Lenz's Law, a law of electromagnetic induction formulated in 1833 by the German physicist H. F. E. Lenz. It states that when an electric current is induced in a conductor, the direction of the current is such that its magnetic effect will oppose the action that gives rise to the current.

Lenz's law applies particularly to electric generators. When a generator induces an electric current, the direction of the current is such as to oppose the rotation of the generator. Therefore, the more electrical energy a generator delivers, the more mechanical energy is required to turn it.