The first connection is one lead of the primary coil winding, and it gets connected to the power supply (typically the 12 volt battery in a car). This lead will usually be marked with a “+”.
The second connection is common between the primary and secondary. The other lead of the primary winding and one lead of the secondary winding are connected to this terminal. This is the point that is connected to ground when the switch closes. It will usually be marked with a “-“.
The third terminal is the high voltage output terminal. It is located in the center of the coil, surrounded by a plastic shroud. Inside the shroud, the terminal looks like a metal cup with an inner diameter of about 5/16”. Refer to the picture below to see the actual terminals. The center high voltage terminal is usually connected via a special cable to the distributor when used in its normal application.
Refer to the pictures below showing the coil connections.
Arcs are drawn from the high voltage terminal and the common “-“ terminal.
The connection to the high voltage terminal is made via a custom piece. It consists of a piece of 1” diameter nylon rod. One end of the rod is machined out to fit over the shroud on the high voltage connector. The other end of the rod has a hole drilled in it to allow a 8-32 threaded standoff to be tightly pressed into place. An 8-32 screw is threaded into one end of the standoff to make the connection to the HV contact within the shroud. The other end of the standoff allows an easy connection to the HV terminal. The pictures below show the details.