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.
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 electrical 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. AC detectors can be used even if no current is flowing through the wire in question.
A non-contact tester which senses electric fields cannot detect voltage inside shielded or armored cables (a fundamental limitation due to the Faraday Cage effect). Another limitation is that DC voltage cannot be detected by this method, since in this case the stray capacitance of a user's body does not pass sufficient current to activate the detector circuit.
These types of testers can be used on series-connected strings of mini Christmas lights to detect which bulb has failed and broken the circuit, causing the set (or a section of it) to not light. By pointing the end of the detector at the tip of each bulb, it can be determined whether it is still connected at least on one side. The first bulb which does not register is likely the one just past the problem bulb. (Burnt-out bulbs will still show as good, if there is a bypass shunt which completes the circuit.) Flipping the set's plug over and reinserting it in the outlet will cause the opposite end of the set or circuit to register instead.