CoachA bulb is a little lamp with a thin thread inside its glass that glows white-hot when electricity flows through it. Look closely at its metal foot: the bulb also has two contact places — the tip at the very bottom, and the metal side of its foot. Two terminals on the battery, two contacts on the bulb: that is no accident.
Examples
Example 16.6 (Why both ends?)
Connect two wires from the battery’s terminal to the bulb’s two contacts, leaving alone: nothing. One wire from and one from , but both to the bulb’s tip: nothing again. The bulb lights only when the loop visits the battery’s two different terminals and the bulb’s two different contacts — a full round trip.