A spark gap consists of two metal electrodes placed close to each other with a gap in between, where an electrical spark can jump. They are used for igniting the fuel in combustion and hybrid cannons. Spark gaps are normally made by screwing two screws through the side wall of the chamber so that the tips of the screws end up very close to each other. The screws should be mounted to keep the heads a good distance a part to ensure that the spark will not jump between the heads outside the chamber. The gap distance must be small enough that the voltage from the ignition source can jump across it. The distance dependence is about 1 mm/kV (25KV/inch) for sharp tipped electrodes. The distance varies with the shape of the electrodes, sharp points will spark across a larger distance for a given voltage than will dull points. For two flat surfaces the distance drops to about 0.3mm/KV (75KV/inch).
The maximum spark distance is also dependent on the pressure of the gases. As the pressure is increased the gap distance a particular voltage will spark across decreases. The pressurized mixture of a hybrid will shorten the max spark length significantly, so a hybrid spark gap should be a fair bit smaller than practical spark length in air to ensure reliability. The relationship between pressure and distance is roughly linear. Increasing the pressure 10X will decrease the distance by roughly 10 fold. So, a BBQ piezo sparker that reliably sparks across a 1/8" gap in a 1X gun will only spark across an ~0.01" gap in a 10X hybrid.
Spark plugs can be used for spark gaps, and are especially useful in metal guns, where the chamber wall would short out the screws of an ordinary spark gap.
Two or more spark gaps can be connected in series to generate multiple sparks, as long as the width of the combined gaps is smaller than the maximum spark distance. Spark gaps can not be wired in parallel, since the spark will then only jump the smallest gap.
(For details see http://burntlatke.quibble.com/short.html)
Spark strips are another popular method of achieving multiple spark gaps.