The Bosch-Honold Magnetic Plug


| October/November 1992



Low tension, or make and break system of ignition'

FIG. 46. Low tension, or make and break system of ignition.

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?