TM 5-4240-501-14&PTHEORIES OF OPERATIONIgnitionFigure 27.Fig. 27 shows the flow of magnetism through the ironcore of the coil as the magnet in the flywheel approachesthe armature. The arrows indicate the direction of flow ofthe magnetic field. You will notice that there is no (orvery little) magnetism flowing through the upper part ofthe core. This is because of the air gap at the top whichcauses a resistance. In this position our breaker pointsclose.Figure 28.The flywheel continues to rotate to the position shown inFig. 28. The magnetism continues to flow in the samedirection and magnitude through the center of the corebecause of primary current. However, the magnetismflows in an opposite direction through the outer portion ofthe core and through the top air gap because of thechange of flywheel position. Since the shunt air gapprovides a path for the flux from the armature legs andthe core, the required current flow through the primarycircuit is low, assuring long breaker point life.Figure 29.At this position our breaker points open, the currentstops flowing in the primary circuit and therefore theelectromagnetic effect ceases. The magnetisminstantaneously changes from the flow shown in Fig. 28to that shown in Fig. 29. Note the opposite direction ofthe arrows indicating a complete reversal of magnetismwhich has happened so fast that the flywheel magnethas not had a chance to move any noticeable amount.The rapid change in magnetism produces 170 volts inthe primary winding. A voltage is also induced in thesecondary but it is in proportion to the turns ratio, i.e., 60to 1 or 10,000 volts. This voltage is more than ample tofire across the spark plug electrodes. This rapidmagnetism change is very short and therefore the flow ofcurrent across the spark plug gap is as long asnecessary, but short enough to afford long electrode life.Thus we achieve our aims of full power plus long life anddependability.1514
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