u00b0 c in a non oxidizing atmosphere the use of coke as the main component results in an increase in contact resistance of the commutator as a whole and in improvements in commutation performance characteristics in addition since coke particles are hard the brush faces are always maintained in a constant surface condition and therefore the wear is slight the sliding motion is stabilized and high efficiency levels can be maintained for a long period of time petroleum derived calcined coke 50 by weight natural graphite 50 by weight and a phenol resin were mixed together and kneaded after kneading the kneaded mixture was dried and ground to an average particle size of 100 u03bcm or smaller the resulting powder was molded into a shape shown in fig1 and fig2 to give a commutator this commutator was measured for resistivity and further placed on a testing apparatus as shown in fig3 and measured for commutator wear rate the resistivity reported herein is the value in the direction perpendicular to the direction of pressure application as measured by the voltage drop method more specifically a copper net to serve as a current terminal was applied to each of both end faces of the test specimen an electric current was carried to the test specimen while applying a pressure of about 1 kg thereto via an insulating material and the voltage drop in the middle of the test specimen was measured using a voltmeter the testing apparatus shown in fig3 is constituted of a motor 13 with the test specimen commutator 1 mounted at the shaft tip thereof a pair of carbon brushes 11 contacting with the commutator 1 and a pair of springs 12 for pushing the carbon brushes 11 against the commutator 1 the commutator wear rate was.