Ballast resistor
The ballast resistor is used in the spark ignition system. It drops the voltage to a lower level to protect the contact points. This is sometimes referred to as a "low energy" ignition system and was the normal system through the 1970s.
Overview
A stronger spark occurs with higher voltage, so lowering the voltage -- though it protects the system -- is not a performance answer. For this reason newer Electronic Ignition systems do not use it. Early EI distributor modules (such as the 1973-1977 Datsun units) could not handle the high current and so continued to use a resistor. In 1978 they upgraded the transistor to handle more current and so were able to get rid of the resistor -- this is called a "high energy" system.
Typical specifications
22460-H7210 RESISTOR FED -0277 [for Points distributor] * 1.17 to 1.43 ohms primary @68F/20C 
22460-H7310 RESISTOR CAL -0277 3-tap type for early EI (low-energy and medium-energy taps) * 1.3 ohms (.0 primary + .9 secondary) @68F/20C 
No Resistor * Used with high-energy ignitors and allow use of 0.7 ohm coil
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