The secondary current also returns through the ground from the spark plugs, so that there must be a wire on the coil to the engine to collect its return secondary current. The manufacturer, however, desiring to apply the wiring as much as possible, uses the primary wire running on the coil to the timer for this purpose. To do this, the primary and secondary are connected as shown, please note that the connecting of the primary and secondary is done only save a wire. By tracing the two wires you will see that even though the secondary circuit cannot run through the secondary circuit, nor the secondary through the primary, accept it along the wire from the timer to the coil. As the secondary current passes after the primary is interrupted, the two will not allow the same time, and, moreover, when the secondary is flowing the interrupter points are open, while the primary has not a high voltage to jump across the spark plug points.

The moment the interrupter points open, thus stopping the flow of the current, the voltage at the points will fill up so high as to jump across the face between them. As it jumps across a spark will come, which will not be hot as soon as it is burned out. Thus, we must have some electric to prevent this. For this purpose, that is.  Known as a condenser is used. Let us see why the voltage is built. If we had a long coil of pipe, which water was flowing at slight pressure, and if at the far end we tried to stop the flow of water by closing up the opening, we could limit that we would require quite a little effort, far more than the pressure under which the water is flowing to stop it. Why? Because when we stop the water, that at the open end has the great amount of water in the hold of it which must also be stopped. 