According to the accepted theory of the alternate current dynamo, the equation of electric current in the armature is γy + R y = periodic function of t , where γ is a constant coefficient of self-induction. This equation is not strictly true, inasmuch as γ is riot in general constant, but it is a most useful approximation. My present purpose is to indicate how the values of γ and of the periodic function representing the electromotive force can be calculated in a machine of given configuration.