The Bosch-Honold Magnetic Plug

By Staff
1 / 3
FIG. 46. Low tension, or make and break system of ignition.
2 / 3
FIG. 47. Magnetic plug.
3 / 3
FIG. 48. Wiring diagram for magnetic plug system.

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

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

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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