Digital paper has been studied as a new medium that appears to offer the advantages of both active displays and paper. A twisting ball display system is a promising candidate technology for digital paper. Dielectric balls with colored hemispheres (black and white) are used as the display elements; each color has a different surface charge. Each ball can be rotated by applying the appropriate electric field. However, obtaining the ideal balls (those that show good rotation characteristics) is a remaining problem. This study proposes a way of clarifying the best ball configuration. Ball mobility is measured in a dielectric liquid under uniform electric fields. Experimental results show a strong relation between the mobility difference between the materials covering the ball and its angular rotation speed.