Operation Of The Jump Spark Coil


| June/July 1988



The following article, condensed from John B. Rathburn's I9I3 book Practical Hand Book of Gas, Oil and Steam, was sent to us by Michael Un win, RR #I, Zephyr, Ont. Canada Lo E IT0.

The spark produced by a coil in good shape should be blue-white with a small pinkish flame surrounding it, when the gap is 1/4 of an inch or less. The sparks should pass in a continuous stream with this length of gap without irregular stopping and starting of the vibrator. Coils giving a sputtering, weak discharge that causes sparks to fly in all directions are broken down and should be remedied. The secondary windings of coils are often punctured or broken down by operating the coil with the high tension circuit open, or by trying to cause long sparks increasing the spark gap over 3/8 of an inch in the open air. Coils are also broken down by allowing excessive currents to flow in the primary coil. Never cause a spark to jump over 3/8 of an inch. High compression in a cylinder shortens the jumping distance of a high tension spark. Coils that will cause a stream of sparks to flow across a gap of 1/2 an inch in open air are often unable to cause a single spark to jump a gap of 1/32 of an inch under a compression of 80 pounds per square inch in the cylinder. Remember that a hot spark causes rapid combustion, and will fire a greater range of mixtures and 'leaner' charges, than a straggling, thin, weak spark. Spark coils that give poor results with a long spark gap under high compression are often benefited by the shortening of the spark gap. Shortening the gap will increase the heat of the spark, and will insure the passing of a spark each time the timer makes contact. A good coil should have no difficulty in igniting a piece of paper inserted between the wires forming the spark gap in the open air.

The adjusting screw affords a means of increasing or decreasing the tension of the vibrator spring, and the amount of battery or magneto current flowing through the primary coil. Increasing the tension of the spring requires stronger magnetization of the core to break the circuit from the contact points. This is turn calls for more current from the battery; hence in order to lessen the demand for current on the battery, the tension should be as little as possible to obtain the necessary spark. An increased tension produces more spark as the magnetization of core is increased, but for the sake of your batteries decrease the tension as much as possible with a satisfactory spark. Almost all operators have a tendency to run with too stiff a vibrator, and hence use too much current. An efficient coil should develop a satisfactory spark with 1/4 to 1/2 an ampere of current in the primary coil. I have often had coils that would work well with 1/2 ampere, that were screwed up so tight that coils were consuming 4 to 5 amperes or 8 to 10 times as much as they should.

A battery ammeter used for testing the current consumed by a coil will save its cost many times over in batteries and burnt points if used at frequent intervals in the primary circuit. An automobile or marine engine should be tested for vibrator adjustment in the following way:

Adjust vibrator so that spring is rather stiff. Start the engine and get it thoroughly warmed up and running at full speed, then slowly and gradually decrease the tension of the spring until misfiring starts in; then slowly increase tension until misfiring stops. Increase tension no farther; this is the correct adjustment.

Poor vibrator adjustment is the cause of much trouble and expense as it uses up the batteries and wastes fuel. The principles of correct adjustment are simple, the adjustment easily made, and there is no possible excuse for the high current consumption and rapid battery deterioration met in every day practice. The usual practice of the average operator is to tighten the vibrator until the spark (observed in the open air) is at its maximum. This is commonly known as 'adjusting the coil', shortly after you hear of him throwing out his batteries as bad.