ABSTRACT
All electrical circuits require two wires to operate. Where water flows through a pipe in one
direction, electricity requires a return path back to the source. This is why batteries and
lightbulbs have two terminals. Interrupting either wire stops the flow of electricity, which is
the basis for a switch. But what if you need to vary the brightness? This is where resistors
come in. When inserted into a circuit, a resistor limits the flow of current. To continue the
water analogy, a resistor is like a short length of narrow pipe that resists flow, as shown
in Figure 21.1 There are also variable resistors-volume and tone controls-that work like
a water valve or faucet.
