to the total shear to which the mixture has been subjected as shown in fig3 total shear in a given mixing machine running at constant speed is linearly related to the mixing time consequently the applicant 39 s discovery permits one skilled in the art to relate standard deviation of volume resistivity to mixing time this knowledge combined with the previously described shape of the mean volume resistivity curve with time permits one skilled in the art to define the proportions of the initial carbon black and rubber mixture and the mixing time which provides the amount of shear required to produce conductive rubber having a volume resistivity close to the target value with small variability from batch to batch shear rate and total shear are terms well known in the art and are defined in the technical literature including the paper mixing of rubber by john m funt imperial college of london department of chemical engineering it will be clear to those skilled in the art that the actual mixing time required to attain the volume resistivities and standard deviations in examples 1 and 2 shown in fig3 will depend on the type of mixer used and the speed and setting of the moving elements the amount of mixing achieved in the examples in ten minutes using a brabender plasticorder may take hours in a mixer generating lower shear such as a roll mill having low relative velocities and wide spacing however the shape of the volume resistivity and standard deviation curves of fig3 will be substantially duplicated if mixing is continued long enough for purposes of the.