We have a weight on the piston, and that weight is chosen so that p2 it acts as though p external is equal to p2.

让活塞上放一重物，让它的效果刚好,等同于外压强。

麻省理工公开课 - 热力学与动力学课程节选

Each weighing, say, one 100th of the weight 1/100 needed to establish p external is equal to p1.

每颗重，比方说，使外压强,达到p1的重量的。

麻省理工公开课 - 热力学与动力学课程节选