Ok, bear with me here as this Q may really not be worthy of the theory area.
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Some time ago i was apprenticed to an individual who liked to use those little two wire testers,( before volt-tics) the ones with the small lighted end that are $1.19 at any hardware store.

The proper way to use these is to touch each lead to what is being tested, however, we used these to determine the hot wires by holding one lead, touching hots with the other.

The result being a dimmer lit tester on contact with the hot conductor.

so did the 'circuit' have to be completed, or was the tester lit by other means???

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