The paper deals with the problem of the internal distribution of eddy current and magnetic flux and the waveforms of these quantities in ferromagnetic laminations under application of an alternating field and with particular surface conditions. The problem discussed is inherently intractable on account of its nonlinear character, and many attempts have been made at an approximate solution. Such solutions, however, have failed to bring out the importance of the surface conditions of magnetization-a factor of considerable significance in thin laminations. The present mode of attack is based on the assumption that, apart from the fundamental, the third harmonic components of the alternating quantities are also significant. The solution of the non-linear equation based on this assumption brings out several features of the complex phenomena which occur and the comparison given between theoretical and experimental results go far to justify the method of solution and the assumption made. The method has the advantage of lending itself to digital computation.