Getting a spark to jump through air is pretty challenging and takes a great deal of energy, even to bridge a gap of just a few millimeters. A disposable camera flash is a pretty impressive little piece of energy ingenuity, squeezing 300 volts out of a 1.5-volt AA battery nearly indefinitely. The flash circuit consists of a fat capacitor, a transistor, a large step-up transformer (which converts the 1.5 volts in the battery to the 300 volts needed to charge the capacitor), and a small step-up transformer (which steps up the 300 volts in the capacitor to almost 1000 volts to trigger the flash—for more information on how transformers work, flip back to Chapter 7, the Ticklebox).

When you press the charging button, the 1.5-volt battery begins feeding the first transformer, using a transistor to create an oscillating current (since the transformer needs a flowing current to work). The high-pitched whine is the sound of that oscillator speeding up as the capacitor nears 300 volts. Once the capacitor is charged, you press the trigger; this sends a small jolt to the secondary transformer, which in turn pumps a big jolt to the middle contact in the spark plug. In a camera flash, this is attached to a metal plate that runs along the back of the bulb, between the two leads. This sudden burst of voltage ionizes the Xenon inside the flash tube (or, in our gun, the tiny air gap between the two points), making it conductive enough for the 300-volt spark to jump between the two end leads. Shazam!