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 light bulbs 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 23.1 There are also variable resistors—volume and tone controls—that work like a water valve or faucet.