1976 Johnson 4HP 4R76, 4W76 Outboards Service Manual, Page 28Get this manual
HIGH TENSION LEAD
Figure 4-1D
As the poles of the magnet sweep further along the flux in the center leg collapsesThe rapid development and collapse of flux in the center leg induces an electric charge in the coils The current in the primary coil flows through the closed breaker points and sets up magnetic flux of its own in the center leg of the coreThe direction of the flux momentarily opposes the establishment of reverse flux in the center leg as the poles of the flywheel magnet reach to the center and third leg of the coilAt this precise moment, the breaker points are opened by cam on the engine crankshaftThis interrupts the flow of current in the primary, and the opposed flux in the center leg collapsesTo hasten the collapse, condenser in the primary circuit absorbs the current which would otherwise arc across the opening breaker points The rapid collapse induces current of sufficient voltage in the secondary circuit to arc across the spark plug air gapThe condenser, which acts like storage tank, consists of thin sheets of metal foil separated by insulation, rolled to save space and enclosed in metal caseOne sheet of foil is grounded to the case, the other is connected through an insulated wire to the insulated stationary breaker pointSPARK PLUGS