[Jacques Reed]

Hi Alastair,

I think it has been covered recently in the forum but I cannot seem to find the thread regarding it.

My basic understanding is that it is to allow more current to flow to the coil for starting purposes when the starter motor is drawing a lot of current ensuring a hot spark. As it heats up, with the engine running and drawing more current through the coil, it increases its resistance and thereby prevents excess current from flowing through the coil and cooking it.

As Mike said the coils are designed to run at about 4.5 volts and not the full 6 Volts of the electrical system. This resistor limits it to that voltage. So it serves two purposes: aids starting and protects the coil.

Various websites have more information on it such as Jalopy Journal, Early Ford Barn etc. Almost everyone says it must be used with original 6 Volt coils. With some aftermarket coils it is important to know the coils internal resistance to work out if a different ohm rated resistor is required.

That's a capsule description and will leave it to you for your own research.
No doubt others can add to this or point you to the original posting.

Cheers,