An indicating coil for applying the oscillograph to the study of commutation

After describing the methods already in use, the author explains a way in which the oscillograph can be applied to show the internal currents in an armature without cutting one of the main coils. Records are given illustrating various points about commutation to illustrate hismethod, and improvements in details are discussed. He also urges the insertion of the indicating coil during construction as a matter of course in all large machines, or others working near the limits of good commutation.