Electrostatic fieldmeter

An electrostatic fieldmeter, also called a static meter is a tool used in the static control industry. It is used for non-contact measurement of electricity charge on an object. It measures the force between the induced charges in equipment and the electric charges present on the surface of an object. This force is converted in volts, measuring both the initial peak voltage and the rate at which it falls away.
Operation:
an electrostatic charge monitoring probe is placed close (1 mm to 5 mm) to the surface to be measured and the probe body is driven to the same potential as the measured unknown by an electronic circuit. This achieves a high accuracy measurement that is virtually insensitive to variations in probe-to-surface distances. The technique also prevents arc-over between the probe and measured surface when measuring high voltages.
Alternative method:
The operation of the an electrostatic field meter is based on the charge-discharge process of an electrically floating electrode: Corona source charges a floating electrode, which discharges with a regular repetition frequency to the earth-electrode. The discharge repetition frequency is the measured variable which is a function of the background electrostatic field. 
Beside static charge control in ESD sensitive environments, another possible application is the measurement of the atmospheric electric field.
F. Roman, V. Cooray, V. Scuka,
The principle of operation of a simple electrostatic field measuring device based on the properties of floating electrodes,
Journal of Electrostatics,
Volumes 40–41,
1997,
Pages 483-488,
ISSN 0304-3886,
https://doi.org/10.1016/S0304-3886(97)00091-0.