In 1834, Russian physicist Heinrich Lenz discovered the directional relationships between induced magnetic fields, voltage, and current when a conductor is passed within the lines of a magnetic field. Lenz's law states:

"An induced electromotive force generates a current that induces a counter magnetic field that opposes the magnetic field generating the current."

Physically, Lenz's law merely tells us what direction an induced current flows in a conductor given a change in the magnetic field through the conductor. The induced current circulates in a sense such that the magnetic field it produces opposes the change in the field that induced the current to begin with. This opposition can be compared to the opposing force two magnets experience when pushed together with like poles facing each other. Mathematically, Lenz's law is used to keep track of the negative sign in Faraday's Law which gives the relationship between a changing magnetic field and the voltage produced by the change in the field.

Of significance to Lenz's law is its relationship to the conservation of energy. Lenz's Law states the fact that energy is conserved for the case of induced currents and the magnetic fields they produce. This can be seen by assuming that if the induced current circulated in a sense that reinforced the change in magnetic field instead of opposing it, this would lead to a greater induced current, which would lead to greater reinforcing of the field. This cycle would continue ad infinitum, thus breaking the law of conservation of energy.