Drops the voltage to the coil when engine is running so points won't burn out as 34Phil said.. Here's some info. https://www.fordbarn.com/forum/showthread.php?t=258063&highlight=coil+resistance+unit and https://www.earlyfordv8.org/forum/viewmessages.cfm?Forum=14&Topic=6378
In all (almost all ?) Ignition systems with points they are designed to run at less than full voltage. Phil
Most cars are wired to bypass the resistor when cranking the engine, and run through the resistor in the run position of the key switch. Giving the coil full voltage when cranking makes for faster cold starts, and on the run position the lower voltage makes the points last longer.
The points will burn up quickly due to the current (amperes), it's not the voltage. About 3 amps is the limit.
The ballast resistor is there to limit current, not voltage. A low resistance coil has a quicker build time, ideal for multi cylinder or high revs. Down side is current draw- hence the ballast resistor. As with many things it's a compromise between what is needed and how to get it. If you watch the trace on a scope it usually shows battery voltage. On thousands of cars but rarely understood. Garpo
I understand the comments regarding ‘current’ (amps) but the resistors also have to do with voltage limitation as it is voltage (I believe) that is the issue with with point arcing upon opening and closing.....the reason that the condenser (capacitor) is also used in the point circuit. If resistors were only used for current control, why would the ignition system bypass the resistor on startup to provide full voltage?........because there is a voltage drop during starter operation.
Well, yes, that is the objective...... My point (no pun intended) is that voltage is a factor, not just current flow. An interesting thing about coil operation is that the coil output (secondary) voltage is only whatever it takes to jump the spark plug gap......and that can be variable depending on a number of factors. The only time that is problematic is if the resistance at the plug gap exceeds the coil’s secondary voltage capacity. Stated another way, a 50,000 volt coil may routinely put out only 15,000 or so volts if the condition of the secondary ignition components is very good (no excessive resistance) and it only takes 15,000 volts to jump the gap. If the resistance is greater, the voltage output is greater up to the limit of the coil’s capacity.
Is this a prewar Ford? They used a 4 volt coil on a 6 volt system with a resistor. Most cars that use a resistor have a bypass wire to feed full voltage for starting. I suspect this resistor is made to have low resistance when cold and increase as it heats up. European cars achieved this without a resistor, the coils had internal resistance that increased as they got hot.
The discussion about current & voltage is more easily understood when you keep Ohms Law in mind. V = Volts, I = Amps, R = Resistance. As with all laws of physics Ohms Law is self enforced and cannot be violated. The primary voltage drops at the coil because there is resistance (the ballast resistor or resistor wire or in this case the "resistance unit"). The current is also reduced because of the resistance. Voltage then rises at the coil, and current flow, when the resistor is bypassed (reduction in resistance), per Ohm's Law. And the same law applies at the spark plug too. The current is induced within the coil due to a collapsing magnetic field. The amount of current induced is a function of the ratio of turns of wire between the primary and secondary coils, and the inductance and resistance of those coils. The ignition coil serves as an energy storage device in addition to a transformer. The gap between the center electrode and the ground strap on the plug creates a resistance, so voltage (potential) rises in the secondary circuit because of that resistance. The higher the resistance, the higher the voltage, up to the capacity of the coil to generate current.
Interesting, I didn't know 6 volt ignitions needed a lower voltage. I don't know if any of you noticed but the color of the wires is coded. If you look closely at the spark plug wires each has a color code. I'll bet these were for the guys "nipping the bottle" or couldn't read to get assembly correct..
Yabut my point (heh) was a set of points don't care a whit about 6 or 12 or 15 volts but they sure as heck care about 8 amps! The kickback or ringing of the secondary winding coil being several hundred volts, it gets (mostly) absorbed by the condenser but it's still a pretty good zap. I was just pointing out that everybody gets exercised about ballast resistors and voltage and what they really need to be thinking about is the heat. Ohm's Law is a good one, and I've always supported it.
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