I'm no electrical engineer, so I'm sure this is a stupid question.

this has been bugging me:

In the old points systems, a resistor wire is used inline between the ignition switch and the coil, which drops the voltage to the coil and the points.

I'd always read that this was to prevent burning the points.


My question...
Why isn't the resistor between the coil and the points, thereby dropping voltage to the points, but getting the full battery voltage to the coil?

 

It won't make any difference. If you put the resistor between the coil and the points, you would get about 3 volts across the resistor, and that would still leave about 9v on the coil. If you put your meter on the coil + you would get 12v there, but on the ground of the coil going out to the points you would get a 3v reading. So you still would have only 9v on the coil.

The resistor is there for a "cushion" of sorts. When voltage is applied to the coil, it's seen as almost a short circuit in the beginning till it charges up. The resistor limits this current so the coil doesn't heat up, and the points last longer like you said.

 

Franklin2 is correct. You're not restoring any voltage by putting the resistor "below" the primary winding of the coil. They're in series; the current (and therefore the voltage drop across the primary winding itself) remain the same.

While it makes no electrical difference, there are two practical reasons why the resistor wire is physically "upstream":

1) The resistor wire has to be somewhat long to build the proper resistance. It would not make sense to try and bundle this in between the relatively short distance between the distributor and the coil.

2) The resistor needs to be bypassed in START. If the resistor wire was in between the coil and the points, it would not be possible to bypass the resistor in a simple manner.

It's actually the other way around. The primary winding of the coil is effectively an inductor, and inductors by nature resist a change in current. When the points close, primary current starts at zero and is slow to rise. As the coil "charges," current increases, until the reactance of the coil approaches zero, and current is limited by the DC resistance of the primary winding, and the ballast resistance. For any inductor, a steady-state current effectively turns the inductor into a small resistor. When you subsequently open the points, you are forcing the inductor current to zero which induces a large voltage across the primary winding. This is reflected across the secondary winding to form the high-voltage, low-current spark we see at the plugs.

You're thinking of a capacitor. When a capacitor is discharged and a voltage is suddenly applied across its terminals, it appears as a short circuit until it charges up, at which point it appears as an open circuit.

 

ah- got it (i think).
thanks for taking the time to answer a noob question guys-