Why does a non-contact voltage detector light up when you touch a plasma ball with the other hand? I am doing a science experiment and we decided to try holding a non-contact voltage detector up to plasma ball.  We were surprised that it would light up when it was 3 ft away from the plasma ball.  I assume that it is detecting the electromagnetic field created by the ball.
Even more interesting, if I hold the voltage detector in one hand and touch the plasma ball with the other, it will still light up.  We created a line of 5 people holding hands, with one person touching the ball and the last person holding the voltage detector.  (About 15-20 ft)
For our test, we were standing on a laminate floor with socks/bare feet.
Unlike my old voltage detector that would beep randomly and for any reason, this voltage detector only beeps and lights up when it was within a centimetre of a live 110v wire.
So, when you touch the globe, is electricity flowing through my body, or what is causing the voltage detector to beep and light up?  We are way outside of the "electromagnetic field", aren't we?
Wikipedia states that it emits radio frequency energy.  How is that different from an electromagnetic field?
Another interesting trick - an LED light panel lights up when brought near the plasma ball.  If you set it on top of the plasma ball and let go, the LED lights go out.  It only lights up when you bring your hand near the LED light or hold it.
So as my hand approaches the plasma ball, why does the LED light start to light up, and much brighter when I hold the LED light?
 A: There are two possible factors that are involved here:


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*The plasma emits electromagnetic waves, which means there will be an oscillating electromagnetic field spreading around the plasma ball, which would explain why the voltage detector beeps 3 feet away. In fact, it is an interesting experiment to study the electromagnetic waves propagating from the plasma ball, have a look here where an experimental configuration similar to yours is used to study different antenna configurations.

*The plasma balls are  dielectric barrier discharges, our bodies are conductors of electricity. Practically by touching the ball, you closed an "effective" electric circuit where displacement current  flows. The circuit starts with anode (the center of the ball), a conductor (the plasma streamer in the ball), a dielectric (the glass of the ball) and another conductor (your body closing the circuit to the ground). This whole configuration represents a RC circuit, where the capacitor is the plasma streamer-glass-your body, and the resistors are the resistance of your body and the plasma streamer. 
Since a current is flowing in your body, there will be a voltage drop across your body and the detector would beep. When you form a chain of people, what you are doing is basically extending the length of the conductor, still a current would flow, but it should become smaller and smaller the larger number of people there are.
When you touch LED, the circuit is closed, so the current flows, when you are away from it and it rests on the top of the ball, the circuit is open and the current can't flow. When you are really close to it, air between you and LED works as a dielectric, such that the "effective" circuit has two capacitors connected in series, one has the glass as dielectric, and one has air as dielectric, the further you move from LED, the higher the impedance of the air capacitor becomes, eventually it becomes an open circuit.
In brief, the effect of the ball is your case is either electromagnetic (doesn't require physical contact), or current related (in case of physical contact).
I would say that the sensitivity of your detectors is really good, what kind of detector is it??
Hopefully that was useful
