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A non-contact voltage tester measures the difference in electrostatic charge between the person holding the tester and the tip. If you hold it near an energized cable it will sound or flash (or both). These devices, however, do have limitations, including detecting ghost voltage (or phantom voltage), which has bled over from a nearby energized cable or device. Contact testers (or continuity testers, as they are called) have probes that are inserted into the slots in a receptacle. They can also test for correct grounding to make sure the receptacle is wired correctly. Be sure to read the manufacturer’s instructions carefully before using these products.