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I reassembled my line tester, and it was then that I got to know how it actually works. I know that when someone touches the metal part at the top, the neon LED inside glows. However, I have seen that when I do not touch it, it still glows which is not important to me for understanding.

The curious part is that when I touch the steel door frame with my second hand, and the line tester is touched with the other hand, it glows more than before. When I am not touching the door frame, it fades a bit. This means that more current was passing, but why is it happening? The steel door frame has paint on it, which should decrease the conductivity on the top surface.

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Why line-tester glows more when I touch the steel door frame?

In order for the neon lamp in the neon line tester to glow, there must be current flowing through it. In order for current to flow through the neon lamp, there must be a complete circuit.

The complete circuit is typically from the power distribution transformer through the line, through the neon line-tester, through your hand, through your body, through your shoes, through the floor, through the earth, through the grounding rod, back to the distribution transformer. One of the most difficult parts of this path for electricity to traverse is through your shoes! Shoes are typically insulating, and so direct current does not flow through them well. But mains power is AC, and that means that it can flow through capacitors (which are insulating to DC). There is capacitance between your feet and the floor, even though your shoes are insulating.

When you touch the steel frame, you provide an alternative (with less impedance) for the current to flow. Instead of flowing from your body, through your shoes to the floor, it flows from your body to the steel frame of the steel frame through to the ground. Less impedance means more current will flow, which means the neon lamp will glow brighter.

The steel door frame has paint on it, which should decrease the conductivity

Yes, the paint is probably non-conductive. However, just as the soles of your feet and the floor form a capacitor that allows AC current to pass between them, your hand and the steel frame form a capacitor. The paint is much thinner that your shoe, and so the capacitor between your hand and the steel frame is quite "efficient" in passing AC. Technically, it has relatively low "impedance".

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  • \$\begingroup\$ The answer doesn't talk about the paint on the door frame, which I think is central to the OP's question. You allude to it but don't state it so an edit would be valuable, thanks. \$\endgroup\$
    – TonyM
    Commented May 22 at 16:15
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When you touch the door frame, even with paint, you have more capacitive coupling to ground/earth/whatever, so more AC current runs through your body and the lamp, and therefore the lamp is brighter.

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With AC voltage, you don't need to have a DC path, which means you can still get current flowing capacitance. Remember a capacitor is any two conductors with a vacuum, air or some other insulator.

So even though you don't have complete contact with the metal surface of the door frame, you can still get a small amount of current through the paint.

You can even get a small amount of current through your body with AC currents. If you've ever held an oscilloscope probe while your floating or holding a metal object you'll notice that a 60Hz voltage is on your body, which is being picked up via your body being an antenna/capacitor.

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  • \$\begingroup\$ The classic way of quickly checking a stereo system is to connect phono leads to a pair of inputs, and touch the central pins of the plugs at the other end. You should hear a hum from each speaker. \$\endgroup\$ Commented May 22 at 19:34

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