Lenz's law: the induced current flows in a direction that opposes the change in magnetic flux causing it. If the flux through a loop is increasing, the induced current creates a magnetic field opposing the increase (opposing the external field). If the flux is decreasing, the induced current supports it. This is a consequence of energy conservation — if the induced current aided the change, it would create a perpetual motion machine. Practical rule: determine the direction of the change in flux, then use the right-hand rule to find the current direction that creates an opposing field.