We took this piece of wood, soaked it in a water and baking soda solution for a few minutes, wiped it off so it wasn't dripping, put two nails in it, connected it to a neon sign transformer, and turned it on. The electricity sortof wandered around for a while, trying to find a path through the wood. It eventually succeeded, and thats the heavy line. The most interesting thing was how indirect it was, it did not take the shortest path.

Of the several attempts, this was the coolest looking one.

Block of wood, 15kV @ 30ma, exposure time about a minute.