Same idea as Van de Graaf for High Voltage Sparks and Van de Graaf for Raising Paper.

Using the two large horizontal plates with one attached to a spring, you charge one of the plates and observe them being drawn together.

Connect the ground to the HANGING plate on the spring, and the high voltage to the BOTTOM plate. (Otherwise, too much charge leaks out through the spring.)

The concept is Coulomb force attraction. If the high voltage plate is negative (as is the case with the large Van de Graaf), it repels negative charges from the hanging plate through the ground wire, leaving it with a net positive charge, hence attracting itself to the bottom negative plate. Conversely true if the high voltage were positive.