By Norman Nock
Originally published in the Austin-Healey Magazine
Coils are normally wound to give a positive earth spark – that is, the spark plug insulated electrode is negative with respect to the engine block or earth. (Fig. 1) We usually refer to this as a ‘negative spark’.
Several distinct advantages are obtained.
We have the same sparking efficiency at considerably lower voltages – approximately at a 10% reduction in the H.T. voltage required to break down the gap. By lowering the voltage, the strain on the insulation throughout the high-tension circuit is considerably reduced – i.e. cable insulation, distributor cap and all mouldings and plugs.
We will point out that if the external connections to the SW and CB terminals of the coil are reversed, current will flow in the opposite direction through the coil, reversing the H.T. spark polarity. In addition the auto-transformer action is lost.
Negative Earth Coil
Standard Lucas coils are all wound for use with positive earth battery: but special coils are available for use on negative earth systems, such coils being connected internally to give a similar spark polarity to those used with the positive earth system.
In emergency the negative earth coil can be used on a positive earth vehicle.
Rotor Wear
Further advantages gained with this negative spark is little or no wear of the rotor arm. In Fig. 2, the picture on the left shows how metal is transferred from the rotor to the fixed electrode on each spark. With the negative spark on the right, the metal transference is in the opposite direction and wear is divided evenly between the four fixed electrodes.
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