Low Tension Magneto

The basics of how low tension magnetos work

| January/February 1984

There are several types of  low tension magnetos that are very popular in use on antique engines. Webster, Sumter, and Wizard were a few of the manufacturers of the oscillating type, identified by the extension springs on the front of the magneto as shown in the photo.

These magnetos were operated by a push rod that moved a lever to produce a movement of the armature around an arc of approximately 30 degrees. Another type of low tension magneto was the rotating armature type, very commonly used. We will discuss the oscillating type at this time, and will explain the rotary type in a subsequent article.

A low tension magneto had only one winding that was needed to produce a flow of current. It is an alternating current but this is of very little significance.

Basic electricity study tells us that all we need to do to produce an electric current is to move a steel bar through the magnetic field between the poles of a permanent magnet. The faster you move the steel bar and the greater the magnetism, the greater the current flow. This is the basic principle of a dynamo, generator or magneto. The current flow does not produce the voltage necessary to create a spark that we can see or feel, as you can with a high tension spark.

The low tension magnetos were usually used with an interrupter or spark igniter, rather than a spark plug. However, with the use of an external coil and interrupter, the low tension magneto could produce a high tension spark to be used with a spark plug. Henry Ford utilized this method on his model T engines.

The principle involved with the igniter was that if you break a connection while current is flowing through a wire, you produce a spark. You may have noticed this when you pull the plug of an electrical appliance or drop cord. The flow of current from a low tension magneto is of extremely short duration when it is at its peak. At this time while current is flowing through the contact points of the igniter, they must be snapped open, very rapidly. This is accomplished through a system of levers and rotating shafts that will be described in more detail later (see figures 1,2, and 3).