Equipment used to detect stray voltage varies, but common devices are electrical tester pens or electric field detectors, with follow-up testing using a low-impedance voltmeter. Electrical tester pens are hand-held devices which detect a potential difference between the user's hand and the object being tested. They generally indicate on contact with an energized object, if the potential difference is above the sensitivity threshold of the device. Reliability of the test can be affected if the user is at an elevated potential him/herself, or if the user is not making firm contact with a bare hand on the reference terminal of the tester.