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Though commutation is one of the important factors in the designing of d-c machines, the existing method of designing the commutation is very crude. It is generally admitted that the best method of checking the quality of commutation experimentally is to plot no-spark zone diagrams and examine them. Therefore, if no-spark zones could be determined through calculation, it would be a great step forward in the designing of d-c machines. The present paper offers an approximate method of calculating the no-spark zones of d-c machines with lap windings. If this method is adopted, it will be possible, by using a medium-sized digital computer, to calculate rapidly no-spark zones from design data. The statements in the paper occur in the following sequence: the general characteristics of commutation, particularly its singularity and the characteristics of no-spark zone; the definition of the so-called Â¿-Â¿diagram based on the knowledge of these characteristics; an approximate plotting of this diagram; an approximate method of calculating the no-spark zones of a practical machine from this Â¿-Â¿diagram; and, finally, a few examples of such calculations.