Lenz's Law and Lenz's law, also known as the rule of contradiction - a rule setting the direction of the magnetic field induced in the phenomenon of electromagnetic induction. It was named after Heinrich Lenz, who formulated it in 1834.
Any change in the magnetic induction B in a given area involves the formation of vortex surrounding electric field E, which in turn (if possible) induces an electrical current opposite of that change.

It says that the current induction (also called secondary shock) raised in the guide under the influence of an alternating magnetic field is always the direction of the magnetic field generated counteracts secondary cause (ie the change of the original magnetic field), which it has occurred.