Operation Of The Jump Spark Coil

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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.

After once getting the vibrator in proper trim the ear will give much information as to the adjustment. A vibrator adjusted too lightly will cause 'skipping' or misfiring with the consequent loss of power. Never attempt to operate a coil that is damp; the coil will be ruined beyond repair. Above all, do not place the coil in a hot oven to dry, as the box is filled with wax, and if this is melted it will run out and reduce the insulation of the coil. Dry coil gradually. If the batteries are new or too strong the vibrator may be held against the core of the coil so that the vibrator will not buzz. If this is the case loosen the screw until it works at the proper speed. If the batteries are weak, the coil may not be magnetized sufficiently to draw the vibrator and break the circuit. If this is the case tighten the screw. If the vibrator refuses to work with the battery in good condition, and if you are sure that the current reaches the coil, look for dirty or pitted contacts on the vibrator. Should the contact points be dirty, clean them thoroughly by scraping with a knife or sandpaper. Water on points will stop the vibrator, as will oil or grease. If contact points are of a uniform gray color on their contact surfaces, and are smooth and flat without holes, pits or raised points, they are in good condition. If pits, discoloration or projections are noted, the contact surfaces should be brought to a square, even bearing by means of a small, fine file. The points should not come into contact on an edge, but should bear on each other over their entire surface. Do not use sand paper to remove pitting, as it is almost impossible to secure an even, flat surface by this means.

It is best to remove contact screw and vibrator blade for examination and cleaning, as the adjustment will be impaired. When replacing contact screw and vibrator blade in coil, be careful that they are in exactly the same relative position as they were before removing. Also be sure that the contacts meet and bear uniformly on their surfaces.

Vibrator Construction

The main objects in view in the construction of a successful vibrator are:

(1) To reduce the weight of the moving parts as much as possible in order to increase the speed of vibration, and to make the trembler instantly responsive to the timer.

(2) To cause the contact points to separate as much as possible in order to cause the maximum spark.

(3) To have the contacts as hard and infusible as possible to resist wear and the action of spark between the contacts.

(4) To make any adjustments that may be required, due to wear, as simple and accessible as possible.