[Illustration: FIG. 138.--Diagram of battery and coil connections for jump spark ignition as applied to a motor cycle. Coils are usually plainly labeled with the abbreviations: “Bat.,” “Pri.,” “Sec.,” indicating that the wires are to be connected to the battery, the primary circuit or contact maker, and the spark plug. The battery and primary wires being for the low tension circuit are easily distinguished from the secondary wire by the small amount of insulation surrounding them.] 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. The wire for the secondary winding consists of about No. 36 B and S gauge silk covered magnet wire, the amount used varying considerably, depending on the desired voltage of the secondary current. When all the wire has been wound on, the ends are brought out to the binding posts, the coil is soaked in shellac dissolved in alcohol and baked, or in melted paraffin or a paraffin compound, and allowed to cool. It is then placed in either a cylindrical hard rubber shell or in a hard wood box. The proportions of such coils vary greatly; for motor cycle use they are made long and of small diameter (10×2-1/2 inches for instance), while for some other purposes short and thick coils are found more convenient. =Ques. How may the coil just described be connected for demonstrating purposes?= Ans. Connect the ends of the secondary winding to fixed insulators and bend the ends so they are about 1/8 inch or less apart. Connect one end of the primary winding to a battery and brush the other end of the primary winding against the other terminal of the battery as indicated in fig. 137. =Ques. What happens when the primary circuit is made?=