Introduction to Electrostatic Voltmeters

A contacting voltmeter is not suitable to measure the voltage associated with the charge on an isolated conductor because the contacting voltmeter will discharge the object, thereby estroying the data that the observer is attempting to measure.

An electrostatic voltmeter performs a non-contacting measurement without modifying or destroying the charge.

The electrostatic voltmeter functions by placing a field sensing probe close to the surface of interest, but without contacting the surface. The electrostatic voltmeter drives the conductive housing of the field sensing probe to a voltage necessary to null the electric field between the probe and the surface of interest. This field-nulling condition is almost always achieved when the voltage on the probe matches the unknown voltage on the surface of interest. By measuring the voltage on the probe, one deduces the (equal) voltage on the surface of interest.

This field-nulling technique is achieved with closed-loop voltage feedback using a high-voltage follower amplifier, which yields very high accuracy, excellent stability, and low drift performance for the electrostatic voltmeter. Accuracies better than 0.1% of full scale are common.

An additional benefit of the field-nulling technique is that the probe can be placed very close to the surface of interest without fear of arc-over between the probe and the surface. This is due to the action of the electrostatic voltmeter to maintain zero electric field strength between these two surfaces. Furthermore, placing the probe very close to the surface of interest results in the ability to resolve very small areas (<1.0 mm) on the surface of interest.