CoachA battery is a small store of electricity. It has two different ends, called its terminals: one marked (plus), the other marked (minus). Both ends matter — electricity plays only when the two of them join the game.
示例
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.