Hints and Tips - Coil Polarity

The Question:

Someone has written to me with a query about ignition coils asking which low-tension terminal (+ or -) on the coil should be connected to the distributor contacts on a positive earth car. All my coils are marked SW (ignition switch) and CB (contact breaker). There are various numbers on the base of the coils and also a letter, some are L, others R or LO.

Obviously somebody thought that polarity matters otherwise why mark the terminals at all? Are coils different for positive and negative earth cars or should positive earth cars connect the SW terminal to the distributor?

The real problem in this case is engine cutting out due to coil overheating, could this be caused by incorrect polarity?

The Answer

There is a good reason spark coils are wound the way they are. Positive and negative ground coils have their windings thrown (wound) in different directions in relationship to each other. Spark coils are always wound in such a manner so that the high tension connection on the coil goes negative when the distributor points open. This means that the rising potential at the spark plug is also negative. Why would one want an operating condition where the rising pulse from the spark coil is negative?

The center electrode in a spark plug runs hotter than its grounded electrode. Since electrons can be striped off of a hot electrode more readily than off of a cold electrode, the center electrode is always made the negative electrode. The result of this choice is that the arc across the spark plug electrodes can be initiated at a slightly lower potential when the center electrode is the negative electrode. (In this manner, thermionic emission aids in the establishment of the initial discharge.) As Pete observed, his vehicle will run with either a negative output or a positive output from the coil. I have operated my TD with the incorrect high tension polarity from the spark coil for over 20 years. So it requires a slightly higher potential to strike the arc - big deal. This polarity reversal occurred when I converted to negative ground so I could more conveniently install contemporary electronic devices. I never bother to fit the 'correct' negative ground spark coil.

After the arc (plasma) has been initiated, the circuity oscillates and the thermionic emission consideration becomes meaningless.

Conclusion -- The coil will work and your MG will run no matter what you do. If you want to be sure that you have the slightly more desirable initial negative output then pay close attention to the polarity of the primary connection. The primary terminal marked (-) must connect to the (-) terminal on the battery.

Best wishes, Carl Cederstrand

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