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.
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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.