<p>It has a floating electrode (2) that is located in small areas of its surface on an insulator (4) attached to a control electrode (3) provided with an opening in which an electrostatic probe (6) with shielding (5) that allows measuring the surface potential without physical contact is applied and which is connected to a measuring block (7) that is in turn connected to an attenuator (8) attached to a controlled high voltage source (9) connected to the control electrode (3).</p>
