INDUCTION COILS
The induction coil has always been a popular piece of apparatus with those
interested in electrical science; the experiments which can be performed
with its aid are very numerous. It is of considerable importance,
especially in its application to such useful purposes as X ray work,
wireless telegraphy and _ignition_ for gas engines. The latter has caused
manufacturers to give much attention to the development of the induction
coil, resulting in many refinements of design and construction.
Induction coils may be divided into two general classes:
1. Primary coils;
2. Secondary coils.
The subject of electromagnetic induction has been fully explained in
chapter X, but it may be said, with special reference to induction coils,
that the operation of the two classes just mentioned is respectively due
to:
1. Self-induction;
2. Mutual induction.
=Self-induction.=--This is the property of an electric current by virtue
of which _it tends to resist any change in its rate of flow_. It is
sometimes spoken of as _electromagnetic inertia_ and is analogous to the
mechanical inertia of matter.
Self-induction is due to the action of the current upon itself during
variations in strength. It becomes especially marked in a coil of wire, in
which the adjacent turns act inductively upon each other upon the
principle of _mutual induction_ arising between two separate adjacent
circuits. Self-induction manifests itself by giving “_momentum_” to the
current so that _it cannot be instantly stopped when the circuit is
broken_, the result being a bright spark at the moment of breaking the
circuit. On account of this spark a primary induction coil is used in low
tension or “make and break” ignition systems.
[Illustration: FIG. 135.--Diagram illustrating the action of mutual
induction between two circuits; the one including a source of electrical
energy and a switch; the other including a galvanometer, but having no
cell or other electrical source. During the increase or decrease in the
strength of the current as on closing or opening the key a current is
_induced_ in the secondary circuit in a direction opposite to that of the
primary current as indicated by the arrows.]
In a single circuit, consisting of a straight wire and a parallel return
wire there is little or no self-induction. When a circuit containing a
primary induction coil and a battery is closed there is no spark because
at the instant of closing the circuit the current is at rest and on
account of self-induction _the current cannot at once rise to its full
value_.
=Mutual Induction.=--This is a particular case of electromagnetic
induction in which the magnetic field producing an electromotive force in
a circuit is due to the current in a neighboring circuit.
The effect of mutual induction may be explained with the aid of fig. 135.
If, as illustrated, a circuit including a battery and a switch, be placed
near another circuit, formed by connecting the two terminals of a
galvanometer by a wire, it will be found that whenever the first circuit,
1, is closed by the switch, allowing a current to pass in a given
direction, a momentary current will be induced in the second circuit, 2,
as shown by the galvanometer. A similar result will follow on the opening
of the battery circuit, the difference being that the momentary induced
current occurring at closure moves in a direction opposite to that in the
battery circuit, while the momentary current at opening moves in the same
direction.
Currents, besides being induced in circuit 2 at _make_ or _break_ of
circuit 1, are also induced when the current in 1 is fluctuating in
intensity.
The most marked results are observed when the make or break is sudden,
_the action being strongest at the break of the current in 1._
The inductive effect of the current in the arrangement shown in fig. 135
is very weak.
=Ques. What name is given to circuit 1?=
Ans. The _primary circuit_.
=Ques. What name is given to circuit 2?=
Ans. The _secondary circuit_.
=Ques. What names are given respectively to the currents in circuits 1 and
2?=
Ans. The _primary_ and _secondary_ or _induced currents_.
=Primary Induction Coils.=--These represent the simplest form of coil, and
are used chiefly in low tension ignition to intensify the spark when a
battery forms the current source.
A primary coil consists of a long iron core wound with a considerable
length of low resistance insulated copper wire, the length of the core and
the number of turns of the insulated wire winding determining the
efficiency. The effect of the iron core is to increase the self-induction.
[Illustration: FIG. 136.--Primary induction coil as used for low tension
ignition. Coils of this type are made in a great variety of form and size.
Ordinarily the winding consists of about six convolutions of No. 14 copper
wire. The winding is usually covered and the ends capped with ebonite
heads so that the core wires are not exposed.]
The spark produced, as previously explained, is due to self-induction, and
it should be remembered that in the operation of the coil, the _spark
occurs at the instant of breaking the circuit_, _not at the instant of
making_.
=Secondary Induction Coils.=--The arrangement shown in fig. 137, may be
considered as a very simple or rudimentary form of secondary induction
coil. In the actual coil, the primary and secondary circuits
(corresponding to 1 and 2 in fig. 135) are made up of coils of insulated
wire, as shown in fig. 143, the primary coil P, being wound over a core C
and the secondary coil S being wound over the primary.
The one property of such an arrangement that makes it of great value for
most purposes is that _the voltage of the induced currents may be
increased or diminished to any extent depending on the relation between
the number of turns in the primary and secondary winding._
This relation may be expressed in the following rule:
_The voltage of the secondary current is (approximately) to the voltage of
the primary current as the number of turns of the secondary winding is to
the number of turns of the primary winding._
[Illustration: FIG. 137.--Production of spark with plain coil. Connect the
ends or leads of the secondary winding to fixed insulators and bend the
ends so they are from one-sixteenth to one-eighth inch apart. Connect one
end of the primary winding to an electric battery, and with the other lead
of the primary winding brush against the other terminal of the battery, as
indicated. When the contact is broken there will be a spark both at the
point of rupture in the primary circuit and at the gap. An electric
impulse is also induced in the secondary circuit when the primary circuit
is closed and the current flowing in it gradually rises to its maximum
value, but this impulse is too feeble to cause a spark to jump across the
gap. Only the impulse induced in the secondary during the dying out of the
current in the primary is utilized.]
For instance, if the voltage of the primary current be 5 volts,
the primary winding have 10 turns and the secondary 100 turns,
then
Secondary voltage: 5 :: 100 : 10 from which
Secondary voltage = 50 volts (approximately)
The watts in each circuit are approximately the same; hence: if,
for instance, the current strength in the primary circuit be 5
amperes, the watts in primary circuit are 5 × 5 = 25.
Accordingly, for the secondary circuit the current strength is:
25 watts / 50 volts = 1/2 ampere (approximately)
From this, it is seen that where the voltage is raised in the
secondary circuit, the current flow is small as compared to that
in the primary circuit; therefore, heavy wire is used in the
primary winding and fine wire in the secondary, as indicated in
figs. 137 and 143.
For most purposes a very much higher secondary voltage is
required than in the example just given.
[Illustration: FIG. 138.--Diagram of battery and coil connections for jump
spark ignition as applied to a motor cycle. Coils are usually plainly
labeled with the abbreviations: “Bat.,” “Pri.,” “Sec.,” indicating that
the wires are to be connected to the battery, the primary circuit or
contact maker, and the spark plug. The battery and primary wires being for
the low tension circuit are easily distinguished from the secondary wire
by the small amount of insulation surrounding them.]
Secondary induction coils may be divided into three general classes:
1. Plain coils;
2. Vibrator coils;
3. Condenser coils.
The plain coil gives but one spark when the primary circuit is made and
broken, while the vibrator coil gives a series of sparks following each
other in rapid succession.
=Plain Secondary Induction Coils.=--Coils of this class are very simple
and consist of:
1. Core;
2. Primary winding;
3. Secondary winding.
The construction of a plain coil, such as would be suitable for ignition
service, is about as follows:
The core is made of soft annealed iron wires (No. 20 B and S
gauge) from one-half to three-quarters of an inch in diameter
and about six inches long. Over this core is slipped a spool of
insulating material (hard rubber or composition), on which is
wound first the primary winding of the coil, which consists of
several layers of about No. 18 B and S gauge silk insulated
magnet wire.
After the _primary winding_ has been wound over the insulated
core, and the ends have been properly brought out through the
heads of the spool to be connected to binding posts thereon, a
layer of insulating material is applied over the primary wire,
and the secondary winding is then wound on.
The wire for the secondary winding consists of about No. 36 B
and S gauge silk covered magnet wire, the amount used varying
considerably, depending on the desired voltage of the secondary
current.
When all the wire has been wound on, the ends are brought out to
the binding posts, the coil is soaked in shellac dissolved in
alcohol and baked, or in melted paraffin or a paraffin compound,
and allowed to cool. It is then placed in either a cylindrical
hard rubber shell or in a hard wood box.
The proportions of such coils vary greatly; for motor cycle use
they are made long and of small diameter (10×2-1/2 inches for
instance), while for some other purposes short and thick coils
are found more convenient.
=Ques. How may the coil just described be connected for demonstrating
purposes?=
Ans. Connect the ends of the secondary winding to fixed insulators and
bend the ends so they are about 1/8 inch or less apart. Connect one end
of the primary winding to a battery and brush the other end of the primary
winding against the other terminal of the battery as indicated in fig.
137.
=Ques. What happens when the primary circuit is made?=
Ans. An electric pressure is induced in the secondary circuit, but of not
enough intensity to cause a spark to jump across the air gap.
[Illustration: FIG. 139.--A Medical coil with armature and attachments
consisting of electrodes, foot place, sponge, induction coil etc. A
current of any degree of intensity may be obtained. The currents furnished
are: 1, primary, 2, secondary; and 3, primary and secondary combined.]
=Ques. What happens when the primary circuit is suddenly broken?=
Ans. A spark is produced both at the point of break in the primary circuit
and at the air gap in the secondary circuit.
=Ques. Why is a spark produced at the air gap at break and not at make of
the primary current?=
Ans. Because when the current is flowing it cannot be stopped instantly on
account of self-induction, that is, it acts as though it possessed weight.
If the reader has charge of a gas engine with a make and break
ignition system, he will often avoid vexatious delays in
locating ignition troubles, if he remember that one of the most
important conditions for obtaining a good spark is that the
_break take place with great rapidity_. This, of course,
involves that the ignition spring be adjusted to the proper
tension.
[Illustration: FIG. 140.--Rhumkorff induction coil. A secondary coil with
vibrator and condenser; a type generally used in the laboratory. The name
Rhumkorff was formerly very widely applied to induction coils for the
reason that some of the earliest coils were constructed by Rhumkorff.]
=Secondary Induction Coils with Vibrator and Condenser.=--A plain
secondary coil, such as just described, will only give feeble sparks for
its size for the following reasons: The inductive effect of the primary
winding in the secondary depends as previously explained on the rate at
which the current in the primary winding decreases or dies out.
If a strong inductive effect is to be produced in the secondary, the
current in the primary must stop suddenly. This is prevented by
self-induction in the primary winding, which opposes any change in the
current strength. The direct result is that, as the primary circuit is
broken, a spark appears at the break, which means that the current
continues to flow after the break has occurred, dying down comparatively
slowly, hence, the inductive effect on the secondary winding is small.
[Illustration: FIG. 141.--Conventional diagram of a condenser. A condenser
is a device designed to absorb or hold an electric charge in about the
same manner as a vessel will hold a liquid. Every conductor of electricity
forms a condenser and its capacity for holding a charge depends upon the
extent of its surface. A condenser is therefore made of conductive
material formed into such shape as to present the maximum surface for a
given amount of material.]
The spark at the break in the primary circuit is even larger than that in
the secondary circuit, and as this primary spark serves no useful purpose,
but, on the contrary, quickly burns away the contact points, such an
arrangement is obviously defective.
The vibrator-condenser coil is designed to overcome this trouble and also
to give a series of sparks following in rapid succession instead of one.
It should be noted that a series of sparks following each other
with considerable rapidity may be obtained with a plain coil by
placing a _mechanical vibrator_ in the primary circuit, as used
on some motor cycle ignition circuits.
The object of the vibrator, of a vibrator-condenser coil, is to rapidly
make and break the primary circuit during the time the primary circuit is
closed externally. It consists of a flat steel spring secured at one end,
with the other free to vibrate. At a point about midway between its ends,
contact is made with the point of an adjusting screw, from which it
springs away and returns in vibrating. The points of contact of blade and
screw are tipped with platinum. One wire of the primary circuit is
connected to the blade and the other to the screw, hence, the circuit is
made when the blade is in contact with the screw and broken when it
springs away.
[Illustration: FIG. 142.--Construction of condenser for an induction coil.
The conducting material used is tinfoil, of which a large number of sheets
are prepared, all cut to the same size. These are placed, one on top of
the other, like the pages of a book, with a thin layer of insulating
material between, usually two sheets of paraffined paper. Numbering the
successive sheets of tinfoil serially, all sheets of even number are
connected together and all sheets of odd number are connected together,
these connections forming the terminals of the condenser. The condenser is
then connected across the break in the primary circuit.]
A condenser is used to absorb the self-induced current of the primary
winding and thus prevent it opposing the rapid fall of the primary
current.
Every conductor of electricity forms a condenser and its
capacity for absorbing a charge depends upon the extent of its
surface. Hence, a condenser is constructed of conductive
material so arranged as to present the greatest surface for a
given amount of material.
The usual form of condenser for induction coils as shown in figs. 141 and
142 is composed of a number of layers of tin foil separated by paraffin
paper, each alternate layer being connected at the ends.
Fig. 143 is a diagram of a vibrator coil. CC represents the core composed
of soft iron wires. PP is the primary winding and SS the secondary. There
is no connection between these windings and they are carefully insulated.
Y is the vibrator or _trembler_ and D the center about which it vibrates.
W is a switch used for opening and closing the primary circuit; B, a
battery of five cells. The point of adjusting screw A rests against a
platinum point R soldered upon the vibrator.
[Illustration: FIG. 143.--Diagram of a vibrator coil. The parts are as
follows: A, contact screw; B battery; C, core; D, vibrator terminal; G,
condenser; P, primary winding; S, secondary winding; W, switch; Y,
vibrator. When the switch is closed, the following cycle of actions take
place: 1, the primary current flows and magnetizes core; 2, magnetized
core attracts the vibrator and breaks primary circuit; 3, the magnetism
vanishes, including a momentary high tension current in the secondary
winding; 4, magnetic attraction of the core having ceased, vibrator spring
renews contact; 5, primary circuit is again completed and the cycle begins
anew.]
If the switch W be closed, the electric current generated by the battery B
will flow through the primary winding. This will cause the core CC to
become magnetized, and the vibrator Y will at once be drawn toward it.
This will break the connection at R. The core, being made of soft iron,
immediately upon the interruption of the current, will again lose its
magnetism, and the vibrator will return to its original position. This
again closes the circuit, after which the operation of opening and closing
it is repeated with great rapidity so long as the switch W remains closed.
[Illustration: FIG. 144.--Circuit diagram of a master vibrator coil. B, is
the battery; C, the unit coils; C1, C2, etc., the condensers; P, the
primary windings and S, the secondary windings; H1, H2, etc., the spark
plugs; T, the timer; MP, the master primary; V, the vibrator; W, the
common primary connection; 1, 2, etc., the stationary contacts of the
timer.]
The cycle of actions may be briefly stated as follows:
1. A primary current flows and magnetizes the core;
2. The magnetized core attracts the vibrator which breaks the primary
circuit;
3. The core loses its magnetism and the vibrator springs back to its
original position;
4. The vibrator, by returning to its original position, closes the primary
circuit and the cycle begins again.
[Illustration: FIG. 145.--The Splitdorf master vibrator coil. As shown in
the illustration the several unit coils are indicated by the figures 1, 2,
3, and 4. A fifth unit V at the left contains the master vibrator. The
primary wires P connect with the timer and the secondary wires S with the
plugs. B B shows the battery connections.]
=Magnetic Vibrators.=--Many types of vibrator are used on induction coils,
the most important requirement being that _the break occur with great
rapidity_. In order to render the break as sudden as possible, different
expedients have been resorted to, all tending to make the mechanism more
complicated, yet having sufficient merit in some cases to warrant their
adoption.
In the plain vibrator, the circuit is broken at the instant the spring
begins to move, hence, the operation must be comparatively slow.
In order to render the break more abrupt some vibrators have two moving
parts, one of which is attracted by the magnetic core of the coil and
moved a certain distance before the break is effected. A vibrator of this
type is shown in fig. 146 and described under the illustration.
[Illustration: FIG. 146.--A hammer vibrator. When at rest, the upward
tension of the spring, which carries the armature A, holds the platinum
points in contact and causes the upper spring C, to leave shoulder of
adjusting screw D, and rest against the heavy brass plate above it. When
the iron core B, attracts the armature A, the downward tension on the
upper spring, C, causes the latter to follow the armature down, holding
the platinum point in contact, until the end of the upper spring C,
strikes the lower shoulder of the adjusting screw, D, which gives it a
“hammer break.” The adjusting screw is held firmly in position by a bronze
spiral spring under shoulder D.]
=Vibrator Adjustment.=--When a vibrator coil is used, the quality of the
spark depends largely upon the proper adjustment of the vibrator. The
following general instructions for adjusting a plain vibrator should be
carefully noted:
1. Remove entirely the contact adjusting screw.
2. See that the surfaces of the contact points are flat, clean
and bright.
3. Adjust the vibrator spring so that the hammer or piece of
iron on the end of the vibrator spring stands normally about
one-sixteenth of an inch from the end of the coil.
4. Adjust the contact screw until it just touches the platinum
contact on the vibrator spring--be sure that it touches, but
very lightly. Now start the engine; if it miss at all, tighten
up, or screw in the contact screw a trifle further--just a
trifle at a time, until the engine will run without missing
explosions.
TABLE OF INDUCTION COIL DIMENSIONS.
Length of spark | 3/8 inch | 1/2 inch | 1 inch | 2 inches
------------------------------------------------------------------------
Size of bobbin | 2-1/8 × | 2-1/2 × | 3 × 3/8 | 4 × 2-3/4 ×
ends | 1-1/4 | 5/16 | | 3/8
------------------------------------------------------------------------
Length of bobbin | 4 | 5-1/2 | 6-1/2 | 6-1/2
------------------------------------------------------------------------
Length and | 4-1/4 × | 6 × 5/8 | 6-1/2 × 3/4 | ----
diameter of core | 7/16 | | |
------------------------------------------------------------------------
Size of base | 7-1/4 × | 9 × 5 × 2 | 14-1/2 × | 12 × 7-1/2 ×
| 3-1/4 × | | 6 × | 3-1/4
| 1-1/2 | | 1-3/4 |
------------------------------------------------------------------------
Size of tinfoil | 4 × 2 | 5-1/2 × | 6 × 4 | 6 × 6
sheets | | 3-1/4 | |
------------------------------------------------------------------------
Number of tinfoil| 36 | 40 | 40 | 60
sheets | | | |
------------------------------------------------------------------------
Size of paper | 5 × 3 | 6-1/2 × | 9 × 5 | ----
sheets | | 4-1/4 | |
------------------------------------------------------------------------
Primary coil | No. 18 | No. 18 |2 layers No. | 2 layers 14
| | |16, silk | B. W. G. silk
| | |covered. | covered.
------------------------------------------------------------------------
Secondary coil | 3/4 lb. | 1 lb. | 1-1/4 lbs. | 2-1/2 lbs.
| No. 40 | No. 40 | No. 38 | No. 36
------------------------------------------------------------------------
TABLE OF SPARKING DISTANCES IN AIR.[13]
| Distance.
Volts. | (Inches.)
-------------------------
5000 | .225
10000 | .47
20000 | 1.00
30000 | 1.625
35000 | 2.00
45000 | 2.95
60000 | 4.65
70000 | 4.85
80000 | 7.1
100000 | 9.6
130000 | 12.95
150000 | 15.00
=Points Relating to Ignition Coils.=--1. Most ignition induction coils or
“spark coils” as they are called, have terminals marked “battery,”
“ground,” etc., and to short circuit the timer for the purpose of testing
the vibrator, it is only necessary to bridge with a screw driver from the
“battery” binding post to the “ground” binding post.
2. In adjusting the vibrator of an ignition coil, the latter should not
require over one-half ampere of current.
[Illustration: FIG. 147 to 161.--Wiring diagrams showing connections of
some standard spark coils. Key: B, to battery; C, to commutator or timer;
G, to ground (engine frame); P, to plugs; S, to switch. 1, 6 terminal
standard non-vibrator coil; 2, 3 terminal standard vibrator coil; 3 and 4,
terminal standard vibrator coil; 5, standard double vibrator coil; 6,
standard triple vibrator coil; 7, standard quadruple vibrator coil; 8,
single dash coil; 9, single dash coil with switch; 10, double dash coil;
11, double dash coil with switch; 12, triple dash coil; 13, triple dash
coil with switch; 14, quadruple dash coil; 15, sextuple dash coil.]
3. A half turn of the adjusting screw on a coil will often increase the
strength of the current four or five times the original amount, hence, the
necessity of carefully adjusting the vibrator. When the adjustment is not
properly made it causes, 1, short life of the battery, 2, burned contact
points, and 3, poor running of the engine.
4. In adjusting a multi-unit coil, if any misfiring be noticed, hold down
one vibrator after another until the faulty one is located, then screw in
its contact screw very slightly.
5. The number of cells in the circuit should be proportioned to the design
of the coil.
If the coil be described by the maker as a 4 volt coil, it
should be worked by two cells of a storage battery or four dry
cells. The voltage of the latter will be somewhat higher, but
since their internal resistance is also greater, the current
delivery will be about the same. Most coils are made to operate
on from 4 to 6 volts.
6. It is a mistake to use a higher voltage than that for which the coil is
designed, because it does not improve the spark and the contact points of
the vibrator will be burned more rapidly, moreover, the life of the
battery will be shortened.