High-Tension Magneto System Theory of Operation – The Secondary Electrical Circuit

in Engine Ignition and Electrical Systems

The secondary circuit contains the secondary windings of the coil, distributor rotor, distributor cap, ignition lead, and spark plug. The secondary coil is made up of a winding containing approximately 13,000 turns of fine, insulated wire; one end of which is electrically grounded to the primary coil or to the coil core and the other end connected to the distributor rotor. The primary and secondary coils are encased in a nonconducting material. The whole assembly is then fastened to the pole shoes with screws and clamps.

When the primary circuit is closed, the current flow through the primary coil produces magnetic lines of force that cut across the secondary windings, inducing an electromotive force. When the primary circuit current flow is stopped, the magnetic field surrounding the primary windings collapses, causing the secondary windings to be cut by the lines of force.


The strength of the voltage induced in the secondary windings, when all other factors are constant, is determined by the number of turns of wire. Since most high-tension magnetos have many thousands of turns of wire in the secondary coil windings, a very high-voltage, often as high as 20,000 volts, is generated in the secondary circuit. The high-voltage induced in the secondary coil is directed to the distributor, which consists of two parts: revolving and stationary. The revolving part is called a distributor rotor and the stationary part is called a distributor block. The rotating part, which may take the shape of a disk, drum, or finger, is made of a non-conducting material with an embedded conductor. The stationary part consists of a block also made of non-conducting material that contains terminals and terminal receptacles into which the ignition lead wiring that connects the distributor to the spark plug is attached. This high-voltage is used to jump the air gap of electrodes of the spark plug in the cylinder to ignite the fuel/air mixture.

As the magnet moves into the E-gap position for the No. 1 cylinder and the breaker points just separate or open, the distributor rotor aligns itself with the No. 1 electrode in the distributor block. The secondary voltage induced as the breaker points open enters the rotor where it arcs a small air gap to the No. 1 electrode in the block.

Figure 4-10. Relation between distributor terminal numbers and cylinder numbers.

Figure 4-10. Relation between distributor terminal numbers and cylinder numbers.

Since the distributor rotates at one-half crankshaft speed on all four-stroke cycle engines, the distributor block has as many electrodes as there are engine cylinders, or as many electrodes as cylinders served by the magneto. The electrodes are located circumferentially around the distributor block so that, as the rotor turns, a circuit is completed to a different cylinder and spark plug each time there is alignment between the rotor finger and an electrode in the distributor block. The electrodes of the distributor block are numbered consecutively in the direction of distributor rotor travel. [Figure 4-10]

The distributor numbers represent the magneto sparking order rather than the engine cylinder numbers. The distributor electrode marked “1” is connected to the spark plug in the No. 1 cylinder; distributor electrode marked “2” to the second cylinder to be fired; distributor electrode marked “3” to the third cylinder to be fired, and so forth.

In Figure 4-10, the distributor rotor finger is aligned with the distributor electrode marked “3,” which fires the No. 5 cylinder of a nine-cylinder radial engine. Since the firing order of a nine-cylinder radial engine is 1-3-5-7-9-2-4-6-8, the third electrode in the magneto sparking order serves the No. 5 cylinder.