Secondary induction coils may be divided into three general classes:
1. Plain coils; 2. Vibrator coils; 3. Condenser coils.
The plain coil gives but one spark when the primary circuit is made and broken, while the vibrator coil gives a series of sparks following each other in rapid succession.
Plain Secondary Induction Coils.—Coils of this class are very simple and consist of:
1. Core; 2. Primary winding; 3. Secondary winding.
The construction of a plain coil, such as would be suitable for ignition service, is about as follows:
The core is made of soft annealed iron wires (No. 20 B and S gauge) from one-half to three-quarters of an inch in diameter and about six inches long. Over this core is slipped a spool of insulating material (hard rubber or composition), on which is wound first the primary winding of the coil, which consists of several layers of about No. 18 B and S gauge silk insulated magnet wire.
After the primary winding has been wound over the insulated core, and the ends have been properly brought out through the heads of the spool to be connected to binding posts thereon, a layer of insulating material is applied over the primary wire, and the secondary winding is then wound on.