Ignition Coil

The ignition coil acts as a transformer, stepping up the voltage whenever there is a drop in the magnetic field. It consists of two windings. Primary windings are made of 100 to 200 turns of copper wire. The copper wires must be insulated in order to prevent shorting within the primary coil. The primary winding coils start at the positive terminal and loop their way around and exit out of the negative terminal. The negative terminal supply is given to the distributor.

Secondary windings contain 20,000 to 30,000 turns of copper wire. These wires also must be insulated to prevent internal shorting. The secondary winding sits inside the loop of the primary winding without any contact between them. To increase the magnetic field of the coil, the secondary winding is wound around a soft iron core.

Inspecting and testing the ignition coil :

From the outside of the coil not much is seen except for if it leaks due to corrosion of the metal casing. Coils with ~1.5 ohm resistance need an external resistor while coils with ~3 ohms do not.
The only way to test its functionality is to pull of the wire which runs from the coil to the distributor at the distributorcap, and held it, with isolated pliers, very close to bare metal and see if it sparks while starting the car. While it does sparks it doesn't guarantee that it will generate enough high voltage (above 20.000 volts) to get a good spark at the sparkplug to ignite the fuel.