Patent ID: 7098644

Claim:
A voltmeter for measuring an unknown voltage Vs on a target surface without contacting the target surface, the voltmeter comprising: a voltage measuring circuit having a detector plate forming a capacitance Cm with the target surface; and a movable shutter for selectively exposing the detector plate to the target surface, wherein the voltage measuring circuit (1) measures, in a first sampling stage, a charge Vs*B, where B is proportional to Cm; (2) measures, in a second sampling stage, a charge Vp*B, where Vp is a known voltage introduced by the voltage measuring circuit; (3) determines the value B based on the known voltage Vp; and (4) determines the value Vs based on the determined value B, and wherein the shutter is placed in a closed position substantially blocking the detector plate from the target surface for resetting the voltage measuring circuit prior to measuring the charge Vs*B and the shutter is moved into an opened position exposing the detector plate to the target surface for measuring the charges Vs*B and Vp*B.