injury. The lamp provides no indication below the strike voltage of the neon lamp, and so cannot detect certain hazardous leakage conditions. Since it relies on capacitance to complete the circuit, direct-current potential cannot be reliably indicated. If the user of the screwdriver is isolated from ground and capacitively coupled to other nearby live wires, a false negative may occur when testing a live circuit, and a false positive when testing a dead circuit. False negatives may also occur in brightly lit areas which make the neon glow hard to see. Non-contact voltage detectors Amplified electronic testers (informally called electrical tester pens, test pens, or voltage detectors) rely on capacitive current only, and essentially detect the changing electric field around AC energized objects. This means that no direct metallic contact with the circuit is required. The user must touch the top of the handle to provide a ground reference (through stray capacitance to ground), at which point the indicator LED will light up or a speaker will buzz, if the conductor being tested is live. Additional energy to light the lamp and power the amplifier is supplied by a small internal battery, and does not flow through the user's body. When the device is placed near a live conductor, a capacitive voltage divider is established, comprising the parasitic capacitance between the conductor and the sensor, and between the sensor to ground (through the user's body). When the tester detects current flowing through this divider, it indicates the presence of voltage. Some amplified testers will give a stronger indication (brighter light or louder buzz) to gauge relative strength of the detected field, thus giving some clues about the location of an energized object. Other testers give only a simple on/off indication of a detected electric field. Professional-grade testers will also have a feature to reassure the user that the battery and lamp are working. Voltage detector pens are made for either line-voltage or lower-voltage (around 50 volt) ranges. A tester intended for mains-voltage detection may not provide any indication on lower-voltage control circuits such as those used for doorbells or HVAC control. Unlike tong ammeters which sense changing magnetic fields, these detectors can be used even if no current is flowing through the wire in question, because they sense the alternating electric field radiating from the AC voltage on the conductor. A non-contact tester which senses electric fields cannot detect voltage