The lower plate of the capacitor is connected to a projection electroscope and the upper plate can be adjusted so that it clears the glass plate. Place a static charge on the plate connected to the electroscope with a Piezio charger. Then carefully insert the glass plate, noting that the potential decreases since the charge is being held constant. Remove the glass plate and observe that the electroscope returns to its original value.
To show the effect of separation, slowly pull the upper plate up and let it back down again.