As a source of high voltage for this great Jacob's Ladder I used 4 MOTs (transformers from microwave ovens). Primaries are connected in parallel
and secondary in series with "grounded" center and supported by the mains voltage. Two of MOTs (1 and 4) are un-grounded. Note any MOT can be ungrounded. You must select a
ones with good secondary isolation. The best type appears with a plastic frame (eg Moulinex). Originally the voltage is 2100V on the live end and 0V at cold end related to earth.
Ungrounded MOTs 1 and 4 do have 4200V at the live end and 2100V at the cold end. Some types of MOTs cannot be ungrounded because of the lower end of the secondary
is pinched between the secondary and the core (see photo below). Ungrounding is done by separating the lower end is from the iron core and inserting insulation under it.
The iron core must not touch anything.
Inlet to the cold end wire end must be somehow attached (eg glue), otherwise you can very easily tear it away.
Several people have asked me how to fix the torn off wire - I can't help you.
Ungrounding may also be a problem for MOTs, where the secondary insulation has holes in the inner side.
Because short-circuit current is too large in MOTs, the trip the breaker or would soon overheated. It is therefore necessary to limit the current. To do this, I used three modified chokes TL. These are ballasts for high pressure sodium lamps 150W TESLA. Since their inductance was too big, I removed The "U" parts of core from them, leaving only the ones of "T" shape. This will reduce the inductance to about 30mH (when the coil connect directly to 230V 50Hz, 20A flows). The chokes limit the maximum drawn current of the Jacob's ladder to 30A (16A circuit breaker can stand this little over a minute before tripping). Short-circuit current at the output is about 750mA. Open output voltage is about 8kV.
I used two iron rods, which are at the narrowest point distant 4 mm, in the broadest 12cm. A useful part of the rods (the one that discharge on) is 75 cm long. Thin copper wires did not succeed, because they tend to melt and bend.