Hardly an Arduino project (hardly a "project" at all, actually), so I'll post it here. Well, the title speaks by itslef.
Enjoy the complete gallery !
Incredible! About those threads of blue lines, are they very dynamically moving and changing shapes?
Thank you ! Yes, these are the sparks generated by the Tesla coil itslef, which are indeed rapildy changing, so that many of them were caught on these long-exposure shots.
On some pictures, it is easy to see a single spark "repeating itslef" many times, with a slightly different position each time, which show that the sparks are produced by very short bursts (a spark lives about a few microseconds, but it take approximately 10^-2 seconds for the Tesla coil to charge one).
Aha! Now I understand! So although we see a bunch of these sparks going off at the same time with our own eyes, they were actually produced one at a time by the machine. I didn't think of that before. I was thinking several sparks were going off at the same time and change rapidly. I then thought you did a SHORT exposure to get your pictures 
liudr:
So although we see a bunch of these sparks going off at the same time with our own eyes, they were actually produced one at a time by the machine.
Exactly !
Sometimes though, two or three sparks can live at the same time (they are both shorter in that case) and form ramifications, but the idea is the same. Below is a picture taken with a shutter speed of 1/30 seconds to mimic what the eye sees.
(it is from the "Zeus Tesla coil" album, if interested.)
So cool! I guess our eyes fool us enough!
Definitely cool Mad Scientist prop there.
Did you make the Tesla coil?
Thank you 
GoForSmoke:
Did you make the Tesla coil?
Indeed. I actually made two coils for now : "Zeus" and "Hyperion". Here are the little beasts :
What precautions do you observe when operating this thing? I have to think that we are looking at something that is actually dangerous, right? Does this cause RF interference? Do you run this at home or at a lab?
JoeN:
What precautions do you observe when operating this thing? I have to think that we are looking at something that is actually dangerous, right? Does this cause RF interference? Do you run this at home or at a lab?
It is indeed quite a dangerous apparatus. Accurate information on the subject can be found on this safety page which has been written by real Tesla coil experts. Here's what I, non- HV expert, do:
The arcs carry something like ~10 amps, but the high-frequency of the current (~300 kHz) changes its distribution so that most of it runs on the surface of conductors. I believe the sparks from my coil can inflict bad burns, but unlikely to kill (but bigger coils could). For this reason, I always turn on and run the coil from a very safe distance, and keep spectators even further.
Another problem is the large quantities of ozone and other nasty gases generated by the breakdown of air components. This is the biggest issue with Tesla coils, actually. The room must be very well ventilated.
Also, the arcs in the main spark gap (closing the high-current primary circuit) are extremely bright and should not be looked at directly without proper protection (like welding goggles).
Finally, there is indeed some interference. Even if there a low-pass filter after the transformer and an EMI suppressor on the power chord, I unplug nearby sensitive electronic material (this is probably unnecessary, but, you know).
I have one coil in my room (nixie pictures taken here), and hopefully, cool neighbours who haven't complained for the noise (...so far)
A proper Mad Scientist -expects- the torches and pitchforks!
But in the New World.. not even a small castle and no Igors.
Have spectators sign an agreement so they are themselves aware of the danger and and not sue you in case of accident happening.