A transformer consists of two coils wound around the same soft iron core. When the primary coil is connected to an alternating p.d. and alternating magnetic field is generated in the soft iron core. The flux linkage in the secondary coil alternates with the magnetic field and the change in flux linkage generates an e.m.f. in the secondary coil. All the magnetic flux produced by the primary coil passes through the secondary coil.The applied p.d. VP = NP(ΔΦ/Δt). The induced p.d. in the secondary coil VS = NS(ΔΦ/Δt). The transformer rule therefore gives VS/VP = NS/NP where N is the number of turns.