If a light bulb is connected
to the output of a flyback supply (without rectifier) or a Tesla Coil it makes
a nice little plasma globe. I was lucky to find a 12cm diameter light bulb at
a surplus store. As with many hv components nobody else seemed to need one of
those - it was quite cheap and the price tag was in DM though we were using Euro
as currency for one and a half years!
First thing I did was to connect the bulb to my flyback
supply. Unluckily this didn't work as well as with normal-sized bulbs, I
hardly got a single discharge inside the bulb. The discharge looks like this:
I couldn't withstand the tempation to point a grounded electrode at the glass bulb - which gave a spectacular discharge but instantly burned a hole into the thin glass. So I bought a new light bulb...
...and placed it on my 2" TC. With nice results - with the spark gap electrodes placed close together I got multiple bluish streamers:
If the spark gap electrodes are spaced farther apart (higher output voltage) the discharges become fatter and white near the filament. The gas filling also starts to glow green. Much cooler than a comercial plasma globe...
The picture below was taken with the TC set to maximum output voltage.
After the TC is turned off the argon atoms remain in a metastable excited state which means that it takes them much longer to return to their ground state than normally. Thus a green afterglow can be seen for some seconds. It's hard to take a foto because the afterglow is quite dim, eletronically enhanced the picture looks like this:
By the way: If you happen to come across a lightbulb which doesn't give any discharge, but instead the glass bulb itself glows greenish - congratulations! You've found a completely evacuated light bulb - which doesn't really make a good toy as it emits x-rays. The radiation flux isn't very high (it doesn't blacken a photographic film even at some minutes of exposure) but there is no such thing as a completely safe level of ionizing radiation.