What is the little green wire or plate on the three-to-two adapter?
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The United States was the first country to establish widespread use of electricity for the public. At the time it was implemented, a 120-volt system seemed to provide enough voltage for users, yet was not enough to cause arcs in small motors. When electrical wiring was installed in European countries some years later, technological advancements allowed those devices to run safely at higher voltages. Therefore, the standard became 220 volts for Europe, while the United States remained at 120 volts. The advantage of 220-volt systems is that the current is almost half as much used by the same appliance on a 120-volt system, and so power losses due to resistance in the house wiring are smaller.
The green wire or metal tab attached to adapters is the grounding wire. Since the adapter is circumventing the ground prong, an alternate means of grounding is needed. If the screw on the outlet plate is grounded, the green wire on the adapter should be attached to it. This way, if there is an electrical short, the current can still flow through the grounding wire. If the screw is not grounded, then the adapter should not be used. An outlet tester that is available at most hardware stores can be used to make sure the screw is grounded.