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In this paper, the authors explain the various phases of the conduction process taking place in the commutator brush contact. At light loads and low speeds, the voltage drop in the contact layer is low (in the order of 0.5 volt). At increased loads, however, the trailing-edge voltage suddenly increases; this is a case of breakdown in the contact layer. Statistically, this is attributed to the fast change of entropy, now defined as the ratio of the incremental energy destroyed in the contact layer to the contact-layer voltage. Since entropy is a function of state, this indicates that a change of conduction state has taken place, bringing about a voltage rise up to 3 volts. Experimental oscillographic studies clearly confirm the existence of this phenomenon, and it should be included in commutation calculations to assess accurately the effects of commutation.