Final answer:
The necessary spark gap for ignition depends on the engine but typically, lower voltages are used for smaller gaps. Magneto systems induce high voltage through electromagnetic induction, with environmental factors such as humidity and conductor shape affecting the voltage required for a spark.
Step-by-step explanation:
The spark gap needed for a spark ignition varies based on the voltage available and the conditions of the surrounding environment. One of the implications of the voltage needed to generate a spark is that it takes about 75 kV to make a spark jump across a 2.5 cm (1 in.) gap, or 150 kV for a 5 cm spark. A smaller gap would require less voltage, and the exact gap needed is specified for each engine and spark plug type. A magneto generates this high voltage by spinning a magnet quickly in front of a wire coil, inducing a current in the coil due to electromagnetic induction. The rapid change in the magnetic field as the magnet spins creates a significant voltage difference across the wires' ends, which can then cause a spark across the gap. Factors such as humidity and the presence of sharp points can affect the voltage required for a spark. On a dry day, higher voltages can be built up due to lower moisture in the air, but sharp points on a conductor can generate sparks at lower voltages because they create stronger electric fields than smooth surfaces. Conversely, in humid air, the air breaks down at a lower field strength, allowing a spark to jump with a smaller voltage.