It can be applied to Transformers to find out the instantaneous polarity of induced emf.
To understand the operation of Transformer with clarity it’s important to visualize the secondary induced emf and flux flow in the transformer core. Emf will be induced in any coil if the flux linkage changes with respect to time, as it happens in Transformer. Now, the polarity of induced emf will depend on the direction of primary flux as well as the rate of change of the primary flux. By Lenz’s law the flux produced by the induced emf will oppose the rate of change of flux of primary.