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Q & A: Hands attracting discharge in plasma balls

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Most recent answer: 10/22/2007
Q:
why does the plasma beams shoot up at your hand when you touch a plasma ball?
- Neal Riley (age 10)
New Zealand
A:
Hi Neal,

The discharges you see in glowing plasma balls occur because of the voltage difference between the metal ball at the center and the inside of the glass holding the gas inside. Glass is an electrical insulator, and so it would charge up rapidly, equalizing the voltage with the inner ball if the voltage were applied as a constant. To keep the discharges going, the inner ball alternates rapidly between positive and negative high voltages, which is the reason why these things hum when they're switched on (it's probably the alternating power supply and also the alternating heating of the gas inside as the current switches on and off rapidly). The pretty glow comes from electrons knocked off of gas molecules which grab electrons back to become electrically neutral. Neon and Argon and Nitrogen are common gases with pretty colored glows.

The thin tubes of discharge stay more or less in the same place even though the current switches on and off rapidly because the heated gas has a lower resistance than unheated gas. This is also why the discharge filaments tend to rise slowly over time, because the hot gas is less dense than the colder remaining gas.

The reason why discharge filaments head for your hand when you place it in contact with the outside of the glass is capacitive coupling between the inside of the glass and the outside and the fact that your hand supplies the other lead of the capacitor, allowing mobile charges in your body (mostly made of saltwater) to flow to cancel charge buildup on the outside of the glass. As charge builds up on the inside of the glass, an equal but opposite charge builds up on the outside because the glass polarizes electrically. The canceling charge on your hand reduces the energy necessary to accomplish this, and so it is energetically favored to put more charge on the inside of the glass near your hand than anywhere else.

The glass cannot conduct a steady current, so there is a limit to how much current you can get in your body and so you are pretty safe doing this. With higher voltages higher frequencies, more energy is deposited in your fingertips however.

Tom

(published on 10/22/2007)

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