The following article is a SIMPLIFIED explanation of ignition systems, although lengthy. There is CONSIDERABLY MORE technical information, including repairs, etc., in the author’s website, particularly here:
A common type of ignition in very early cars consisted, typically, of wooden boxes, with a vibrating electrical contact which sent battery energy into a coil of a modest number of turns of wire wound on an iron core located inside the box. There was another coil of wire wound on that iron core, with thousands of turns, and these turns ‘transformed’ the lower voltage of the ‘primary winding’, fed by the battery, to a few thousand volts, and this was applied to the spark plugs. In most early engines the spark was applied continuously. Somewhat later a rotating switch was used. It had a contact or close by contact, one for each cylinder, and this was called a ‘distributor’. The only problem with the vibrating contact system is that the spark output can not be set to ‘fire’ the spark plug at a very specific piston position, although the distributor method helped SLIGHTLY. With the very low compression ratio and very low power output of these engines, together with the very long stroke and large diameter pistons, these ignition methods worked OK. As engines improved in various areas, the need for better and more precise ignition came about.