Commutation condition at full - load a large D.C. machine can be checked by Hopkin son’s method. In this method two identical D.C. machine are coupled, both mechanically and electrically are tested simultaneously. One machines are made to run as a motor and it drives the other machine as generator. Though both machine operated on fulls load yet power taken by circuit is equal to machine loss. This test give information about armature reaction, commutation temperature rise.