Understanding ignition basics and building your own ignition system.
Figure 2: If you and wrap the wire into a coil, the magnetic flux lines complement each other.
Figure 1: Whenever electrical current flows through a conductor such as a wire, circular magnetic lines of force from around that conductor.
Figure 3: If an iron core is placed in the coil, the flux lines will travel through the iron. This is an electro magnet.
Figure 4: Suppose we wrap two coils around an iron core and induce an electrical current into one coil. We'll call it the "primary coil." This will cause a magnetic field or flux lines to build around the iron core. As the flux lines are building and cut across the wires in the "secondary" coil, electricity will flow in the secondary coil. This is called induction.
Figure 5: A round head rivet is placed in the cam gear. The rivet acts as a contact to complete the circuit from the battery and around through the primary winding. This contact point or switch needs to be adjustable so that the timing of the spark will fire just as the piston comes to the top of the compression stroke. This adjustment can be as simple as just bending the contact finger that touches the rivet head. On large engines it is very desirable to have some means to retard the timing for starting, and then advancing it for running.
Figure 6: One way to build and ignition system.
Figure 7: The simplest high tension ignition system is the single spark ignition system shown here.