HOW DO YOU make an electric discharge climb a pair of wires? In practice, it is quite easy. The two vertical wires are spaced close together at the bottom and slightly splayed apart to increase the gap as the sparks rise.

So why do they rise at all? Surely the spark would always take the shortest route rather then extend itself as it travels upwards?

But the spark discharge is actually taking the shortest path, or rather, the easiest path from one electrode to the other. Initially, the discharge does take the shortest path which is at the bottom of the wires. But the continuous spark discharge is hot and heats up the air around it. This heated ionised air rises, carrying the discharge up with it until the gap between the two electrode wires is too large to maintain the spark. The discharge then starts at the bottom again and the cycle continues.

Fig. 1: this shows the spark plug firing arrangement for the Commodore V6 double-ended ignition coil. The two spark plugs are fired togther(in series), so quite a high output voltage is needed.

Back in September 1995 we produced a Jacob’s Ladder circuit which has been popular ever since. But just recently our attention was drawn to a number of mains-powered discharge circuits on the internet. While quite spectacular they are also quite dangerous.

We got to thinking: how can we produce something just as spectacular but not mains-powered? Our original Jacob’s Ladder circuit was based on a conventional 12V ignition coil and we realised that today’s cars have very powerful ignition systems.

So why not revise the circuit with a higher-powered coil out of a late model car?

In practice, it turned out to be not quite so simple. While all current model cars use engine management and high-energy ignition systems, they use a wide variety of ignition coil arrangements.