[Illustration: FIG. 53.—“K-W” MAGNETO IN SECTION] To start an engine, the crank shaft must be turned at sufficient speed to drive the magneto fast enough to produce a spark. With large engines this is often a difficult matter, so it is very usual to equip a magneto with an _impulse starter_. One part of this is attached to the magneto shaft and the other to the engine shaft that drives the magneto; the two are connected by a spring. When starting, a catch holds the armature and prevents it from turning. The drive shaft turns, however, and in so doing winds up the spring. At a certain point the catch is automatically released, and the spring then throws the armature over at a speed that gives a good spark. A spark is thus assured, even though the engine is being cranked very slowly. CHAPTER VII BATTERY IGNITION SYSTEMS While the greater number of tractor engines use magneto ignition, many use battery and coil systems, which are the same in general principle as magneto systems, but produce magnetism in a different manner. Copper is a _nonmagnetic_ metal; that is, magnetism will not flow through it, nor can it be magnetized. If a pile of iron filings is stirred with a copper wire there will be no effect, as might be expected; but if a current of electricity flows through the wire, the iron filings will cling to it, as shown in Figure 54, as if it were a real magnet. [Illustration: FIG. 54.—MAGNETISM IN A COPPER WIRE] It is one of the principles of electricity that when a current flows through a wire, the wire is surrounded by magnetism, which continues as long as the current flows; when the circuit is broken and the current stops flowing, the magnetism dies away. The magnetism produced is feeble and can be very greatly increased by winding the wire around an iron bar. The magnetism produced by the current then flows into the bar, and that, like the core of the winding of a magneto, throws out magnetism of its own. This is indicated in Figure 55. By changing the intensity of the electric current, or by cutting it off, the strength of the magnetism can be made to change, and this change of strength can produce a sparking current. [Illustration: FIG. 55.—MAGNETISM FROM ELECTRICITY] The principle employed is illustrated in Figure 56. A is a coil of wire wound around one end of an iron bar and connected with a battery; B is an entirely separate coil of wire wound around the other end of the bar, with its ends separated by a short distance. By closing the battery switch the current will be permitted to flow in coil A, and the bar will become magnetized; the magnetism that it throws out will be felt by coil B. When the switch is opened the current stops flowing and the magnetism dies out of the bar; these changes in strength will create an electric current in coil B, which will form a spark as it passes across the space between the ends. [Illustration: FIG. 56.—PRINCIPLE OF SPARK COIL]