Tesla coil sparks (the
6 inch coil
The new 6 inch coil has performed better than expected with sparks reaching
8 feet (0.5 million volts). The photo above shows 7 foot sparks to
the closest point of the ladder. This is with a large topload and a tank capacitance of
92 nF (0.092 uF) at a power in excess of 5 kVA. A lot of smoke comes
from my spark gap at runs of over 5-10 seconds at the highest powers.
I suspect it is metal oxide from the tungsten stationary electrodes and
the brass dome nuts and steel bolt heads that form the rotary
electrodes. There was no sign of any polycarbonate frame or
wheel overheating. Despite the leaf blower cooling, the tungsten rods
become hot enough to have a visible glow though the yellow polypropylene
box after turn off. The 8 foot (96 inch) spark is exactly 3 times
as long as the 32 inch secondary coil that generates it.
spark length by John Freau's formula for 3 kW is 1.7 x sqrt 3000 = 96 inches. For 5 kW
it is 120 inches.
Earlier photos reaching up to 6 feet with a capacitor of 48 nF and
a smaller topload at a power of about 5 kVA. They
are 10 second exposures showing a variety of types of sparks and
streamers. Lots of ground rail strikes
resulting in arc-overs from the primary (the bright spot in the right
photo). These improved with a smaller toroid under the larger one. In
these early photos, It is all supported by a stylish barstool and is
precarious, particularly as the toroid is supported by a heavy 12 inch
loudspeaker frame including magnet.
This 10 second exposure demonstrates the shaping of the high
electromagnetic fields around the main coil that directs away them from
the axis of the coil. The strong wind at the time shows the multiple
sparks going down the initial ionised path of each main strike (banjo
effect). This is best seen on the enlarged
4 inch coil
A shot outside the shed with the smaller coil (above).
These are from a demo at a Fete at our local
school in March 2003.
This is my first public display and used an 8ft square faraday cage
constructed out of angle iron and chicken wire. The Fete had a medieval
theme so the theatrics involved Merlin the magician giving a magic class
demonstration. Then, using a butchers chain mail gauntlet earthed
to the cage, I put my hand into the cage and turn on giving the effect of
lightning bolts coming from my hand which is close enough not to show
any other streamers. The 'lightning' finger had a high intensity blue
LED pulsating inside the gauntlet. I then disappeared and changed into a scientist lab
coat and gave a more contemporary explanation and a more conventional
with sparks to the cage 3 feet from the edge of the toroid.
Definitely a hands over the ears display.
(click to enlarge)
I need a bigger shed as I am running out of safe standing room! (above)
(click to enlarge)
On the wrong side of a Faraday cage (above), doing a demo to the staff at
Scitech in Feb 2003 with the 4
inch coil. An earthed PVC
rod with steel wool on the end gives a sparkler effect if the sparks are
hot enough. To the right is Scitech's TC which needs some work. My 6
inch coil will probably go to Scitech to replace theirs.
Video of 4 inch coil
Run mouse over picture to
view an MPEG video (570K) of the 4 inch Tesla coil in action.
Two inch sparks onto my hand. There is some motion artifact with my hand
but not the equipment or sparks. This is done with a clear knowledge of
earth paths and at very low power levels on a small coil with no toroid
capacitance. Wetting my hand beforehand reduces the sensation to a mild
prickle. This is potentially dangerous and should not be attempted on a
large coil regardless of power input.