A glass tube filled with lightning sparked plenty of attention after it was posted on Reddit this month, but what's causing the dancing electrified bolts that seem to have no source? Turns out, the seemingly sci-fi contraption is a demonstration of the effect of electric fields and the same principles that make neon lights glow.
The video doesn't show it clearly, but the holder of the tube is standing near a Tesla coil, which is essentially two open circuits connected to a "spark gap," or a gap between two electrodes that creates a spark. The coil can generate an electric field that can have thousands of volts but very little current, which makes it safer than, for example, touching a wall socket, which has a lot of current but little voltage. That's because a large electric current — the amount of energy traveling in an electrical flow — can be dangerous even if the voltage (the pressure that pushes the electrons along a current) is low. But a small current is much less dangerous even under high voltage.
Carl Willis, a nuclear engineer in New Mexico who builds similar art projects, told Live Science in an email that one type of Tesla coil can produce an electric field around it that changes direction tens or hundreds of thousands of times per second. If you put a tube full of gas at low pressure near it, the electric field will excite the atoms of gas, stripping electrons from them. The result? The positively charged ions (those stripped atoms of gas) and electrons (which are negatively charged) move to opposite sides of the tube. [Nikola Tesla vs. Thomas Edison: Who Was the Better Inventor?]
Initially, this generates the sparks we see because the ionization process — as the gas gets stripped of its electrons — produces light. Ordinarily, the light wouldn't last long because the ions and electrons would all gather at either end of the tube, and that would be that. But when the electric field reverses direction, they don't have time to settle into their new positions. Instead, some of the electrons and ions recombine to form neutral atoms, which get ionized again as the now-reversed field drives the charged particles to change direction.
The gas inside the Reddit user's tube seems to be xenon, and the pressure inside the tube is on the order of a pound per square inch, Willis noted. (Xenon is characteristically bluish.)
How does this connect to neon lights?
If, instead of putting the tube in an electric one connected two electrodes to it — one at either end — the gas could be charged with an ordinary current source, like a wall socket, rather than a Tesla coil. This is exactly what happens in neon signs and fluorescent lights. (The latter appear white because the inside of the bulb is coated with a material that glows white in the presence of UV light, which is what gets generated inside the bulb.) Electrons from the current source smack into the electrons surrounding atoms of gas, exciting them and resulting in the release of a photon — light. Neon's characteristic wavelength is reddish.
In typical fluorescent bulbs, mercury gas is mixed with argon, krypton or xenon, depending on the brand. (Xenon lights are popular in car headlights, for example.) In that way, the current from the electrodes is serving the function that the electric field does for the xenon-filled tube in the video. In fact, one can get both fluorescent and neon lights to light up in the presence of an electric field – as demonstrated by Florian Dussopt Design Studio's "EM Table (opens in new tab)," which generates a small electromagnetic field around it.
One could do this at home — Tesla coils are widely available, and a number of artists sell discharge tubes and electric-field setups here (opens in new tab).
Originally published on Live Science.