The visitor can get a shock from the metal disk if it is positively charged or negatively charged. The plastic base, the disk, and the rubbing cloth-like everything else in the universe-contain negative and positive electronic charges, called electrons and protons. Most of the time, there are equal numbers of both kinds of charge. But if an object has more of one kind of charge than the other, then it is electrically charged and can give you a shock. Use the handle to pick up the metal disk. Rub the white square of the teflon briskly with the cloth attached to the exhibit. Put the disk back on the square and touch the disk with your finger.
You wil hear a crackling sound and get a small shock. Watch carefully, and you may also see a spark.