Your vehicle's ignition coil uses 12-volt power coming in from a wire that gets electricity from the battery and alternator. The coil is also grounded using a similar wire, going either to the block or the firewall or chassis. It is essential that both wires be hooked up correctly and in good shape for the coil to work properly.
Inside an ignition coil, there are hundreds of rows of tiny copper wire, all coiled up and wrapped in a circle. Once the 12 volts travels through this series of wire, the output is many times greater than the input--10,000 to 20,000 volts. Performance applications (racing ignitions, for example) might reach 40,000 or even 50,000 volts. The increase in voltage is the reason the ignition coil gets hot after a very short time.
The new voltage travels to the center of the distributor cap through a spark plug wire connected to the coil. From there, the rotor inside the distributor spins around, and the constant voltage from the coil is distributed to the spark plug that needs to fire. This cycle continues until something fails, such as the connections in the distributor cap or wire going bad, or the coil itself expiring.
One of the oldest ways to test the coil is to take the wire off the distributor side and hold it near a suitable ground, such as the block or a valve cover. The key is then turned to the “on” position, and if there is a spark out of the wire, then the coil is good. While this method will indeed work, use extreme caution because of the high voltage involved. Always wear safety goggles and insulated gloves, and use pliers with rubber-coated handles to hold the wire. Be sure not to hold it there too long, because you are essentially sending a power to the ground and the coil could short out.
