The Bosch-Honold Magnetic Plug

October/November 1992
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FIG. 46. Low tension, or make and break system of ignition.

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Reprinted from The Automobile Instructor, a 1911 manual written by Clyde H. Pratt. Submitted by Claude B. Andersen, 1 148 La Casa Ave., Yuba City, California 95991.

The reason we address the low tension or 'make and break' system first is because it is very simple electrically and easy to understand. It is used but very little, however, because of the many mechanical parts necessary on each cylinder. Unless you have a car with this system on, it is not necessary to spend much time on the remainder of this lesson. Just read it through carefully.

If an ELECTRIC CURRENT is broken in any way, either by a switch or the simple separating of any two wires in the circuit, a small spark will be formed at that point. If this current was broken inside of the cylinder it would make the spark there and explode the gas. On many of the earlier types of automobiles and a few of those still being made this is exactly what is done. Fig. 46 will give you an idea of how this system works. It is called the 'Make and Break' system because the electric current is made and then broken inside of the cylinder. It is also called the 'Low Tension' system, because there are no induction coils used to raise the voltage (electric pressure).

The working of the Make and Break system is as follows: In Fig. 46, B is the source of current, which may be either dry cells or storage battery. A is the switch, C is a simple primary coil of wire wound on the soft iron core D, E is the engine cylinder, F is a little hammer which is free to move toward or away from G. The spring S holds F as far away from G as the cam R will permit. R is the cam wheel which rotates and makes the electrical connection inside of the cylinder once each revolution of the cam by forcing F against the point G. When the cam R turns just beyond this point at which F and G are held in contact, F slips off the catch on the cam and is forced away from G by the spring S, thus making an electric spark inside of the cylinder. H is the insulation which prevents the electric current from flowing from G to the cylinder E, except when the cam R forces F against G.

If you will look carefully at Fig. 46 you will see that there is a complete circuit when the switch A is 'on' and F is forced against G, thus:-From battery B through A, through C, through G, through F, from F to cylinder E (E is all metal and therefore an electrical conductor), from E back to batteries B through ground wire W, thus making a complete circuit.

When the piston is at the top of the cylinder on the end of compression stroke the cam R is so set that it will let F slip off the point and break the circuit between F and G, and then make a spark to ignite the gas. On a four cycle engine the cam R will be geared to turn half as fast as the crankshaft. Why?

The simple coil C is put in series in the circuit to make a hotter spark inside the cylinder when the circuit is broken. It also acts as a resistance in the circuit, and keeps the electric current very low even when the spark plug points F and G are in contact. If any of the wires between the switch and igniter G should have the insulation worn off and come in contact with the engine in any way, the electricity would flow back over to the wire W without going through G and F. This would then cause a short circuit.

If we use a low tension magneto with this system of ignition it is wired as shown on the right of Fig. 46. J is the magneto, K the switch. One binding post O of J is connected by ground wire M to E. The other binding post P is connected to K, from K to wire L, which is connected to G, G to F (when the cam is in right position), F to E, E to the magneto at O, which completes the entire circuit from one binding post on the magneto back to the other. If a magneto is used with this system no simple coil is required as the armature answers the purpose of a coil. If this system be used on a four cylinder engine, the wire L should be connected to the insulated part G in each cylinder. There would be a cam R for each cylinder.

There are a very few ignition systems of this 'Make and Break' type in which an ELECTRO-MAGNET (when an electric current is passed through the coil C in Fig. 46 the iron core D becomes an electro magnet, and stays a magnet only as long as the current is flowing) is used to separate the points corresponding to F and G in Fig. 46.

The most important of these electro magnetic plug systems is the Bosch-Honold magnetic plug system, and this is given here only for reference, as the system is used but little. The plug used in this system is shown in Fig. 47. The two points which make and break the circuit are 27 (a continuation of 1) and 20. 27 is movable toward and away from point 20, about the steel knife edge about one inch from its upper end. 3 is a flat 'U' shaped spring which presses 27 against 20 when in normal condition. 2 is a pole piece, or electromagnet; 4 is an iron sleeve; 5 is a magnetic coil; 6 current conducting ring; 7 current conducting rivet; 8 mica washer; 9 binding post nut for electrical wire; 10 current carrying plate; 11 insulating brush; 12 mica ring; 13 upper magnet yoke piece; 14 detachable brass piece; 15 separating brass piece; 16 internal ring nut; 17 center ring; 18 mica plates; 19 packing washer; 20 contact point; 21 contact piece or plug body; 22 steatite cone; 23 hexagon head with thread of plug; 24 packing ring for coil; 25 lower magnet yoke piece; 26 connecting screw for the winding.

When an electric current is sent through this plug it enters through the binding post 9, then goes to plate 10, then to rivet 7, then to ring 6. From here the current flows through the winding 5, then by contact screw 26 to the coil body 25; from the coil body to the interrupter lever 1 by way of the knife edge, then-when 27 is in contact with 20 (which it is when normal)-to the cylinder through 21. As soon, however, as the current starts to flow the pole piece 2 becomes a magnet and draws the upper part of lever 1 towards it, thus throwing the bottom end of the lever 27 away from 20. As this occurs the circuit is broken between 20 and 27, causing a spark.

With this plug on a single cylinder engine a switch would have to be fastened to the engine in such a way that when a spark is required in the cylinder the switch would be thrown on (connected). A switch connected to the engine so as to connect the circuit at every second revolution (in case of a single cylinder) is called a TIMER. The Timer will be taken up later. This plug is usually used on four cylinder engines together with a special magneto. The wiring for this system is shown in Fig. 48.

The armature wire is grounded to the magneto base at E; the other end is connected to the distributor A (revolving switch) through shaft F, spring H and wire I. The wire I can be connected to any one of the wires that lead from the distributor to the plugs by the brass piece A. As shown in the diagram, I is connected to plug No. 1. At the next quarter turn I would be connected to plug 3, then to 4, then to 2. From the plugs the circuit is completed by means of the cylinder and engine frame back to E again. When the contact points at B are together a part of the current flowing through the armature does not go through the outside circuit (H, I and plugs) at all, but is taken off at C and short circuited back to D. When the contact at B is broken all of the current is suddenly sent through the one of the plugs which is connected in the circuit by distributor A, thus making a spark as the two contact points on the plug separate.

In any system of wiring be sure that all the wires are clean at the ends where the contacts are to be made. See that every binding post is tight. The principal troubles that are likely to arise with the Make and Break system are usually in the make and break mechanism. The points of contact may become dirty so that they will not make an electrical circuit when pressed together. In this case take apart and clean the points. The insulation H in Fig. 46 may become dirty so that the current will flow from G direct to the cylinder without going through F. This causes a short circuit which can be remedied by removing the insulation and cleaning well with gasoline and waste.

It is well to look at the wiring occasionally to see that the wires are not rubbing on some part of the machine and wearing the insulation off. In general see that everything in the ignition system is as it should be as far as you can without tearing it down, and you will have very little trouble with it.

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