Operation Of The Jump Spark Coil

By Staff

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

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

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.

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