Physics demonstration on the dinning room table laboratory.
I built this Marx generator, for a classroom demonstration.

See the next image for the turned off device in the light.

The upper spark is approximately 300 KV.
This was invented by Erwin Marx in 1923.
It uses a high voltage DC source to charge
a bunch of capacitors in parallel through
high value resistors (I used 1M Ohm).
Then they are all discharged in series to
multiply the voltage 20x. The connection in
series is made due to the very low resistance
of ionized air in the spark gaps. As soon
as the bottom gap sparks there's twice the
voltage across the second gap (first two
caps now connected in series) and the high
value resistors are in comparison open circuit,
this causes the second gap to sparks etc. etc.
This cascade continues until the caps are
all connected in series. Thus, in this 20 step
design there is 20x the high voltage source.