The round third prong on a grounded plug connects to a ground wire. A cheater plug or a three prong adapter is an adapter that allows a NEMA 5-15P grounding type plug to connect to NEMA 1-15 R non grounding receptable.
The purposes of the third prong is to act as a ground connection to improve safety in the event of either a short circuit within an appliance that otherwise might result in delivering shock to the user, or a surge delivered to the appliance due to lightning or another malfunctioning attached to the prong.
