a flashlight with wires that were miles long, the resistance in the wires would be so high that the flashlight wouldn't work.
The thicker a wire, the lower the resistance it has. This may be somewhat counterintuitive. You might imagine that a thick wire requires much more electricity to "fill it up." But actually the thickness of the wire makes available many more electrons to move through the wire.
I've mentioned voltage but haven't defined it. What does it mean when a battery has 1.5 volts? Actually, voltage—named after Count Alessandro Volta (1745–1827), who invented the first battery in 1800—is one of the more difficult concepts of elementary electricity. Voltage refers to a potential for doing work. Voltage exists whether or not something is hooked up to a battery.
Consider a brick. Sitting on the floor, the brick has very little potential. Held in your hand four feet above the floor, the brick has more potential. All you need do to realize this potential is drop the brick. Held in your hand at the top of a tall building, the brick has much more potential. In all three cases, you're holding the brick and it's not doing anything, but the potential is different.
A much easier concept in electricity is the notion of current. Current is related to the number of electrons actually zipping around the circuit. Current is measured in amperes, named after André Marie Ampère (1775–1836), but everybody calls them amps, as in "a 10-amp fuse." To get one amp of current, you need 6,240,000,000,000,000,000 electrons flowing past a particular point per second.
The water-and-pipes analogy helps out here: Current is similar to the amount of water flowing through a pipe. Voltage is similar to the water pressure. Resistance is similar to the width of a pipe—the smaller the pipe, the larger the resistance. So the more water pressure you have, the more water that flows through the pipe. The smaller the pipe, the less water that flows through it. The amount of water flowing through a pipe (the current) is directly proportional to the water pressure (the voltage) and inversely proportional to the skinniness of the pipe (the resistance).
In electricity, you can calculate how much current is flowing through a circuit if you know the voltage and the resistance. Resistance—the tendency of a substance to impede the flow of electrons—is measured in ohms, named after Georg Simon Ohm (1789–1854), who also proposed the famous Ohm's Law. The law states
I = E /