Noncontacting fieldmeter and voltmeter

Ideal meters have a high input impedance and low capacitance to avoid draining the source charge. 

7 In practice, the sensing electrode is placed behind a small hole of about 1 mm diameter. 

Two limiting electrostatic conditions of interest in potential measurement are constant voltage and constant charge flat surfaces. In both cases a uniform electric field is desired between the source and the sensing electrode so that the calibration of the instrument is retained over a wider range of separation distances (Blitshteyn, 1984). 

For a constant voltage source, the charge on the test surface will be altered by the capacitance of the meter however, the voltage of the test surface can be correctly read from the meter (reading in V/m multiplied by the separation distance in m). For closely spaced parallel plates, the original charge density on the test surface is calculated by the relation 

Equation 3.40 shows that the charge density on the test surface is increased by the presence of the meter (and ground plate) and must be reduced by the ratio of capacitance before and after introduction of the fieldmeter. 

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