This instrument is a Digital Static Field Meter, which measures Voltages associated with electrostatic charge. Here we have two paddles. One is an insulator and the other is a conductor. If we rubbed the paddles together and hold them over the Field Meter, you can see that the insulative paddle holds many thousands of volts. In this case, a positive charge. The conductive puddle holds a much lower negative charge, but it is still charged. If we place both paddles on a grounded surface and hold them over the Field Meter again, you'll see that the insulator is still holding a large positive charge, while the conductor is no longer holding a charge. By placing it on a grounded surface, the charge was transferred to the mat and away to ground.
In any ESD protected area, insulators can be controlled by doing the following: keeping insulators a minimum of 30 centimeters from all ESD sensitive items at all times; replace regular insulative items with an ESD protected version; or, periodically apply a coat of Topical Antistat. When none of this is possible, the insulator is termed process essential, and therefore neutralization using an ionizer should become a necessary part of the ESD Control Program. Once the ionizer is turned on, it will send out thousands of matching pairs of both positive and negative ions in the airflow. If we hold the positively-charged insulated paddle into the airflow, it will attract negative ions, thereby neutralizing the charge on the surface. If we then hold it back over the Digital Field Meter, you will see that the charge has disappeared.