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Both Faraday's law and Ampere–Biot–Savart's law neatly come together in Lenz's law, written in 1835 by the Estonian‐born physicist Heinrich Lenz (1804–1865). Lenz's law states that electromagnetic‐induced currents and forces will try to cancel the originating cause.

For example, if a conductor is forced to move, cutting lines of magnetic force, a voltage is induced in it (Faraday's law). Now, if the conductors' ends are closed together so that a current can flow, this induced current will produce (according to Ampere–Biot–Savart's law) a force acting upon the conductor. What Lenz's law states is that this force will act to oppose the movement of the conductor in its original direction.

Here, in a nutshell, is the explanation for the generating and motoring modes of operation of an electric rotating machine. This law explains why, when the load in a generator is increased (i.e. more current flows in its windings, cutting the magnetic field in the gap between rotor and stator), more force is required from the turbine to counteract the increase in induced larger forces and keep supplying the larger load.

Figure 1.5-3 Lenz's law as it applies to electric rotating machines. Basic numerical relationships and a simple rule are used to determine the direction of the induced forces and currents.

Similarly, Lenz's law explains the increase in the supply current of a motor as its load increases. Figure 1.5-3 neatly captures the main elements of Lenz's law as it applies to electric rotating machines.