Fig. 7: Schematic diagram of a field mill at the macroscale.
From: Electrostatically actuated MEMS resonators for magnetic and electric field sensing: a review

As the grounded shutter (or shield) rotates at a constant speed, the charge on the A and B electrodes change in opposite directions. The currents induced by these charges are generated in opposite directions. The field mill detects electric fields with high sensitivity by sensing the potential difference between the A and B electrode sets