One of the archetypes of hv devices carved deeply into my mind is the Jacobs
Ladder (rising arc). So as soon as my 4 MOT power supply had been finished I've
decided to build one. Unfortunately operating a Jacobs Ladder at a voltage as
low as 8kV is a little tricky - if the electrodes are a little too close together
the arc gets stuck and doesn't rise, if the electrodes are spaced apart a bit
too much the arc doesn't ignite at all.
One way of avoiding this problem is to put a candle under the electrodes which
produces ions and makes it possible to space the electrodes farther apart. But
this solution kinda sucks. Stefan Kluge had the idea of putting a helper electrode
between the main electrodes thus forming a triggered spark gap. The helper electrode
is connected to one of the main electrodes via resistors (2 x 2.5MOhm). This
way it's even possible to get a Jacobs Ladder to work at 4kV!
The electrode holders are designed to allow adjustment of both electrode spacing
and electrode angles. The main electrodes are made of 6mm brass bars with 720mm
arc path length. Originally the electrodes were 150mm longer but they started
swinging due to the electrostatic forces, so I had to saw them off.
Some nice arc pictures. On the upper ends the electrodes are spaced apart 180mm. The third from the left shows a "jumping" arc. The forth is a long time exposure.
Below are three interesting fotos where the remains of arcs can be seen. The left one shows the old version of my Jacobs Ladder with candle and long electrodes.