Albert Graham.

Two methods of electric ignition are in general use to explode the compressed charge behind the piston of a gas or gasoline engine. These are commonly called the "make and break" spark method and the "jump-spark " method. Of these two perhaps the " make and break" contact method will give the most general satisfaction, although to make a positive assertion on this point is to assume a great deal, since the jump-spark method allows of great latitude in instantaneous adjustment, known as "timing," by means of which the explosion is made to occur at different points of the stroke. The principle difficulty with the former method is the rapid deterioration of the sparking points due to the pitting caused by the spark, while on the other hand the spark-plug used in the jump-spark method occasionally becomes covered with soot (unburned carbon deposited after the explosion and caused by insufficient air being mixed with the gas or gasoline vapor, thus preventing complete combustion) which short circuits the current, on the other hand the oil used in lubricating the cyclinder is sometimes deposited on the spark-ng terminals of the plug and burned on as a glaze by the heat. This forms an effectual insulating medium through which the spark may be unable to penetrate owing to the increased resistance.

In order that a spark may pass between any terminals separated by an air gap, it is necessary that the tension of the current be raised to such a point that the resistance of the intervening layer of air will be overcome, and the current passing across will heat the suspended matter into a highly incandescent state which gives rise to the spark.

If we join to the terminals of a cell two wires and complete the circuit by joining the free ends, we will notice that a small spark is formed when the ends are suddenly separated. Suppose we now include in the circuit an electric bell, when the bell is ringing sparks of considerably larger size will be seen at the vibrating armature spring contact, even with the same cell as before. Now this increase in the size of the spark is caused by the effect of the magnets in the following manner.

When the bell is in a passive state and the current is first turned on by pressing a button or other means of closing the circuit, the magnets do not instantly acquire their full strength, but require a very short interval of time to "build up." When this state is reached they attract the armature, thus breaking the circuit. But the lines of magnetic force which are at this time coursing through the magnet cores and armature do not cease immediately, and it is the cutting of these lines of force by the coiled conductors (wire) as they fall from their maximum value to zero that generates sufficient tension to make the spark jump the gap between the terminals. It is, in fact, as if the current possessed inertia which required the application of an external force to bring it to rest.

Now this is exactly why a spark coil is provided for a gas engine with contact points that are suddenly separated. It consists of a core of soft iron wires closely bound together and surrounded by a coil of insulated magnet wire of many tnrns. The core is built up of small wires because it makes a much stronger magnet, due to various reasons that need not be discussed at present.

To make this core, it will require 2 1/2 lbs. of No. 14 soft iron wire, cut into lengths of 7". About 250 pieces will be required. When all are cut, bundle them together as closely as possible and wrap firmly with small steel or iron wire. Lay a piece of thin sheet iron over a good bed of coals in a stove and upon this place the bundle, leaving it there until the entire mass is brought to an even cherry red heat, not too bright. Then by means of a pair of tongs place the red hot bundle just on the top of the fire brick where it will cool very slowly, pushing it back further and further from the coal pit at intervals of about half an hour. This will serve to thoroughly anneal the wires and give them a high permeability.

After they are cooled off at the back of the stove, bring them into the air and allow to cool to the temperature of the room. Then bind the ends firmly so that the first binding wire may be removed. Now begin to wrap the bundle with a strip of foolscap paper cut into strips 5 1/2" wide, cementing it in place with shellac. When a layer about 1/16" is laid on, tie a thread around it to keep it from unrolling and bake in a warm oven until perfectly hard. This paper tube will serve to keep the wires in place but the binding wires now on the ends must not be removed entirely until the coil is finished.

Cut out the base from cherry or some other hard wood, making the coil ends B-B of the same stock. Bore a hole in each end as shown so that it will just go over the paper tube. Force the ends on and secure by a little shellac, after making both bases parallel so that they will each lie flat on the base A. The binding wires on the ends may be temporarily removed while these ends are being put in place, but should be replaced immediately thereafter.

Now find the centre of the iron wire core C with a pair of dividers, and drill a small hole in each end, countersinking for the lathe centers. Swing the spool on the centres and drive either by a clamp-dog or a piece of wire wrapped around the core end and then fastened to the face plate. The binding wires on the core ends will prevent the pressure of the tail centre spreading the wire.

One of the spool ends has a vertical hole drilled from the top to the center which accommodates the end of the inside layer of wire. Pass the wire through this hole before beginning to wind leaving enough protruding to attach to the binding post E. Wind the space between the ends full of No. 18 single cotton covered magnet wire, which will require about 6 1/4" pounds. Secure the end of the last layer to the other binding post. These binding posts should have a wood-screw end which can readily be screwed into place and exert a good pressure on the bared end of the wire beneath them, thus preserving a good contact. Give the last layer two or three good coats of shellac varnish, allowing each to dry in an oven before applying the next. This will serve to exclude moisture from the coils. A layer of cord may be wrapped on outside of this if desired, which, when stained black with india ink or shoe-polish, dried, and polished with wax makes a very handsome finish.

Mount the spool on the base by four screws passing through it and finish by giving two coats of varnish. This coil should be connected in series between the battery and the sparking device of the engine. It is very strong and will give a good fat spark if the connections are well made. A word of advice to those fitting ont gasoline or gas engines with a make and break contact ignitor. Do not attach one terminal to one end of the spark plug and the other to some remote part of the engine frame where the current will have all the extra resistance of the metal, frame joints and dirty, oil-begrimed bearings of the other vibrating contact point to pass through. Drill a hole into the end of the moving or oscillating pin and tap it so that it will take a binding post. Then make your other connection to this post, wrapping the wire once or twice around the shaft or even the binding post itself, tying it there with thread or another wire. This will allow the wire to bend with the movement of the shaft and not be broken off. The amateur will find that this method will give him less trouble than the ordinary method, his spark will be better for the lower resistance, and with proper care the connecting wire need not be broken by the vibrations. This one point in particnlar is where fully half of the present day engine troubles lurk, The jump-spark coil will be described in the next number of Amateur Work.