Jacob’s Ladder

Here’s a little Jacob’s Ladder I built from a household dimmer, an old car ignition coil, based on this design. It generates sparks just under an inch long, which probably means the voltage is around 20,000V.

Here are a couple of photographs. This one is an eight second exposure, but it really manages to capture the fact that the spark isn’t on continuously, but consists of a series of pulses.

long exposure

This one is probably a one second exposure, but leaving the garage lights on:

coil with lights on

It takes a lot of tuning of the shape between the uprights to get the rising arc effect – often you just get a series of random sparks at different heights, which also looks pretty cool.

I had both a fuse and a circuit breaker in line, just in case. Really the “low” voltage side should be enclosed, as it’s all at 230V, so pretty lethal (not that the HV side isn’t dangerous, but that needs to be exposed). The web page listed omits two important things for anyone who wants to build such a Jacob’s ladder.

  • First, for a coil from a negative earth car, the spark gap should connect between the centre of the coil and the negative terminal. This may be obvious, but most diagrams you see show the spark going to the bottom terminal on the diagram, and this diagram is the other way up.
  • Second, when you disconnect the power, the capacitor is still charged at 230V. There’s no leakage path, so it can stay that way for a long time. You NEED to discharge the capacitor after you unplug, or you can get a 230V shock from the terminals of the plug. The capacitor probably isn’t large enough to kill you, but I’m sure it can hurt. You should add a switch with a resistor to short out the capacitor, or at the very least remember to short out the plug terminals (giving a good spark at the HV end) after unplugging.

But anyway, a fun evening.