Microwave oven transformers (MOTs) are HV transformers powered from 230V mains. They give 2100VAV on the secondary winding. A single MOT is rated at about 1kVA, but it draws several amps from mains at short circuit and the power is usually limited by breakers :) Obviously it is possible to connect a few MOTs in series to raise the output voltage, but in fact connecting more than two most requires insulation improvement (mineral oil sink). Every MOT has one end of the HV winding connected with its core, so the best way to connect two MOTs in series is to connect the cores together. An interesting idea to increase the arc length is to use microwave oven capacitors (MOC) as resonant caps. The idea comes from here: 4hv.org. MOCs capacitance is about 1uF and they are rated at 2100VAC. Connecting four caps just like you see on the schematic below makes an LC resonant circuit with the MOT's secondary windings. This causes increase of voltage and current in the arc, so it can be much longer.
On the two pictures below you can see an arc made by only one MOT. Maximum length is about 10cm.
The next two pictures shows an arc made by two seried MOTs with four MOCs as resonant capacitors, the arc length is even more than 50cm!
I used only a long extension lead as a ballast, the transformers was drawing about 20A from 230V mains. High voltage at this power is very dangerous but arcs are really big and impressive :)
Obviously this stuff is ideal as a power supply of a big Jacob's ladder, so I made it too. Below you can see the MOTs with caps and more photos of big arcs. There are also three movies at the bottom of the site.