Early Ignition Systems--Electrical Ignition Best--Fundamentals
of Magnetism Outlined--Forms of Magneto--Zones of Magnetic
Influence--How Magnets are Made--Electricity and Magnetism
Related--Basic Principles of Magneto Action--Essential Parts of
Magneto and Functions--Transformer Coil Systems--True High
Tension Type--The Berling Magneto--Timing and Care--The Dixie
Magneto--Spark Plug Design and Application--Two-Spark Ignition--
Special Airplane Plug.
EARLY IGNITION SYSTEMS
One of the most important auxiliary groups of the gasoline engine
comprising the airplane power plant and one absolutely necessary to
insure engine action is the ignition system or the method employed of
kindling the compressed gas in the cylinder to produce an explosion and
useful power. The ignition system has been fully as well developed as
other parts of the engine, and at the present time practically all
ignition systems follow principles which have become standard through
wide acceptance.
During the early stages of development of the gasoline engine various
methods of exploding the charge of combustible gas in the cylinder were
employed. On some of the earliest engines a flame burned close to the
cylinder head, and at the proper time for ignition a slide or valve
moved to provide an opening which permitted the flame to ignite the gas
back of the piston. This system was practical only on the primitive form
of gas engines in which the charge was not compressed before ignition.
Later, when it was found desirable to compress the gas a certain degree
before exploding it, an incandescent platinum tube in the combustion
chamber, which was kept in a heated condition by a flame burning in it,
exploded the gas. The naked flame was not suitable in this application
because when the slide was opened to provide communication between the
flame and the gas the compressed charge escaped from the cylinder with
enough pressure to blow out the flame at times and thus cause irregular
ignition. When the flame was housed in a platinum tube it was protected
from the direct action of the gas, and as long as the tube was
maintained at the proper point of incandescence regular ignition was
obtained.
Some engineers utilized the property of gases firing themselves if
compressed to a sufficient degree, while others depended upon the heat
stored in the cylinder-head to fire the highly compressed gas. None of
these methods were practical in their application to motor car engines
because they did not permit flexible engine action which is so
desirable. At the present time, electrical ignition systems in which the
compressed gas is exploded by the heating value of the minute electric
arc or spark in the cylinder are standard, and the general practice
seems to be toward the use of mechanical producers of electricity rather
than chemical batteries.
ELECTRICAL IGNITION BEST
Two general forms of electrical ignition systems may be used, the most
popular being that in which a current of electricity under high tension
is made to leap a gap or air space between the points of the sparking
plug screwed into the cylinder. The other form, which has been almost
entirely abandoned in automobile and which was never used with airplane
engine practice, but which is still used to some extent on marine
engines, is called the low-tension system because current of low voltage
is used and the spark is produced by moving electrodes in the combustion
chamber.
The essential elements of any electrical ignition system, either high or
low tension, are: First, a simple and practical method of current
production; second, suitable timing apparatus to cause the spark to
occur at the right point in the cycle of engine action; third, suitable
wiring and other apparatus to convey the current produced by the
generator to the sparking member in the cylinder.
The various appliances necessary to secure prompt ignition of the
compressed gases should be described in some detail because of the
importance of the ignition system. It is patent that the scope of a work
of this character does not permit one to go fully into the theory and
principles of operation of all appliances which may be used in
connection with gasoline motor ignition, but at the same time it is
important that the elementary principles be considered to some extent in
order that the reader should have a proper understanding of the very
essential ignition apparatus. The first point considered will be the
common methods of generating the electricity, then the appliances to
utilize it and produce the required spark in the cylinder. Inasmuch as
magneto ignition is universally used in connection with airplane engine
ignition it will not be necessary to consider battery ignition systems.
FUNDAMENTALS OF MAGNETISM OUTLINED
To properly understand the phenomena and forces involved in the
generation of electrical energy by mechanical means it is necessary to
become familiar with some of the elementary principles of magnetism and
its relation to electricity. The following matter can be read with
profit by those who are not familiar with the subject. Most persons know
that magnetism exists in certain substances, but many are not able to
grasp the terms used in describing the operation of various electrical
devices because of not possessing a knowledge of the basic facts upon
which the action of such apparatus is based.
Magnetism is a property possessed by certain substances and is
manifested by the ability to attract and repel other materials
susceptible to its effects. When this phenomenon is manifested by a
conductor or wire through which a current of electricity is flowing it
is termed "electro-magnetism." Magnetism and electricity are closely
related, each being capable of producing the other. Practically all of
the phenomena manifested by materials which possess magnetic qualities
naturally can be easily reproduced by passing a current of electricity
through a body which, when not under electrical influence, is not a
magnetic substance. Only certain substances show magnetic properties,
these being iron, nickel, cobalt and their alloys.
The earliest known substance possessing magnetic properties was a stone
first found in Asia Minor. It was called the lodestone or leading stone,
because of its tendency, if arranged so it could be moved freely, of
pointing one particular portion toward the north. The compass of the
ancient Chinese mariners was a piece of this material, now known to be
iron ore, suspended by a light thread or floated on a cork in some
liquid so one end would point toward the north magnetic pole of the
earth. The reason that this stone was magnetic was hard to define for a
time, until it was learned that the earth was one huge magnet and that
the iron ore, being particularly susceptible, absorbed and retained some
of this magnetism.
Most of us are familiar with some of the properties of the magnet
because of the extensive sale and use of small horseshoe magnets as
toys. As they only cost a few pennies every one has owned one at some
time or other and has experimented with various materials to see if they
would be attracted. Small pieces of iron or steel were quickly attracted
to the magnet and adhered to the pole pieces when brought within the
zone of magnetic influence. It was soon learned that brass, copper, tin
or zinc were not affected by the magnet. A simple experiment that serves
to illustrate magnetic attraction of several substances is shown at A,
Fig. 57. In this, several balls are hung from a standard or support, one
of these being of iron, another of steel. When a magnet is brought near
either of these they will be attracted toward it, while the others will
remain indifferent to the magnetic force. Experimenters soon learned
that of the common metals only iron or steel were magnetic.
[Illustration: Fig. 57.--Some Simple Experiments to Demonstrate Various
Magnetic Phenomena and Clearly Outline Effects of Magnetism and Various
Forms of Magnets.]
If the ordinary bar or horseshoe magnet be carefully examined, one end
will be found to be marked N. This indicates the north pole, while the
other end is not usually marked and is the south pole. If the north pole
of one magnet is brought near the south pole of another, a strong
attraction will exist between them, this depending upon the size of the
magnets used and the air gap separating the poles. If the south pole of
one magnet is brought close to the end of the same polarity of the other
there will be a pronounced repulsion of like force. These facts are
easily proved by the simple experiment outlined at B, Fig. 57. A magnet
will only attract or influence a substance having similar qualities. The
like poles of magnets will repel each other because of the obvious
impossibility of uniting two influences or forces of practically equal
strength but flowing in opposite directions. The unlike poles of magnets
attract each other because the force is flowing in the same direction.
The flow of magnetism is through the magnet from south to north and the
circuit is completed by the flow of magnetic influence through the air
gap or metal armature bridging it from the north to the south pole.
FORMS OF MAGNETS AND ZONE OF MAGNETIC INFLUENCE DEFINED
Magnets are commonly made in two forms, either in the shape of a bar or
horseshoe. These two forms are made in two types, simple or compound.
The latter are composed of a number of magnets of the same form united
so the ends of like polarity are laced together, and such a construction
will be more efficient and have more strength than a simple magnet of
the same weight. The two common forms of simple and compound magnets are
shown at C, Fig. 57. The zone in which a magnetic influence occurs is
called the magnetic field, and this force can be graphically shown by
means of imaginary lines, which are termed "lines of force." As will be
seen from the diagram at D, Fig. 57, the lines show the direction of
action of the magnetic force and also show its strength, as they are
closer together and more numerous when the intensity of the magnetic
field is at its maximum. A simple method of demonstrating the presence
of the force is to lay a piece of thin paper over the pole pieces of
either a bar or horseshoe magnet and sprinkle fine iron filings on it.
The particles of metal arrange themselves in very much the manner shown
in the illustrations and prove that the magnetic field actually exists.
The form of magnet used will materially affect the size and area of the
magnetic field. It will be noted that the field will be concentrated to
a greater extent with the horseshoe form because of the proximity of the
poles. It should be understood that these lines have no actual
existence, but are imaginary and assumed to exist only to show the way
the magnetic field is distributed. The magnetic influence is always
greater at the poles than at the center, and that is why a horseshoe or
U-form magnet is used in practically all magnetos or dynamos. This
greater attraction at the poles can be clearly demonstrated by
sprinkling iron filings on bar and U magnets, as outlined at E, Fig. 57.
A large mass gathers at the pole pieces, gradually tapering down toward
the point where the attraction is least.
From the diagrams it will be seen that the flow of magnetism is from one
pole to the other by means of curved paths between them. This circuit is
completed by the magnetism flowing from one pole to the other through
the magnet, and as this flow is continued as long as the body remains
magnetic it constitutes a magnetic circuit. If this flow were
temporarily interrupted by means of a conductor of electricity moving
through the field there would be a current of electricity induced in the
conductor every time it cut the lines of force. There are three kinds of
magnetic circuits. A non-magnetic circuit is one in which the magnetic
influence completes its circuit through some substance not susceptible
to the force. A closed magnetic circuit is one in which the influence
completes its circuit through some magnetic material which bridges the
gap between the poles. A compound circuit is that in which the magnetic
influence passes through magnetic substances and non-magnetic substances
in order to complete its circuit.
HOW IRON AND STEEL BARS ARE MADE MAGNETIC
Magnetism may be produced in two ways, by contact or induction. If a
piece of steel is rubbed on a magnet it will be found a magnet when
removed, having a north and south pole and all of the properties found
in the energizing magnet. This is magnetizing by contact. A piece of
steel will retain the magnetism imparted to it for a considerable length
of time, and the influence that remains is known as residual magnetism.
This property may be increased by alloying the steel with tungsten and
hardening it before it is magnetized. Any material that will retain its
magnetic influence after removal from the source of magnetism is known
as a permanent magnet. If a piece of iron or steel is brought into the
magnetic field of a powerful magnet it becomes a magnet without actual
contact with the energizer. This is magnetizing by magnetic induction.
If a powerful electric current flows through an insulated conductor
wound around a piece of iron or steel it will make a magnet of it. This
is magnetizing by electro-magnetic induction. A magnet made in this
manner is termed an electro-magnet and usually the metal is of such a
nature that it will not retain its magnetism when the current ceases to
flow around it. Steel is used in all cases where permanent magnets are
required, while soft iron is employed in all cases where an intermittent
magnetic action is desired. Magneto field magnets are always made of
tungsten steel alloy, so treated that it will retain its magnetism for
lengthy periods.
ELECTRICITY AND MAGNETISM CLOSELY RELATED
There are many points in which magnetism and electricity are alike. For
instance, air is a medium that offers considerable resistance to the
passage of both magnetic influence and electric energy, although it
offers more resistance to the passage of the latter. Minerals like iron
or steel are very easily influenced by magnetism and easily penetrated
by it. When one of these is present in the magnetic circuit the
magnetism will flow through the metal. Any metal is a good conductor for
the passage of the electric current, but few metals are good conductors
of magnetic energy. A body of the proper metal will become a magnet due
to induction if placed in the magnetic field, having a south pole where
the lines of force enter it and a north pole where they pass out.
We have seen that a magnet is constantly surrounded by a magnetic field
and that an electrical conductor when carrying a current is also
surrounded by a field of magnetic influence. Now if the conductor
carrying a current of electricity will induce magnetism in a bar of iron
or steel, by a reversal of this process, a magnetized iron or steel bar
will produce a current of electricity in a conductor. It is upon this
principle that the modern dynamo or magneto is constructed. If an
electro-motive force is induced in a conductor by moving it across a
field of magnetic influence, or by passing a magnetic field near a
conductor, electricity is said to be generated by magneto-electric
induction. All mechanical generators of the electric current using
permanent steel magnets to produce a field of magnetic influence are of
this type.
BASIC PRINCIPLES OF MAGNETO OUTLINED
The accompanying diagram, Fig. 58, will show these principles very
clearly. As stated on an earlier page, if the lines of force in the
magnetic field are cut by a suitable conductor an electrical impulse
will be produced in that conductor. In this simple machine the lines of
force exist between the poles of a horseshoe magnet. The conductor,
which in this case is a loop of copper wire, is mounted upon a spindle
in order that it may be rotated in the magnetic field to cut the lines
of magnetic influence present between the pole pieces. Both of the ends
of this loop are connected, one with the insulated drum shown upon the
shaft, the other to the shaft. Two metal brushes are employed to collect
the current and cause it to flow through the external circuit. It can
be seen that when the shaft is turned in the direction of the arrow the
loop will cut through the lines of magnetic influence and a current will
be generated therein.
[Illustration: Fig. 58.--Elementary Form of Magneto Showing Principal
Parts Simplified to Make Method of Current Generation Clear.]
The pressure of the current and the amount produced vary in accordance
to the rapidity with which the lines of magnetic influence are cut. The
armature of a practical magneto, therefore, differs materially from that
shown in the diagram. A large number of loops of wire would be mounted
upon this shaft in order that the lines of magnetic influence would be
cut a greater number of times in a given period and a core of iron used
as a backing for the wire. This would give a more rapid alternating
current and a higher electro-motive force than would be the case with a
smaller number of loops of wire.
[Illustration: Fig. 59.--Showing How Strength of Magnetic Influence and
of the Currents Induced in the Windings of Armature Vary with the
Rapidity of Changes of Flow.]
The illustrations at Fig. 59 show a conventional double winding
armature and field magnetic of a practical magneto in part section and
will serve to more fully emphasize the points previously made. If the
armature or spindle were removed from between the pole pieces there
would exist a field of magnetic influence as shown at Fig. 57, but the
introduction of this component provides a conductor (the iron core) for
the magnetic energy, regardless of its position, though the facility
with which the influence will be transmitted depends entirely upon the
position of the core. As shown at A, the magnetic flow is through the
main body in a straight line, while at B, which position the armature
has attained after one-eighth revolution, or 45 degrees travel in the
direction of the arrow, the magnetism must pass through in the manner
indicated. At C, which position is attained every half revolution, the
magnetic energy abandons the longer path through the body of the core
for the shorter passage offered by the side pieces, and the field thrown
out by the cross bar disappears. On further rotation of the armature, as
at D, the body of the core again becomes energized as the magnetic
influence resumes its flow through it. These changes in the strength of
the magnetic field when distorted by the armature core, as well as the
intensity of the energy existing in the field, affect the windings, and
the electrical energy induced therein corresponds in strength to the
rapidity with which these changes in magnetic flow occur. The most
pronounced changes in the strength of the field will occur as the
armature passes from position B to D, because the magnetic field
existing around the core will be destroyed and again re-established.
During the most of the armature rotation the changes in strength will be
slight and the currents induced in the wire correspondingly small; but
at the instant the core becomes remagnetized, as the armature leaves
position C, the current produced will be at its maximum, and it is
necessary to so time the rotation of the armature that at this instant
one of the cylinders is in condition to be fired. It is imperative that
the armature be driven in such relation to the crank-shaft that each
production of maximum current coincides with the ignition point, this
condition existing twice during each revolution of the armature, or at
every 180 degrees travel. Each position shown corresponds to 45 degrees
travel of the armature, or one-eighth of a turn, and it takes just
three-eighths revolution to change the position from A to that shown at
D.
ESSENTIAL PARTS OF A MAGNETO AND THEIR FUNCTIONS
The magnets which produce the influence that in turn induces the
electrical energy in the winding or loops of wire on the armature, and
which may have any even number of opposed poles, are called field
magnets. The loops of wire which are mounted upon a suitable drum and
rotate in the field of magnetic influence in order to cut the lines of
force is called an armature winding, while the core is the metal
portion. The entire assembly is called the armature. The exposed ends of
the magnets are called pole pieces and the arrangement used to collect
the current is either a commutator or a collector. The stationary pieces
which bear against the collector or commutator and act as terminals for
the outside circuit are called brushes. These brushes are often of
copper, or some of its alloys, because copper has a greater electrical
conductivity than any other metal.
These brushes are nearly always of carbon, which is sometimes
electroplated with copper to increase its electrical conductivity,
though cylinders of copper wire gauze impregnated with graphite are
utilized at times. Carbon is used because it is not so liable to cut the
metal of the commutator as might be the case if the contact was of the
metal to metal type. The reason for this is that carbon has the peculiar
property in that it materially assists in the lubrication of the
commutator, and being of soft, unctuous composition, will wear and
conform to any irregularities on the surface of the metal collector
rings.
The magneto in common use consists of a number of horseshoe magnets
which are compound in form and attached to suitable cast-iron pole
pieces used to collect and concentrate the magnetic influence of the
various magnets. Between these pole pieces an armature rotates. This is
usually shaped like a shuttle, around which are wound coils of insulated
wire. These are composed of a large number of turns and the current
produced depends in great measure upon the size of the wire and the
number of turns per coil. An armature winding of large wire will deliver
a current of great amperage, but of small voltage. An armature wound
with very fine wire will deliver a current of high voltage but of low
amperage. In the ordinary form of magneto, such as used for ignition,
the current is alternating in character and the break in the circuit
should be timed to occur when the armature is at the point of its
greatest potential or pressure. Where such a generator is designed for
direct current production the ends of the winding are attached to the
segments of a commutator, but where the instrument is designed to
deliver an alternating current one end of the winding is fastened to an
insulator ring on one end of the armature shaft and the other end is
grounded on the frame of the machine.
The quantity of the current depends upon the strength of the magnetic
field and the number of lines of magnetic influence acting through the
armature. The electro-motive force varies as to the length of the
armature winding and the number of revolutions at which the armature is
rotated.
THE TRANSFORMER SYSTEM USES LOW VOLTAGE MAGNETO
The magneto in the various systems which employ a transformer coil is
very similar to a low-tension generator in general construction, and the
current delivered at the terminals seldom exceeds 100 volts. As it
requires many times that potential or pressure to leap the gap which
exists between the points of the conventional spark plug, a separate
coil is placed in circuit to intensify the current to one of greater
capacity. The essential parts of such a system and their relation to
each other are shown in diagrammatic form at Fig. 60 and as a complete
system at Fig. 61. As is true of other systems the magnetic influence is
produced by permanent steel magnets clamped to the cast-iron pole pieces
between which the armature rotates. At the point of greatest potential
in the armature winding the current is broken by the contact breaker,
which is actuated by a cam, and a current of higher value is induced in
the secondary winding of the transformer coil when the low voltage
current is passed through the primary winding.
[Illustration: Fig. 60.--Diagrams Explaining Action of Low Tension
Transformer Coil and True High Tension Magneto Ignition Systems.]
[Illustration: Fig. 60A.--Side Sectional View of Bosch High-Tension
Magneto Shows Disposition of Parts. End Elevation Depicts Arrangement of
Interruptor and Distributor Mechanism.]
It will be noted that the points of the contact breaker are together
except for the brief instant when separated by the action of the point
of the cam upon the lever. It is obvious that the armature winding is
short-circuited upon itself except when the contact points are
separated. While the armature winding is thus short-circuited there will
be practically no generation of current. When the points are separated
there is a sudden flow of current through the primary winding of the
transformer coil, inducing a secondary current in the other winding,
which can be varied in strength by certain considerations in the
preliminary design of the apparatus. This current of higher potential or
voltage is conducted directly to the plug if the device is fitted to a
single-cylinder engine, or to the distributor arm if fitted to a
multiple-cylinder motor. The distributor consists of an insulator in
which is placed a number of segments, one for each cylinder to be fired,
and so spaced that the number of degrees between them correspond to the
ignition points of the motor. A two-cylinder motor would have two
segments, a three-cylinder, three segments, and so on within the
capacity of the instrument. In the illustration a four-cylinder
distributor is fitted, and the distributing arm is in contact with the
segment corresponding to the cylinder about to be fired.
[Illustration: Fig. 61.--Berling Two-Spark Dual Ignition System.]
TRUE HIGH-TENSION MAGNETOS ARE SELF-CONTAINED
[Illustration: Fig. 62.--Berling Double-Spark Independent System.]
The true high-tension magneto differs from the preceding inasmuch as the
current of high voltage is produced in the armature winding direct,
without the use of the separate coil. Instead of but one coil, the
armature carries two, one of comparatively coarse wire, the other of
many turns of finer wire. The arrangement of these windings can be
readily ascertained by reference to the diagram B, Fig. 60, which shows
the principle of operation very clearly. The simplicity of the ignition
system is evident by inspection of Fig. 62. One end of the primary
winding (coarse wire) is coupled or grounded to the armature core, and
the other passes to the insulated part of the interrupter. While in some
forms the interrupter or contact breaker mechanism does not revolve, the
desired motion being imparted to the contact lever to separate the
points of a revolving cam, in this the cam or tripping mechanism is
stationary and the contact breaker revolves. This arrangement makes it
possible to conduct the current from the revolving primary coil to the
interrupter by a direct connection, eliminating the use of brushes,
which would otherwise be necessary. In other forms of this appliance
where the winding is stationary, the interrupter may be operated by a
revolving cam, though, if desired, the used of a brush at this point
will permit this construction with a revolving winding.
During the revolution of the armature the grounded lever makes and
breaks contact with the insulated point, short-circuiting the primary
winding upon itself until the armature reaches the proper position of
maximum intensity of current production, at which time the circuit is
broken, as in the former instance. One end of the secondary winding
(fine wire) is grounded on the live end of the primary, the other end
being attached to the revolving arm of the distributor mechanism. So
long as a closed circuit is maintained feeble currents will pass through
the primary winding, and so long as the contact points are together this
condition will exist. When the current reaches its maximum value,
because of the armature being in the best position, the cam operates the
interrupter and the points are separated, breaking the short circuit
which has existed in the primary winding.
The secondary circuit has been open while the distributor arm has moved
from one contact to another and there has been no flow of energy through
this winding. While the electrical pressure will rise in this, even if
the distributor arm contacted with one of the segments, there would be
no spark at the plug until the contact points separated, because the
current in the secondary winding would not be of sufficient strength.
When the interrupter operates, however, the maximum primary current will
be diverted from its short circuit and can flow to the ground only
through the secondary winding and spark-plug circuit. The high pressure
now existing in the secondary winding will be greatly increased by the
sudden flow of primary current, and energy of high enough potential to
successfully bridge the gap at the plug is thereby produced in the
winding.
THE BERLING MAGNETO
[Illustration: Fig. 63.--Type DD Berling High Tension Magneto.]
The Berling magneto is a true high tension type delivering two impulses
per revolution, but it is made in a variety of forms, both single and
double spark. Its principle of action does not differ in essentials from
the high tension type previously described. This magneto is used on
Curtiss aviation engines and will deliver sparks in a positive manner
sufficient to insure ignition of engines up to 200 horse-power and at
rotative speeds of the magneto armature up to 4,000 r. p. m. which is
sufficient to take care of an eight-cylinder V engine running up to
2,000 r. p. m. The magneto is driven at crank-shaft speed on
four-cylinder engines, at 1-1/2 times crank-shaft speed on six-cylinder
engines and at twice crank-shaft speed on eight-cylinder V types. The
types "D" and "DD" BERLING Magnetos are interchangeable with
corresponding magnetos of other standard makes. The dimensions of the
four-, six- and eight-cylinder types "D" and "DD" are all the same.
The ideal method of driving the magneto is by means of flexible direct
connecting coupling to a shaft intended for the purpose of driving the
magneto. As the magneto must be driven at a high speed, a coupling of
some flexibility is preferable. The employment of such a coupling will
facilitate the mounting of the magneto, because a small inaccuracy in
the lining up of the magneto with the driving shaft will be taken care
of by the flexible coupling, whereas with a perfectly rigid coupling the
line-up of the magneto must be absolutely accurate. Another advantage of
the flexible coupling is that the vibration of the motor will not be as
fully transmitted to the armature shaft on the magneto as in case a
rigid coupling is used. This means prolonged life for the magneto.
The next best method of driving the magneto is by means of a gear keyed
to the armature shaft. When this method of driving is employed, great
care must be exercised in providing sufficient clearance between the
gear on the magneto and the driving gear. If there should be a tight
spot between these two gears it will react disadvantageously on the
magneto. The third available method is to drive the magneto by means of
a chain. This is the least desirable of the three methods and should be
resorted to only in case of absolute necessity. It is difficult to
provide sufficient clearance when using a chain without rendering the
timing less accurate and positive.
[Illustration: Fig. 64.--Wiring Diagrams of Berling Magneto Ignition
Systems.]
Fig. 64, A shows diagrammatically the circuit of the "D" type two-spark
independent magneto and the switch used with it. In position OFF the
primary winding of the magneto is short-circuited and in this position
the switch serves as an ordinary cut-out or grounding switch. In
position "1" the switch connects the magneto in such a way that it
operates as an ordinary single-spark magneto. In this position one end
of the secondary winding is grounded to the body of the motor. This is
the starting position. In this position of the switch the entire voltage
generated in the magneto is concentrated at one spark-plug instead of
being divided in half. With the motor turning over very slowly, as is
the case in starting, the full voltage generated by the magneto will
not in all cases be sufficient to bridge simultaneously two spark gaps,
but is amply sufficient to bridge one. Also, this position of the switch
tends to retard the ignition and should be used in starting to prevent
back-firing. With the switch in position "2" the magneto applies
ignition to both plugs in each cylinder simultaneously. This is the
normal running position.
Fig. 64, B shows diagrammatically the circuit of the type "DD" BERLING
high-tension two-spark dual magneto. This type is recommended for
certain types of heavy-duty airplane motors, which it is impossible to
turn over fast enough to give the magneto sufficient speed to generate
even a single spark of volume great enough to ignite the gas in the
cylinder. The dual feature consists of the addition to the magneto of a
battery interrupter. The equipment consists of the magneto, coil and
special high-tension switch. The coil is intended to operate on six
volts. Either a storage battery or dry cells may be used.
With the switch in the OFF position, the magneto is grounded, and the
battery circuit is open. With the switch in the second or battery
position marked "BAT," one end of the secondary winding of the magneto
is grounded, and the magneto operates as a single-spark magneto
delivering high-tension current to the inside distributor, and the
battery circuit being closed the high-tension current from the coil is
delivered to the outside distributor. In this position the battery
current is supplied to one set of spark plugs, no matter how slowly the
motor is turned over, but as soon as the motor starts, the magneto
supplies current as a single-spark magneto to the other set of the
spark-plugs. After the engine is running, the switch should be thrown to
the position marked "MAG." The battery and coil are then disconnected,
and the magneto furnishes ignition to both plugs in each cylinder. This
is the normal running position. Either a non-vibrating coil type "N-1"
is furnished or a combined vibrating and non-vibrating coil type
"VN-1."
SETTING BERLING MAGNETO
The magneto may be set according to one of two different methods, the
selection of which is, to some extent, governed by the characteristics
of the engine, but largely due to the personal preference on the part of
the user. In the first method described below, the most advantageous
position of the piston for fully advanced ignition is determined in
relation to the extreme advanced position of the magneto. In this case,
the fully retarded ignition will not be a matter of selection, but the
timing range of the magneto is wide enough to bring the fully retarded
ignition after top-center position of the piston. The second method for
the setting of the magneto fixes the fully retarded position of the
magneto in relation to that position of the piston where fully retarded
ignition is desired. In this case, the extreme advance position of the
magneto will not always correspond with the best position of the piston
for fully advanced ignition, and the amount of advance the magneto
should have to meet ideal requirements in this respect must be
determined by experiment.
_First Method:_
1. Designate one cylinder as cylinder No. 1.
2. Turn the crank-shaft until the piston in cylinder No. 1 is in the
position where the fully advanced spark is desired to occur.
3. Remove the cover from the distributor block and turn the armature
shaft in the direction of rotation of the magneto until the distributor
finger-brush comes into such a position that this brush makes contact
with the segment which is connected to the cable terminal marked "1."
This is either one of the two bottom segments, depending upon the
direction of rotation.
4. Place the cam housing in extreme advance, i.e., turn the cam housing
until it stops, in the direction opposite to the direction of rotation
of the armature. With the cam housing in this position, open the cover.
5. With the armature in the approximate position as described in "3,"
turn the armature slightly in either direction to such a point that the
platinum points of the magneto interrupter will just begin to open at
the end of the cam, adjacent to the fibre lever on the interrupter.
6. With this exact position of the armature, fix the magneto to the
driving member of the engine.
_Second Method:_
1. Designate one cylinder as cylinder No. 1.
2. Turn the crank-shaft until the piston in cylinder No. 1 is in the
position at which the fully retarded spark is desired to occur.
3. Same as No. 3 under First Method.
4. Place the cam housing in extreme retard, i.e., turn the cam housing
until it stops, in the same direction as the direction of rotation of
the armature. With the cam housing in this position, open the cover.
5. Same as No. 5 under First Method.
6. Same as No. 6 under First Method.
WIRING THE MAGNETO
The wiring of the magneto is clearly shown by wiring diagram.
First determine the sequence of firing for the cylinders and then
connect the cables to the spark plug in the cylinders in proper
sequence, beginning with cylinder No. 1 marked on the distributor block.
The switch used with the independent type must be mounted in such a
manner that there will be a metallic connection between the frame of the
magneto and the metal portion of the switch.
It is advisable to use a separate battery, either storage or dry cells,
as a source of current for the dual equipment. Connecting to the same
battery that is used with the generator and other electrical equipment
may cause trouble, as a "ground" in this battery causes the coil to
overheat.
CARE AND MAINTENANCE
_Lubrication:_
Use only the very best of oil for the oil cups.
Put five drops of oil in the oil cup at the driving end of the magneto
for every fifty hours of actual running.
Put five drops of oil in the oil cup at the interrupter end of the
magneto, located at one side of the cam housing, for every hundred hours
of actual running.
Lubricate the embossed cams in the cam housing with a thin film of
vaseline every fifty hours of actual running. Wipe off all superfluous
vaseline. Never use oil in the interrupter. Do not lubricate any other
part of the interrupter.
_Adjusting the Interrupter:_
With the fibre lever in the center of one of the embossed cams, as at
Fig. 65, the opening between the platinum contacts should be not less
than .016" and not more than .020". The gauge riveted to the adjusting
wrench should barely be able to pass between the contacts when fully
open. The platinum contacts must be smoothed off with a very fine file.
When in closed position, the platinum contacts should make contact with
each other over their entire surfaces.
When inspecting the interrupter, make sure that the ground brush in the
back of the interrupter base is making good contact with the surface on
which it rubs.
_Cleaning the Distributor:_
The distributor block cover should be removed for inspection every
twenty-five hours of actual running and the carbon deposit from the
distributor finger-brush wiped off the distributor block by rubbing with
a rag or piece of waste dipped in gasoline or kerosene. The
high-tension terminal brush on the side of the magneto should also be
carefully inspected for proper tension.
LOCATING TROUBLE
Trouble in the ignition system is indicated by the motor "missing,"
stopping entirely, or by inability to start.
It is safe to assume that the trouble is not in the magneto, and the
carburetor, gasoline supply and spark-plugs should first be
investigated.
[Illustration: Fig. 65.--The Berling Magneto Breaker Box Showing Contact
Points Separated and Interruptor Lever on Cam.]
If the magneto is suspected, the first thing to do is to determine if it
will deliver a spark. To determine this, disconnect one of the
high-tension leads from the spark-plug in one of the cylinders and place
it so that there is approximately 1/16" between the terminal and the
cylinder frame.
Open the pet cocks on the other cylinders to prevent the engine from
firing and turn over the engine until the piston is approaching the end
of the compression stroke in the cylinder from which the cable has been
removed. Set the magneto in the advance position and rapidly rock the
engine over the top-center position, observing closely if a spark occurs
between the end of the high-tension cable and the frame.
If the magneto is of the dual type, the trouble may be either in the
magneto or in the battery or coil system, therefore disconnect the
battery and place the switch in the position marked "MAG." The magneto
will then operate as an independent magneto and should spark in the
proper manner. After this the battery system should be investigated. To
test the operation of the battery and coil, examine all connections,
making sure that they are clean and tight, and then with the switch, in
the "BAT," rock the piston slowly back and forth. If a type "VN-1" coil
is used, a shower of sparks should jump between the high-tension cable
terminal and the cylinder frame when the piston is in the correct
position for firing. If no spark occurs, remove the cover from the coil
and see that the vibrating tongue is free. If a type "N-1" coil is used,
a single spark will occur. The battery should furnish six volts when
connected to the coil, and this should also be verified.
If the coil still refuses to give a spark and all connections are
correct, the coil should be replaced and the defective coil returned to
the manufacturer.
If both magneto and coil give a spark when tested as just described, the
spark-plugs should be investigated. To do this, disconnect the cables
and remove the spark-plugs. Then reconnect the cables to the plugs and
place them so that the frame portions of the plugs are in metallic
connection with the frame of the motor. Then turn over the motor, thus
revolving the magneto armature, and see if a spark is produced at the
spark gaps of the plugs.
The most common defects in spark-plugs are breaking down of the
insulation, fouling due to carbon, or too large or small a spark gap. To
clean the plugs a stiff brush and gasoline should be used. The spark
gap should be about 1/32" and never less than 1/64". Too small a gap may
have been caused by beads of metal forming due to the heat of the spark.
Too long a gap may have been caused by the points burning off.
If the magneto and spark plugs are in good condition and the engine does
not run satisfactorily, the setting should be verified according to
instructions previously given, and, if necessary, readjusted.
[Illustration: Fig. 66.--The Dixie Model 60 for Six-Cylinder Airplane
Engine Ignition.]
Be careful to observe that both the type "VN-1" and type "N-1" coils are
so arranged that the spark occurs on the opening of the contacts of the
timer. As this is just the reverse of the usual operation, it should be
carefully noted when any change in the setting of the timer is made. The
timer on the dual type magneto is adjusted so that the battery spark
occurs about 5 deg. later than the magneto spark. This provides an
automatic advance as soon as the switch is thrown to the magneto
position "MAG." This relative timing can be easily adjusted by removing
the interrupter and shifting the cam in the direction desired.
THE DIXIE MAGNETO
[Illustration: Fig. 67.--Installation Dimensions of Dixie Model 60
Magneto.]
The Dixie magneto, shown at Fig. 66, operates on a different principle
than the rotary armature type. It is used on the Hall-Scott and other
aviation engines. In this magneto the rotating member consists of two
pieces of magnetic material separated by a non-magnetic center piece.
This member constitutes true rotating poles for the magnet and rotates
in a field structure, composed of two laminated field pieces, riveted
between two non-magnetic rings. The bearings for the rotating poles are
mounted in steel plates, which lie against the poles of the magnets.
When the magnet poles rotate, the magnetic lines of force from each
magnet pole are carried directly to the field pieces and through the
windings, without reversal through the mass of the rotating member and
with only a single air gap. There are no losses by flux reversal in the
rotating part, such as take place in other machines, and this is said
to account for the high efficiency of the instrument.
[Illustration: Fig. 68.--The Rotating Elements of the Dixie Magneto.]
And this "Mason Principle" involved in the operation of the Dixie is
simplified by a glance at the field structure, consisting of the
non-magnetic rings, assembled to which are the field pieces between
which the rotating poles revolve (see Fig. 68). Rotating between the
limbs of the magnets, these two pieces of magnetic material form true
extensions to the poles of the magnets, and are, in consequence,
_always_ of the _same_ polarity. It will be seen there is no reversal of
the magnetism through them, and consequently no eddy current or
hysteresis losses which are present in the usual rotor or inductor
types. The simplicity features of construction stand out prominently
here, in that there are no revolving windings, a detail entirely
differing from the orthodox high-tension instrument. This simplicity
becomes instantly apparent when it is found that the circuit breaker,
instead of revolving as it does in other types, is stationary and that
the whole breaker mechanism is exposed by simply turning the cover
spring aside and removing cover. This makes inspection and adjustment
particularly simple, and the fact that no special tool is necessary for
adjustment of the platinum points--an ordinary small screw-driver is the
whole "kit of tools" needed in the work of disassembling or
assembling--is a feature of some value.
[Illustration: Fig. 69.--Suggestions for Adjusting and Dismantling Dixie
Magneto. A--Screw Driver Adjusts Contact Points. B--Distributor Block
Removed. C--Taking off Magnets. D--Showing How Easily Condenser and High
Tension Windings are Removed.]
With dust- and water-protecting casing removed, and one of the magnets
withdrawn, as in Fig. 69, the winding can be seen with its core resting
on the field pole pieces and the primary lead attached to its side. An
important feature of the high-tension winding is that the heads are of
insulating material, and there is not the tendency for the high-tension
current to jump to the side as in the ordinary armature type magneto.
The high-tension current is carried to the distributor by means of an
insulated block with a spindle, at one end of which is a spring brush
bearing directly on the winding, thus shortening the path of the
high-tension current and eliminating the use of rubber spools and
insulating parts. The moving parts of the magneto need never be
disturbed if the high-tension winding is to be removed. This winding
constitutes all of the magneto windings, no external spark coil being
necessary. The condenser is placed directly above the winding and is
easily removable by taking out two screws, instead of being placed in an
armature where it is inaccessible except to an expert, and where it
cannot be replaced except at the factory whence it emanated.
CARE OF THE DIXIE MAGNETO
The bearings of the magneto are provided with oil cups and a few drops
of light oil every 1,000 miles are sufficient. The breaker lever should
be lubricated every 1,000 miles with a drop of light oil, applied with a
tooth-pick. The proper distance between the platinum points when
separated should not exceed .020 or one-fiftieth of an inch. A gauge of
the proper size is attached to the screwdriver furnished with the
magneto. The platinum contacts should be kept clean and properly
adjusted. Should the contacts become pitted, a fine file should be used
to smooth them in order to permit them to come into perfect contact. The
distributor block should be removed occasionally and inspected for an
accumulation of carbon dust. The inside of the distributor block should
be cleaned with a cloth moistened with gasoline and then wiped dry with
a clean cloth. When replacing the block, care must be exercised in
pushing the carbon brush into the socket. Do not pull out the carbon
brushes in the distributor because you think there is not enough tension
on the small brass springs. In order to obtain the most efficient
results, the normal setting of the spark-plug points should not exceed
.025 of an inch, and it is advisable to have the gap just right before a
spark-plug is inserted.
The spark-plug electrodes may be easily set by means of the gauge
attached to the screwdriver. _The setting of the spark-plug points is an
important function which is usually overlooked, with the result that the
magneto is blamed when it is not at fault._
TIMING OF THE DIXIE MAGNETO
[Illustration: Fig. 69A.--Sectional Views Outlining Construction of
Dixie Magneto with Compound Distributor for Eight-Cylinder Engine
Ignition.]
In order to obtain the utmost efficiency from the engine, the magneto
must be correctly timed to it. This operation is usually performed when
the magneto is fitted to the engine at the factory. The correct setting
may vary according to individuality of the engine, and some engines
may require an earlier setting in order to obtain the best results.
However, should the occasion arise to retime the magneto, the procedure
is as follows: Rotate the crank-shaft of the engine until one of the
pistons, preferably that of cylinder No. 1, is 1/16 of an inch ahead of
the end of the compression stroke. With the timing lever in full retard
position, the driving shaft of the magneto should be rotated in the
direction in which it will be driven. The circuit breaker should be
closely observed and when the platinum contact points are about to
separate, the drive gear or coupling should be secured to the drive
shaft of the magneto. Care should be taken not to alter the position of
the magneto shaft when tightening the nut to secure the gear or
coupling, after which the magneto should be secured to its base. Remove
the distributor block and determine which terminal of the block is in
contact with the carbon brush of the distributor finger and connect with
plug wire leading to No. 1 cylinder to this terminal. Connect the
remaining plug wires in turn according to the proper sequence of firing
of the cylinders. (See the wiring diagram for a typical six-cylinder
engine at Fig. 70.) A terminal on the end of the cover spring of the
magneto is provided for the purpose of connecting the wire leading to a
ground switch for stopping the engine.
A special model or type of magneto is made for V engines which use a
compound distributor construction instead of the simple type on the
model illustrated and a different interior arrangement permits the
production of four sparks per revolution of the rotors. This makes it
possible to run the magneto slower than would be possible with the
two-spark form. The application of two compound distributor magnetos of
this type to a Thomas-Morse 135 horse-power motor of the eight-cylinder
V pattern is clearly shown at Fig. 71.
[Illustration: Fig. 70.--Wiring Diagram of Dixie Magneto Installation on
Hall-Scott Six-Cylinder 125 Horse-Power Aeronautic Motor.]
SPARK-PLUG DESIGN AND APPLICATION
[Illustration: Fig. 71.--How Magneto Ignition is Installed on
Thomas-Morse 135 Horse-Power Motor.]
With the high-tension system of ignition the spark is produced by a
current of high voltage jumping between two points which break the
complete circuit, which would exist otherwise in the secondary coil and
its external connections. The spark-plug is a simple device which
consists of two terminal electrodes carried in a suitable shell member,
which is screwed into the cylinder. Typical spark-plugs are shown in
section at Fig. 72 and the construction can be easily understood. The
secondary wire from the coil is attached to a terminal at the top of a
central electrode member, which is supported in a bushing of some form
of insulating material. The type shown at A employs a molded porcelain
as an insulator, while that depicted at B uses a bushing of mica. The
insulating bushing and electrode are housed in a steel body, which is
provided with a screw thread at the bottom, by which means it is screwed
into the combustion chamber.
[Illustration: Fig. 72.--Spark-Plug Types Showing Construction and
Arrangement of Parts.]
When porcelain is used as an insulating material it is kept from direct
contact with the metal portion by some form of yielding packing, usually
asbestos. This is necessary because the steel and porcelain have
different coefficients of expansion and some flexibility must be
provided at the joints to permit the materials to expand differently
when heated. The steel body of the plug which is screwed into the
cylinder is in metallic contact with it and carries sparking points
which form one of the terminals of the air gap over which the spark
occurs. The current entering at the top of the plug cannot reach the
ground, which is represented by the metal portion of the engine, until
it has traversed the full length of the central electrode and overcome
the resistance of the gap between it and the terminal point on the
shell. The porcelain bushing is firmly seated against the asbestos
packing by means of a brass screw gland which sets against a flange
formed on the porcelain, and which screws into a thread at the upper
portion of the plug body.
The mica plug shown at B is somewhat simpler in construction than that
shown at A. The mica core which keeps the central electrode separated
from the steel body is composed of several layers of pure sheet mica
wound around the steel rod longitudinally, and hundreds of stamped steel
washers which are forced over this member and compacted under high
pressure with some form of a binding material between them. Porcelain
insulators are usually molded from high-grade clay and are approximately
of the shapes desired by the designers of the plug. The central
electrode may be held in place by mechanical means such as nuts,
packings, and a shoulder on the rod, as shown at A. Another method
sometimes used is to cement the electrode in place by means of some form
of fire-clay cement. Whatever method of fastening is used, it is
imperative that the joints be absolutely tight so that no gas can escape
at the time of explosion. Porcelain is the material most widely used
because it can be glazed so that it will not absorb oil, and it is
subjected to such high temperature in baking that it is not liable to
crack when heated.
The spark-plugs may be screwed into any convenient part of the
combustion chamber, the general practice being to install them in the
caps over the inlet valves, or in the side of the combustion chamber, so
the points will be directly in the path of the entering fresh gases from
the carburetor.
Other insulating materials sometimes used are glass, steatite (which is
a form of soapstone) and lava. Mica and porcelain are the two common
materials used because they give the best results. Glass is liable to
crack, while lava or the soapstone insulating bushings absorb oil. The
spark gap of the average plug is equal to about 1/32 of an inch for coil
ignition and 1/40 of an inch when used in magneto circuits. A simple
gauge for determining the gap setting is the thickness of an ordinary
visiting card for magneto plugs, or a space equal to the thickness of a
worn dime for a coil plug. The insulating bushings are made in a number
of different ways, and while details of construction vary, spark-plugs
do not differ essentially in design. The dimensions of the standardized
plug recommended by the S. A. E. are shown at Fig. 73.
[Illustration: Fig. 73.--Standard Airplane Engine Plug Suggested by S.
A. E. Standards Committee.]
It is often desirable to have a water-tight joint between the
high-tension cable and the terminal screw on top of the insulating
bushing of the spark-plug, especially in marine applications. The plug
shown at C, Fig. 72, is provided with an insulating member or hood of
porcelain, which is secured by a clip in such a manner that it makes a
water-tight connection. Should the porcelain of a conventional form of
plug become covered with water or dirty oil, the high-tension current is
apt to run down this conducting material on the porcelain and reach the
ground without having to complete its circuit by jumping the air gap and
producing a spark. It will be evident that wherever a plug is exposed to
the elements, which is often the case in airplane service, that it
should be protected by an insulating hood which will keep the insulator
dry and prevent short circuiting of the spark. The same end can be
attained by slipping an ordinary rubber nipple over the porcelain
insulator of any conventional plug and bringing up one end over the
cable.
TWO-SPARK IGNITION
On most aviation engines, especially those having large cylinders, it is
sometimes difficult to secure complete combustion by using a
single-spark plug. If the combustion is not rapid the efficiency of the
engine will be reduced proportionately. The compressed charge in the
cylinder does not ignite all at once or instantaneously, as many assume,
but it is the strata of gas nearest the plug which is ignited first.
This in turn sets fire to consecutive layers of the charge until the
entire mass is aflame. One may compare the combustion of gas in the
gas-engine cylinder to the phenomenon which obtains when a heavy object
is thrown into a pool of still water. First a small circle is seen at
the point where the object has passed into the water, this circle in
turn inducing other and larger circles until the whole surface of the
pool has been agitated from the one central point. The method of
igniting the gas is very similar, as the spark ignites the circle of gas
immediately adjacent to the sparking point, and this circle in turn
ignites a little larger one concentric with it. The second circle of
flame sets fire to more of the gas, and finally the entire contents of
the combustion chamber are burning.
While ordinarily combustion is sufficiently rapid with a single plug so
that the proper explosion is obtained at moderate engine speeds, if the
engine is working fast and the cylinders are of large capacity more
power may be obtained by setting fire to the mixture at two different
points instead of but one. This may be accomplished by using two
sparking-plugs in the cylinder instead of one, and experiments have
shown that it is possible to gain from twenty-five to thirty per cent.
in motor power at high speed with two-spark plugs, because the
combustion of gas is accelerated by igniting the gas simultaneously in
two places. The double-plug system on airplane engines is also a
safeguard, as in event of failure of one plug in the cylinder the other
would continue to fire the gas, and the engine will continue to function
properly.
In using magneto ignition some precautions are necessary relating to
wiring and also the character of the spark-plugs employed. The conductor
should be of good quality, have ample insulation, and be well protected
from accumulations of oil, which would tend to decompose rubber
insulation. It is customary to protect the wiring by running it through
the conduits of fiber or metal tubing lined with insulating material.
Multiple strand cables should be used for both primary and secondary
wiring, and the insulation should be of rubber at least 3/16 inch thick.
The spark-plugs commonly used for battery and coil ignition cannot
always be employed when a magneto is fitted. The current produced by the
mechanical generator has a greater amperage and more heat value than
that obtained from transformer coils excited by battery current. The
greater heat may burn or fuse the slender points used on some battery
plugs and heavier electrodes are needed to resist the heating effect of
the more intense arc. While the current has greater amperage it is not
of as high potential or voltage as that commonly produced by the
secondary winding of an induction coil, and it cannot overcome as much
of a gap. Manufacturers of magneto plugs usually set the spark points
about 1/64 of an inch apart. The most efficient magneto plug has a
plurality of points so that when the distance between one set becomes
too great the spark will take place between one of the other pairs of
electrodes which are not separated by so great an air space.
[Illustration: Fig. 74.--Special Mica Plug for Aviation Engines.]
SPECIAL PLUGS FOR AIRPLANE WORK
Airplane work calls for special construction of spark-plugs, owing to
the high compression used in the engines and the fact that they are
operated on open throttle practically all the time, thus causing a great
deal of heat to be developed. The plug shown at Fig. 74 was recently
described in "The Automobile," and has been devised especially for
airplane engines and automobile racing power plants. The core C is built
up of mica washers, and has square shoulders. As mica washers of
different sizes may be used, and accurate machining, such as is
necessary with conical clamping surfaces, is not required, the plug can
be produced economically. The square shoulders of the core afford two
gasket seats, and when the core is clamped in the shell by means of
check nut E, it is accurately centered and a tight joint is formed. This
construction also makes a shorter plug than where conical fits are used,
thus improving the heat radiation through the stem. The lower end of the
shell is provided with a baffle plate O, which tends to keep the oil
away from the mica. There are perforations L in this baffle plate to
prevent burnt gases being pocketed behind the baffle plate and
pre-igniting the new charge. This construction also brings the firing
point out into the firing chamber of the engine, and has all the other
advantages of a closed-end plug. The stem P is made of brass or copper,
on account of their superior heat conductivity, and the electrode J is
swedged into the bottom of the stem, as shown at K, in a secure manner.
The shell is finned, as shown at G, to provide greater heat radiating
surface. There is also a fin F at the top of the stem, to increase the
radiation of heat from the stem and electrode. The top of this finned
portion is slightly countersunk, and the stem is riveted into same,
thereby reducing the possibility of leakage past the threads on the
stem. This finned portion is necked at A to take a slip terminal.
In building up the core a small section of washers, I, is built up
before the mica insulating tube D is placed on. This construction gives
a better support to section I. Baffle plate O is bored out to allow the
electrode J to pass through, and the clearance between baffle plate and
electrode is made larger than the width of the gap between the firing
points, so that there is no danger of the spark jumping from the
electrode to the baffle plate.
This plug will be furnished either with or without the finned portion,
to meet individual requirements. The manufacturers lay special stress
upon the simplicity of construction and upon the method of clamping,
which is claimed to make the plug absolutely gas-tight.