Tesla coil sparks
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Tesla coil sparks (the best bit)
                     
6 inch coil

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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 Tufnol 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.  Predicted maximum 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.


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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.

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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 view.

4 inch coil

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A shot outside the shed with the smaller coil (above).

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These are from a demo at a Fete at our local high 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 display with sparks to the cage 3 feet from the edge of the  toroid. Definitely a hands over the ears display.

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I need a bigger shed as I am running out of safe standing room! (above)

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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.

Mini coil

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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.

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This page was last updated June 06, 2003