In the first case the work expended on displacing the conductor is entirely converted into heat, given off in resistance R; in the second case part of the work expended goes on increasing the magnetic energy of the field which arises round the inductor. The work expended on displacing the conductor in unit time equals EI, where E is the e.m.f., identical in both cases, arising as the result of the conductor's displacement in the magnetic field, and I is the current in the circuit. Current I₁ in the first case is greater than current I₂ in the second case, since the inductor's e.m.f. retards the velocity of increase of the current. Consequently too, work in unit time EI₁ > EI₂.
This is also true for uniform motion of the conductor, but only up to the establishment of steady current in the circuit, after which the presence of self-induction in the circuit does not have any effect.
XI. Optics