The circuit in a magneto ignition system in which the primary current flows. Primary current is produced when lines of flux from the rotating magnet cut across the turns of wire in the primary winding of the magneto coil. At the instant the primary current is greatest, a set of breaker points opens the circuit and the flow stops immediately. The magnetic field caused by the primary current collapses, cuts across the many turns of wire in the secondary winding of the coil, which is wound around the primary winding, and induces a high voltage in it.

The primary circuit includes the primary winding of the coil, the breaker points, a condenser, and an ignition switch. The breaker points, condenser, and switch are in parallel with each other and in series with the coil.