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Form factor is significant in the study of transformer losses; as is well known, hysteresis loss is small when the form factor is large, and vice versa. Every wave shape has a definite value of form factor; but the converse is not true, for a particular value of form factor does not indicate a particular wave shape. A wave may contain a third harmonic equal to seventy five per cent of the fundamental and still have the same form factor as a true sine wave. Form factor, therefore, has no general significance as an indicator of wave form or wave distortion. A general expression for form factor is derived in terms of the relative amplitudes and phase positions of its harmonic components; curves are drawn showing the variation of form factor with the amplitude and phase of the third harmonic. Various wave forms are shown, very unlike in appearance, having the same form factor.