I don't know if you know how a spark plug works, but for the electrical current to fly into operation in a spark plug, the two points of contact have to be gapped.

我不知道你们是否知道,火花塞是怎样工作的，在电流通入火花塞中使它运转之前，两个接触点之间需要有空隙。

耶鲁公开课 - 文学理论导论课程节选

Because there are ions moving back and forth, there's a current that flows and there's a electrical potential that's generated.

由于离子的进进出出,就产生了电流流动,并且形成了电势差

耶鲁公开课 - 生物医学工程探索课程节选