If the polarity is reversed, that is the secondary circuit has a negative ground, a higher voltage is required to cause a spark at the gap. This because the centre electrode is considerably hotter than the ground electrode. The coolant in the water jacket of the block carries away more heat from the ground electrode than from the centre electrode. Electrons flow much easier from a hot surface to a cold surface than from a cold surface to a hot one.It is easier and requires less voltage for the centre electrode to give off electrons, or negatively charged particles because electrons more readily leave a heated surface.Also, the cooler ground electrode offers less resistance to current flow. If secondary polarity is incorrect, 20 to 40 percent more voltage is required to complete the secondary ignition circuit. Experiments have proven that 14 percent more voltage is required to ignite a fuel-air mixture when the centre electrode is positive instead of negative in polarity. These same experiments show that 30 percent more voltage is required when polarity is reversed with the centre electrode heated. e.g. if an ignition system were capable of producing 18,000 volts,but required 15,000 volts and had polarity reversed,the ignition requirement would be increased by 30 percent. 15,000 volts by 30%=4,500 volts. Add this 4,500 volts to the required 15,000 volts, and we find we have a voltage requirement of 19,500 volts or 1500 above the capabilities of the ignition system. Accordingly, available voltage from the coil would be insufficient to ignite the fuel-air mixture in the combustion chamber.
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