This picture shows my first Tesla Coil using a 4" secondary with 1000 turns, and an RQ style static gap at roughly 1.2kW.

The HV supply voltage is 6kv @ 200mA. The measuring stick is marked in feet (300mm) intervals. The spark leaving the left side of the picture hits an earthed target 28 inches from the edge of the toroid.

The apparent large number of sparks is due to the 3 second exposure time used here, and the high repetition rate of the static gap which makes the sparks jump about frantically.


The longest spark produced with the static gap is shown here. I was lucky to catch this on film.

Although the arc leaves the picture, the distance was later measured as 31 inches from a discharge point on the toroid to the tip of an earthed metal target.

The exposure time for this picture was very short so not much of the coil can be seen but the arc still shines brightly.

This picture shows the same coil being tested at 900 watts with a new experimental rotary spark gap.

This rotary is a 200BPS full synchronous type which uses 8 unevenly spaced electrodes in an attempt to process power more efficiently in the primary charging circuit.

The top toroid is 24" in outside diameter, and is finished with a smooth surface. There is a foil bump on the right side surface, to act as a spark director.

Maximum spark length is around 36". Notice the shorter, brighter strike to a grounded object.


In this photo the secondary has been replaced by one with 1500 turns. (Approximately 3 times the inductance). This increases the output voltage and is also believed to provide a better impedance match to the burning arcs.

A small 18" toroid was fitted to keep the resonant frequency high so that the same primary circuit could be used.

The 1200watt power input, small toroid, and 6 second exposure all contribute to the large number of short arcs visible in this picture. Maximum length was around 38".


Spark lengths were confirmed with a measuring stick after the test:

Strikes to left wall = 36",
Strikes to right wall = 49",
Strike up to ceiling = 54",
Arc to metal ladder = 41".

Click here for more technical details about this Tesla Coil.

This photo shows the 4"x21" 1500turn secondary and unevenly spaced rotary being run at 1800 WATTS. Power was provided by a 10kV power transformer with external inductive ballast.

The 3 toroids provide a high Csec. This lowers the resonant frequency and allows a correspondingly larger Cpri (70nF) to be used for increased primary energy.

The top toroid is a smooth 4"x24" toroid with a breakout bump on the right edge.

Exposure time was 4 seconds for this photograph, and the sparks were confined by the garage walls.

The white dots above the TC are thought to be due to intense light from the rotary gap reflecting inside the camera lens. They were not observed by anyone present during the run.

Sparks in excess of 66" captured on video the evening before the Corby 2000 UK Teslathon.

The coil was running at approximately 1800 watts in the church hall. Unfortunately the open space of the church hall seemed to encourage arcs to head for the workings underneath the primary coil.

This frame was taken seconds before the rotary gap motor was struck by an arc from the toroid.

The motor developed an internal short and cut the test short before full power was reached on the variac.

Close-up of corona from a copper rod placed on top of the toroid.

The sharp pointed end of the rod causes breakdown of the air at this location before anywhere else on the surface of the toroid.

Streamers are short and frantic, but the display becomes impressive at high input powers. Notice the pink colour which indicates that these discharges did not connect with ground.


Movie files for download…

Best results are achieved by first downloading the file to disk, and then playing at double normal size.



1800 watt testing with 1500turn secondary, 3 toroids and 70nF primary capacitance. This video shows almost constant strikes to walls, ceiling, and other items. Notice the extra long spark heading up to the ceiling in the first few seconds, and the white hot arc to a set of aluminium ladders lying at the bottom right.

Running time = 6 seconds,
Frames / sec = 12.5,
Size = 240 x 180,
File size = 564 KB,
Download = 3 minutes,


The same Tesla Coil being tested before the Corby 2000 UK Teslathon. Power level is less than 1800 watts, and the longest sparks are in excess of 66 inches. Notice the downward arc from the left side of the toroid at the end of the video. This arc went around the primary and struck the case of the RSG motor causing a winding to fail !

Running time = 6 seconds,
Frames / sec = 25,
Size = 384 x 288,
File size = 767 KB,
Download = 4 minutes,

In the first movie the streamer growth can be examined in some detail by advancing the video one frame at a time. This clearly shows that long sparks evolve over several milliseconds, and also that hot arcs rise due to convection. 

Unloading of ventilation ducting for my new bigger toroid ;-)
You can never make a toroid too big. Right guys ???

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