Copyright Goodheart-Willcox Co., Inc. Auto Fundamentals 204 Several hundred turns of heavier copper wire are wrapped around the secondary winding. Each end is connected to a primary circuit terminal on the coil. This wire is also insulated. The turns of heavier wire form the primary windings. The relative positions of the windings and core are shown in Figure 10-3. The core, with both the secondary and primary windings attached, is placed inside a laminated iron shell. The job of the shell is to help concentrate the magnetic lines of force that will be developed by the windings. This entire unit is then placed inside a steel, aluminum, or plastic case. In some coil designs, the case is filled with oil or a paraffin-like material. In other designs, the coil windings are encased in heavy plastic. The coil is sealed to prevent the entrance of dirt or moisture. The primary and secondary coil terminals are carefully sealed to withstand vibration, heat, moisture, and high voltages. Coil Operation When the ignition switch is turned on, the current flows through the primary windings of the coil to ground. See Figure 10-4A. As you learned from Chapter 8, Electrical System Fundamentals, when a current flows through a wire, a magnetic field is built up around the conductor. Since there are several hundred turns of wire in the primary windings, a strong magnetic field is produced. This magnetic field surrounds the secondary windings as well as the primary windings. See Figure 10-4B. If there is a quick and clean interruption of current flow on its way to ground after passing through the coil, the magnetic field will collapse into the laminated iron core. As the field collapses through the primary wind- ings, the voltage in the primary windings will be increased. This is called self-induction, since the primary windings produce their own voltage increase. The voltage induced in the primary windings is about 200 volts, since these windings consist of only several hundred turns of wire. Self-induction does not affect the operation of the primary windings. As the magnetic field collapses, it passes through the secondary windings, producing a tiny current in each turn of wire. This is known as induction. The secondary windings possess thousands of turns of wire. Since they are in series, the voltage of each turn of wire is multiplied by the number of turns. This can produce a voltage exceeding 100,000 volts. The high voltage produced by the secondary windings exits the high-tension coil terminal and is directed to the spark plugs. Primary connections Primary windings Secondary windings Iron core Secondary (high- voltage) output Insulation Goodheart-Willcox Publisher Figure 10-3. Cutaway of an ignition coil. Note the relative positions of the primary winding, secondary winding, and iron core. Although ignition coils may be shaped differently, they all contain the parts shown here. This coil is typical of the coils used on a coil-on-plug system, discussed later in this chapter.