Does anyone remember the old point system, setting the dwell for proper operation? Well the dwell sets the length of time the points are closed and the coil gets electrical flow to build up the electro magnetic flux field of the coil. When the points opens the field collapses extremely fast inducing a voltage in the secondary proportional to the ratio of turns in the primary to secondary windings and coil flux density.
The amount of magnetic flux induced in the coil is based on how long and how much current is allowed to flow in the primary windings allowing the magnetic flux to saturate the metal core.
For an external resistor coil system When starting the vehicle the battery voltage is likely down to around 9 volts. This the time the starter circuit has a direct wire bypassing the ballast resister. This gives the coil good current flow for starting. Once the the engine is running the the starter circuit wire is off and the current goes through the ballast resister. When the engine is idling the current flow to the coil is very high as the points are closed for long periods of time(electricity speaking) . This high current flow warms up the ballast resister and thus the resistance rises and lowers the available voltage to the coil. This keeps the secondary voltage to the plugs within range and the coil from frying.
when the engine rpm rises to 2000 RPM the points are closed less than 1/3 the time as at idle. The ballast resister cools off as the current flow drops and allows more voltage to the coil in decreased resistance in the resistor to keep the coil flux high and the secondary voltage to the plugs within range(12000 volts etc)
this means that the resistance in the primary circuit has to be current sensitive to increase resistance at lower rpm and lower it at high rpm to keep the output voltage in specs.
The primary winding themselves increase resistance as the coil warms up. This is a normal property of most metals as the temperature rises the resistance increases. This is what makes an incandescent tungsten filament work in a light bulb.
a coil that has no external ballast resister has the primary winding very current sensitive. At high currents as the temperature rises the resistance increases and reduces current flow in the primary circuit. When the current flow drops off at high engine rpm the primary winding resistance drops and allows the flux to stay constant in the coil making up for decreased length of time the points are closed.
so you see until the electronic ignition system came out all coils needed current sensitive resistance in them or external for the engine to run properly. The only other choice wold be to have more than one coil.
This also points out it is important to use the proper coil and resister in the ignition system for proper results. ie coil and resister must be matched for the voltage used. Using a 6 volt coil with a resister pulled off another engine on a 12 volt system would certainly give off spec performance.
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