Abstract: Apparatus for measuring the surface potential of moving electrostatically charged elements uses at least one electrically conducting detector mounted on the outer periphery of a grounded roller. An insulated ring secured around each end of the roller in an edge contacting relationship with the charged element under measurement provides a friction drive for the roller and maintains the detector in a spaced and non-contacting position from the charged element. As the roller is rotated, the detector is alternately exposed to the charged element and to a grounded shell arranged in a spaced and non-contacting manner from both the charged element and the roller and builds an A.C. voltage on a sampling capacitor. The output of the sampling capacitor is connected to a circuit which determines the envelope of the A.C. voltage and provides a differential D.C. voltage representative of the surface potential of the charged element. This D.C.