THE UNIVERSITY
m
OF ILLINOIS
LIBRARY
n
ft
m
Digitized by the Internet Archive
in
2014
http://archive.org/details/theoryofignitionOOdutt
THEORY OF THE IGNITION COIL
BY
MATI LAL DUTT
B. S., University of Illinois,
1911
THESIS
Submitted
in Partial Fulfillment of the
Requirements
Degree of
MASTER OF SCIENCE
IN ELECTRICAL ENGINEERING
IN
THE GRADUATE SCHOOL
OF THE
UNIVERSITY OF ILLINOIS
19 12
O'
for the
1*5*
ft
UNIVERSITY OF ILLINOIS
THE GRADUATE SCHOOL
Way 31,
I
19
£2
HEREBY RECOMMEND THAT THE THESIS PREPARED UNDER MY SUPERVISION BY
MAT I LAL DUTT
TECCHY OF THE
ENTITLED
BE ACCEPTED
DEGREE OF
AS FULFILLING
THIS
IGIIIT IOIT
COIL
PART OF THE REQUIREMENTS FOR THE
MASTER OP SCIENCE IU ELECTRICAL
In
EIT CITIES?. TUG
Charge
of
Major
Work
ead of Department
Recommendation concurred
%
in
Committee
on
Final Examination
KM
CONTENTS
Pago
I
Introduction
II
Physical Theory and Principles
of Operation
III
1
3
Fundamental Equations in
Transients
8
IV
Numerical Applications
16
VI
Conclusions
26
THEORY OF THE MODERN IGNITION COIL
I
Introduction
The modern ignition coil, otherwise known as
Induction coil. Induct orium, or Rhumkorff coil was regarded
a few years ago principally as a useful laboratory apparatus
and was chiefly used to illustrate the principles of electro-
magnetic induction.
To-day it has become an article of ne-
cessity to thousands of people, and the industry of manu-
facturing induction coils has become a very important one.
Among the uses to which induction coils are put, may he
mentioned the wireless telegraph, the excitation of X-ray
tubes, igniting the explosive mixture of gas and gasoline
in the internal combustion engines of various types, electric
gas lighting, etc.
In spite of the extensive use of the induction
<Joil
and the trouble arising from ignorance in the handling
of such coils, very little has been written regarding its
principles and operation from an engineering and practical
point of view.
Of course, all text books on physics give an
elementary explanation of its action, and a few highly mathematical articles, written more from the stand point of the
mathematician than from that of the engineer, have been
published.
The practical engineer having ordinary mathematic
al knov/ledge,
finds very little information on the subject
and his knowledge is limited to the usual text books. 3ven
z
this meager information is so scattered that it is quite be-
yond the reach of practical engineers.
The attempt of this
thesis will be to present the whole subject in a logical
manner and develop some equations from fundamental principles
to be of use to the every day engineer.
3
II
Physical Theory and Principles of Operation
When we examine the development of the ignition
coil, we find that the evolution followed the general law
"that improvements in experimental instruments advance along
definite lines
a process of evolution in which rudimentary
"by
forms are successively replaced
"by
developed machines." Most people
more and more completely
who are interested in the
induction coil think that Faraday made the discovery of
electro-magnetic induction with his rude ring transformer,
"but
the fact is that Joseph Henry, a young teacher in the
Albany Academy, was an independent discoverer of electro-
magnetic induction.
This is proven
"by
an account of the
manner in which he had independently performed a similar
experiment in the previous autumn "before receiving an account
of Faraday's work.*
But V/illiam Sturgeon, an eminent English
physicist, was the first person to construct a coil which
was practically the same in the general form as the induction
coil of to-day.
The next great improvement was that of in-
serting a condenser across the ""break" of the primary circuit of the coil.
physicist.
This was rendered by Fizeau,
French
This application of the condenser in the primary
circuit made possible the universal use of the induction
coil.
The modern ignition coil is essentially the same as
the Rhumkorff coil or induction coil.
It consists of two
coils, primary and secondary, and a central iron core.
The
4
latter is usually composed of a "bundle of soft iron wires
(insulated from each other by shellac).
The primary coil
which is wound around this core is made of a few turns of
thick insulated copper wire;
the secondary coil which
surrounds the primary coil is made of very fine, well in-
sulated wire of considerable length having a lp-~ge number
of turns.
Fig.
I
represents
the picture of such a coil
showing the details.
The cur-
rent flowing from the battery
B through the binding screw A,
the contact screw b and the
hammer h, enters the primary coil,
Frg.1
where it acts inductively on the
secondary coil.
out at f
screw
C.
,
Having traversed the primary coil it comes
and returns to the battery through the binding
When the current flows through the primary coil it
magnetises the iron core which attracts the iron hammer or
vibrator h, thus breaking the primary circuit at e.
This
interruption of the primary circuit causes the magnetic flux
to die away.
The hammer is then released thus completing
the electric circuit, and the current flows again.
This is
repeated at a great frequency which depends on the inertia
of the
hammer and the constants of the coil.
The repeated
appearance and disappearance of the current in the primary
coil induces an alternating electro-motive force of high voltage
in the secondary coil.
The strength of this electro-motive
force depends on tho ratio of the number of turns in the
secondary coil to that of the primary coil.
When the electro-
motive force is large enough to overcome the resistance between the needle points at p, a spark passes.
To make the break more sudden and to set up an
electrical oscillation in the circuit it is usual to put a
condenser across the "break points" of the primary circuit.
This condenser is thus so connected that it does not affect
the current rush in making but only in breaking.
Owing to
the magnetic induction in the coil, the current in the primary
coil cannot rise suddenly to its maximum strength.
It follows
the law,
where
i
=
primary current in amperes
E
=
impressed electro-motive force in volts
r = resistance of primary coil in ohms
self induction in henries
I
6 = 2.718
t
=
time in seconds
The current therefore requires an appreciable fraction of a
second to flow through the jprimary coil containing inductance
and resistance only.
Consequently at the make of the primary
circuit the rate of change of flux is relatively slow.
In the
case of the break, however, the primary circuit contains in-
ductance, resistance and capacity, as a consequence the rate
of change of flux in the case of break is greater than in the
€
case of make.
Hence the maximum voltage across the terminals
of the secondary coil occurs at the time of primary "break.
The object of the condenser is primarily two-fold.
It enables the primary circuit to he broken more or less
8parklessly and it serves as a spring which absorbs the electro
magnetic energy, which is equal to
break.
—
,
at the time of the
This absorbed energy is sent back from the condenser
as a reversed current into the primary coil causing magnetic
lines to appear in the opposite direction.
Thus the electro-
motive force in the secondary coil is increased and an in-
tensified spark occurs at the spark gap.
More clearly, if a condenser is connected across
the "break points" of the primary circuit, the energy in the
magnetic field dies down and charges the condenser for a short
time;
the condenser then discharges sending a reversed current
through the coil, thus setting up an electrical oscillation.
The electro-motive force set up in the secondary coil is then
the result of the stoppage of the primary current and its
immediate reversal in direction, and this is equivalent to
the removal of a certain number of lines of induction from
the secondary circuit, and their immediate insertion into it
in the opposite direction.
Hence when a condenser is used the
secondary electro-motive force would be just double the electro
motive force in the case when no condenser is used (neglecting losses).
"The condenser acts by setting up electrical
oscillations, and it does away with the spark or largely di-
minishes it, in virtue of the fact that the condenser acts at
7
at the moment of ""break" as if it were a shunt circuit of
negative self-induction, only with this difference
— that
instead of dissipating energy like a circuit of resistance
and induction it returns to the primary circuit in the form
of a reversed current, and increases the total change of in-
duction through the secondary circuit"
e
FUNDAMENTAL BCffJAflONS IN TRANSIENTS
The induction coil
stripped of all mechanical
detail may
"be
represented
diagrammatically
"by
Pig. 2.
X P
E is the constant electromo-
tive force impressed, L and
F.g.2
r are the
self-induction
and resistance of the primary coil.
C is a
across the switch or interrupter
e
of the condenser.
S.
capacity connected
is the electromotive force
Also, I^is the self-induction of the secondary
r^of the coil, and
coil, and r4 is the sum of the resistances,
r of the
of the
spark gap, and M is equal to the mutual induction
two coils, primary and secondary.
the
From the fundamental law that in a closed circuit
to zero, we
algehraic sum of the electromotive forces is equal
folinstant after opening the switch S, that the
find at the
lowing equations are true.
/^fo/fip/y /ny
V*
*
epu&S-i'orr
I?
f
L' M
6y
&
L/ arte/
e^ca^/or? Z &y
+ * L ~ eL '
—
f
_
/
-
^
?
,
=°
~~
3
<
_
-
-
-
-f
9
~cl*
~
[n
J £&t x
f]
/va,
nr c
cot-
+
cut
e—e
>
(^J^ *fe*^#^*f-'
=
?V 3
-f
a
m
2-
-r-
bm
7
c
+
O
= o
_
__
_
_
to
+
y
fu/e
are
h
+
o
v
— —
:
/
-/-//J
Z
5* jore
*»« yroc As
^3
^7€"
o-A
=
genera/
>
"\3
f
-
9
eocro-//c/7
/fie
<?re
43
3o/c/A/o/y
o-/
&
^/'/^ferer? // <z./ e.pua-//&r7
/S
-
//
Sohs A//urir?0
express sons for
~7fre
g£
^naf <g/ "7
t
t
<?pc<,/,orr
M si.
TA<
mi*i*t
oy
four**
=
°
L,=
c
I
7#<?
£>tj/>s///t,//r7y
/ Z-
One/ /3
/
Ct/r re/771
ctrref
sf>e<?S & /
cases
-
/7a/
^x /37
ao'/t'err
cc^Jorr
circuit" a/ faSVtlc/,
/primary
t-
Cans ^rr^
sn/efrct^err
7?7re C
of
l/a/ves
7%e €Xf?r>e sst'ons
seear?c/ary
/?
a^/rry
fjr-o&S-'C* /<fy)
c /-e ar/y
ttrt
//
/7f
,
for
Career? /
cer /-a
*"*
,
°*
S
in/vrrufter
Or
m
Ae
*3
>
/72
Gnat #3 "7
Corraferrse*-
/'rr
£=<>•
,
ffms
/7?ac/* f/Ca ^/Os?s
Cans/Werec?
/pr/sr? a ry
cq/? <5e
~£
( ttacr^/r
/o
ft*
-e<pc/a~f/o/7s
C/rarpe f
o^f
77T<>r>
<s/-/iz/r/-
Co/pl//g /c
-ttpy
s /&rr c/
fre
C/7SE.
1
/7ssume
/V^ 3
or
=
r>
r?o
qener a /
ooi/
Case.
o
//?
^
'
c/rra//-
primary
~tfce
T/re
.
e^e^a/^'orrs
f
7/7 e
orta*
Same
Z-
OS
cosr?e>
is?
Tfre
;
/s
0&
*2juct / / or7
Aiu//fjo/y/'ny
ore
r?o/af/o/7S
The
T/r-crj
cut
sSe
-
tes/'s /-o/7ce
secor? c/<try
77f e
rt
such a
he/?h€senf-S
har/na
-
€
4>y
JL,
c?s?af
epca// on
'S~
6y — /V
<?ya'*7
o'/
- ML,
/7 <£aCt'r?o
2-
a*
So/rmo
T/?/s
I
<?^<-
7
i/
oon 6
£71.
wr'/Jfan
differentia/
—
<3
=
as
A-e/ore
<rycal/cn
6y
Tfce
cto*/'/,ary
l+S-C
'7
/3
L
a
-
-
/s
—a
7
,
——— ^ //
t
-
)
/9
-
r»
^
3
—
n
= c
x
rfrirtJ
3
/
V
"7
c
o
/r/tcyra/e
Lfi,
Jsy / s
O&C
/
7f
so
777a
?%af
&f
T/us
L it
;
~7k-
y?r //77& ry
//&/<?
7r/-e
S'e"r>cr/rt/o r-y
rr?c7 y/ /r?e^rr?
c/o&eaf
Qc/rrenY-
t^a ry
CC/rreny*-
C/S
O^rr-
Tfye
for
0/7
//a a/
c//
z
C a r rr r> /~
s'&
/r?/s?/' /77i//*
t
af>/>*-e c /a A/^
e^f g/oas/t A>
jzes-o^
-
I
CC/r rer> /
e ypn/co //y
/S
f
/
S
g/so
-
readA^^
ry*J
&/7
& //f>r r?o / /r?p
C orrer?)/
—_Z~
,
— J~
f
Co
OS
rr?
'f?
ar / r? c?
oS/fr/ried
I
=o
2y
(/ C(U
y
JT a^s/
-^(ry
srA/cr?
C(i lrAf
y?r/rr?&ry
/?e/rs*e en
~7r/
one/
y7r-//7? ary Area,/:
Tie,
7
TsjQS
r->
—
-
o
t
to
0/7 e /s
600/ A^r&rr
S-e c*
0/
Arr?e
•
sry7<rr>
—
C?onsfa nj)
f?r//r>Qry
ct/r re/771
A
/&,
K
-
i
^>-er / oc// Ca//y
/Pr/rrrat-y
Gr/vc/ l
M
+-
sAc/v's-
/e
/-/'rrre
/J?e/7
e^>uo. //of?
ir?
7%/S
Me
r^z/or
Castr
0y£
£>/~ 77? e
i^
f
/V//^
7/7?
&€ <?as?4fory
,
sAor7~ ^e"cor?c/a/-y
Cc/rrfrr/-
fcoAer-e
<2
\=
V
rs
Cf/Sf. 2
ac/ye7/7/~a<^ec>css
7
,
if
is
'Of?
Ao
SuAj/fAote-
As ^pos/V/Ve- /
see-
~fA?e
T^fr-e/vm
AA?e
-77
/A7/S
sActv s
<?'Sc///a//f7^
gats*/ Ac
CL
f
= o
//?
AAe
^fs?r/-a/ ey<^a A/ or?
:
By /ns^c// osp
^J/yf?
/7 -
I
y&/?£>//c>/77es7an^
/?0
me As
A
as7
tc+rc/e^
~7Abz
&r?£A /s^
ratrt'ca/
y-ea/.
77)
y^^/^a-A
7%a^/-
t^S
e.jcy?re ss/as?
stvcsA^o./
A&s? s^-y c/er?A/y
osc/AAa A/ftp
£
/rpt/tyc /A?r?
A
A'a/A /r
ce/ry-e
.
/s"
~Z#e
NUMERICAL EXAMPLES
To illustrate the use of the equations previously deduced, the constants of an induction coil were determined for
application
numerical
The resistances of "both the primary
.
were determined
coils
and the secondary
"by
means of a re-
sistance "bridge which is familiar to every electrical engineer.
The method used in measuring the inductance was the well known
Each of the primary
impedance method.
find
the secondary coils
An alternating electro-mot^
was connected as shown in Fig. 4.
9
force was impressed on the circuit,
The total current in the coil and the
voltmeter was measured in each case,
1
\Q
\V)
and this current was later corrected
"by
deducting the current taken
the
the voltmeter was determined
Jv =
r
where
"by
is the
was also determined
Z=
-j-
"by
^
the
voltmeter resistance, and E is
the impressed electro-motive force.
coil
^'3'
The current flowing through
voltmeter.
equation
"by
The frequency of the circuit
a frequency meter.
therefore the reactance, x
»
The impedance of the
J7?
-
R2
where R
x
the resistance of the coil, the self-induction L =
2
ir
=
where
f
f * the frequency of the electro-motive force.
The unknown capacity was determined
"by
comparing the
deflection caused
"by
its full charge in a galvanometer with the
deflection caused
"by
a known capacity charged with same voltage
in the same galvanometer*
G is a galvanometer.
The connections are shown in Fig. 5.
The unknown capacity was charged by the
battery and then discharged into the galvanometer circuit and the
deflection noted.
Then the standard capacity of a "known value
was connected in place of C x and the de-
flection was noted egain.
Then
where d is the deflection of the
galvanometer and
C the
#
capacity.
Fig 5
The following constants were determined:
the primary resistance r = .1696 ohms
the primary reactance x = .612 ohms
the secondary resistance
the secondary reactance
*
x-^
3060 ohms
6580 ohms
-
the capacity C * .122 microfarads
the primary inductance L
.0017 henry
the secondary inductance
*
and the mutual inductance M
=
•
17.5 henries
^/.75 L
.1495 (assumed)
General Case
Substituting these constants of the coil under invesitgation, previously determined, in the solution of the
general differential equation 8 and the auxiliary equation 9.
a
=
L,n
-f-A.n,
'7
5 X
/(>?5
+ OO/yx 3o(,Q
/e
for
or7c/
jb
t^e.
<g
<?&f
(S^J _
f =
Scj6s^-/^u/n^
one/
y
=
+ o)
Q^ats7
- 3.67 x/o
ftrese
/z-
yo one/
\z&/c,es
^> /<?
y y
3
?
ye/
H/t?
f
f
y
=
3 oo
y
-
- /so
y
-
-
•/
6a/
*? =
/
5~o
-
/3 s o o
One/
y-fT*?
=
/
-
?T)
=
J£
-
777
3
=
T/re re-fore
i
—
o
f
C^£.
=
3^3.
€5
-
-
3^5
-
- 5/5 +
=
- S/S-* /3&000
6S
Soba-ZtZ-u / tnj
£*2L
=
6-5"
3
- 3
e
So/
/3&ooo
j.
J.
Z
/s?
-t-
S/rese
J 3&ooo
'
vq/c/<t $
of
/r?^^/??^,
o/^cy
m.
/9
-est
_
—
F rem
eyuaf/or?
l
//,/.,_ /y
etc
_
=
iu£L
s,* t
/3 8ooo
£Cos
(
'3eooofr
S /S £&"7 ('/ 3 8 000C + ATjj^
/7Z €
£J
5"
,
i
•
„
oo 6 4
9
i
rfr
i
AT r
and
&t,&s/ t Sa/,r?p
Or?*
^
^Xfress Sorts
(*f„ + f,-0rr- z^J
l
f
/6.3 x to
=
"7
Vte
£ eS
-fit*-
&6ore
-+«Z E
f
i
f<?7 c a J,ort
Cfi,**/*^,
f
c
A,j
z.t) ,
focrrc,
t9o*o «,o* SSoO***:*^)
-S/SY~
-
•
/
9/Z 5~
*>*r>t/o//fy /rtj
,
ore. 2£-/l"icf-
mr^/a/
<a<p<sa/,*r7S
p-f
y^r e
Co,/
trrtcftr
/rtresAyoStorj.
/7V
-t
- o
/enow
?Vf
-
-
•
<£/
4*5/7,
3/3
+
+
TTrese.
c&c /a/
jBuj-
a
I
-
—
/72
— C5
/7f
-&5t
/7j
€
J-
Case
Co rrtr? f~ ore
f7* €
'
-5/5-6
/7
TZere
t ,
—
^3
/9£ ?
'
3
8PS
&3
ot<^
o
-
(7/vc/
27^ a rye/ Z,& ^
t4a77<?
/*7-&
.
//?e
yoAra^-e.
^3
etm^/p
fAaS'e
C*oS
soSst*/-
?^/e~
6 a t/-&
_
(/3eoao^) _
%f
— (,
/3&O0O S/s?
? 3&0o<7*-j
t>/5
<?**e
/e77/rr<j
= o
£ <o One/ 27 /
€~^' S
£~ d S i-f.
one/
TJrere /or?
Cor? rer-rrea/ As/#F
<?<^c(a./ tOr?S
-T-
&
^^
'
X
&3
r?s/a*cY Zo
/?e>7^
Z
6
* *°~
S~/6~
-for
AS/fy
or~e
fye
<*>'
^^c^^Z/o^s
Zfre
/r?
-
fag.
j
<fr=rc.
- /t-tgo &j_
e^ wo / t'orrs
-
<2/7£>/
3
-
&A"'/r4
o^7<^
-
000 &i £/n
3
^l~or?7
--<£>/
7%/s
/?
o
#3
S/1
Cos
f
SZ7 075&C. /?7<3 /-/'co/
7~bere^ore
Os =
i
/o/orf&cf
S/r?ce.
Ct-/fz>A>rr-sas?7£
Or?
62
•
#;>
/7X /3S-00
Their v&/ues
Current
+
S7,
O
=
I
X/O 4%
So/r/rry
-rof-mj
C
x/o~
Me
&/* c/o£y"7f
Z/rr?e
f/re
»
Coti C'3S>CO(7-& )
Cr?Ar?o*/r7
cZ/r<<?t/y f>r-of*r Z/'orya
/
—
—
yc?/7////fs
-/o~7% e
-
tz>
3o
so/v^
77 Crsr? 6-r r 07*-
^Or^s
2/
-ft> r
Con re r?/ fr? C€
'.
-6
—
-
<£>
5
/
x /o~ * +
/7
-
f
//z
2? ana* 30 ,
S7,
- ~
+
SfS
=
£>/
x
/o~ 6
O
//'ntf
5/s
h
^
+ ?08x/0- G E~ S &'fas {,38000*)
'
—
= -S
/r?
-h 'f
€
t, Li
(0
£ ~6
£
Ore
<j
—/ t/t,S£
/Ven
—
^hTo
(\?s
f/3&ooot J
cadres
-
oy is 7%e
-
-
-
33
f*oi£?ss?se >-
£3
Subs-ti
express >on s
Or?(/
TVe
Cc/rrer? /
^decono/ary
~7Vre
Cor? c/erf^er-
C*oi/ <p reirtous /y
7
eo csa^r or??
-from
f/nc/
7r?e
for
,
of zfre
Cons£ar>fs
/fie
-first Case
Tfre
/r?
f-uLinq
/
,
/8
y
l/rx/er-
/r? i/-es/r
ZO
Or?a
f
C c/rr-rof
/»-//t?C7 r
Cf/<2r-<?e
for
/7
C/c /vrrr?/'/"'/
c?a // 0/7
.
Trley
are
-f^/ne/
ra^' orr
/nfe<?
f
c/oS/r7<7
C?/7af
rpr
rffe
EC
-
&
—
.
<£
/
x /<?~ ^
rf
7
=
/
*e x
-
r-
iC
J //
/
3
C/'r-Cts//-
'
^-frptr
=
~
s
ts-e
r*
sve>
O
6
one/ 3 £
f
Cue-
c?e>y(
-
-
3S
-
36
An o*r
f*7&
avians
5~
~
)
f
/<?
_
XZO~^
&Gcsa//or? S
/fpft
£?C>/r-/s7&
ua-Z^Or? s
6/
/o~ 6
=
S/n r7z
f
f
0^) -
Ce>r7S far? /s
£>r-/rr7ar-y
r7
#
-r
Z37& Co* f/3 08oo-t y-
~7fie
'
f/3ff0O0 *
^ir?
=
i.
75>
r7
=
G,
~& - C
Ao/tse
2.
=
t
-0
2*
g
1
o/so
e
s
/
ZZ x /o~ C $/r7 (,3&g>oo
-/CtA,
=
'
Cos (/3&&0O t +
t?
6/ x/o~ C
-
+
3J
-
3&
______
4o
^)
-
-
= _£L
_r
//7
<?<SV7_7tr/7
ser
/O S/o ff 388
^' J<?4>arj4? 7
Cot
~7$e
Curves ore
7%e>
/?s-/sr?ar-y
sAa^r?
p
Curves? f
/s
<2,
/4e
T^e
u-
or
t.
s. s.
roRM
a
Conclusions
VI
The curve q in Fig. 6 shows the condenser charge
after the primary circuit is broken at switch S Pig. 2.
constant term
-
6
in
.61 x 10"
The
equation 31 represents the
normal condenser charge corresponding to the impressed electromotive force.
The two transient terms show how the condenser
charge varies from instant to instant.
The vector sum of
these three terms is represented by the curve q.
The electrical
oscillation is started by the energy stored up in the magnetic
field around the coil.
At the instant of the primary break,
this energy is dissipated as a contact spark if no condenser
is used.
But it charges a condenser when it is connected
across the "break points," and consequently a current flows
in the syrtem because the time-rate of the condenser charge
is the current flowing in the circuit.
Hence this energy
charges the condenser to a higher voltage than the impressed
constant electro-motive force.
This electrical instability
cannot exist and the condenser discharges pnd sends a reversed
current through the coil which sets up a magnetic field al-
most as strong as the original field, if we neglect the losses
in the resistance and the imperfect mutual induction.
The
energy stored in the magnetic field thus set up charges the
condenser again which discharges and sends current through
the coil again and so on.
Thus the electrical oscillation
is set up whose period is a very small fraction of a second
and generally depends on the constants of the circuit.
If we refer to the simpler case, when
r-^
r
=
o,
we find from equation 17, the period of oscillation equal to
7
77-
fTL
<~
.
- r/^jc^
%
in
L, L^, and M remain constant.
a particular coil the terms
The deduction is that the
period of oscillation for the coil is directly proportional
to the square-root of the capacity.
Therefore the frequency
is inversely proportional to the square-root of the capacity.
Hence smaller the capacity, the larger the frequency.
The period of the mechanical oscillation of the
vibrator of the induction coil is much larger than the period
of the electrical oscillation.
As soon as the hammer is attract
ed, the primary circuit is "broken and the electrical oscilla-
tion, previously described, starts and goes on for some time
before the hammer completes the circuit again.
It is a phys-
ical impossibility for a mechanical vibrator to have the same
period of oscillation as that of an electrical circuit of this
nature, therefore a series of reversals of magnetic lines takes
place in the coil and several sparks pass between the terminals
of the secondary coil during one break and make of the primary
circuit.
It might also happen that when the current dies down
in the act of charging the condenser, for the first time, the
hammer is released.
But before it goes a reasonable distance
in the space between the attracted position of the hammer and
the contact screw, the reversed current from the condenser
magnetises the iron core in the coil.
This magnetised iron
ZQ
iron core, though of reversed polarity, reattracts the hammer
to a certain extent and dampens or hinders its oscillation
and keeps it longer in the space
.
The reversed current dying
down charges the condenser and the hammer is released again
and so on.
This process continually repeated keeps the hammer
in space while sparks pass between the terminals of the sec-
ondary coil, until the strength of the current becomes so
small that it cannot hold the hammer any longer in space.
Then the hammer recompletes the primary circuit and some more
current flows through it, at the same time short-circuiting
This phenomenon is possible because the
the condenser.
frequency of this oscillation is so great.
Thus what happens
is almost equivalent to holding the hammer in space
is not noticeable when the frequency is great.
This
.
But by in-
creasing the capacity, the frequency becomes smaller and it
is noticed that the hammer becomes sluggish.
The action,
explained above, causes it as is evident from the discussion.
The most interesting feature of this phenomenon is
that the primary coil takes a fresh charge of energy only when
the oscillation cannot continue while working in unison with
the other mechanical factors involved in the operation.
The term
.1762 x 1CT 6
£"* 615
x
Cos (138000 t)
of equation 31 represents the amount of the condenser charge
which causes the oscillation and
is its period.
= .000046
second
138000
It is also interesting to note that the period
being .000046 of a second, the exponential
- StSt
terms
£
-4>5t
£
f
Z9
decrease very slowly and therefore the oscillations practically retain their initial strength for a large number of re-
versals of magnetic lines.
All this is true when ideal conditions are obtainable, but such conditions do not exist in practice.
The break
of the primary current is always accompanied by a spark even
when a condenser is connected across the "break points".
L i*
is lost.
The oscillation
Thus a part of the energy --
—
therefore starts with a smaller value of q and
i
and dies
down faster than the theoretical curves.
Almost all ignition coils, which are seen in the
market today, are different modifications of the induction
coil.
In the ignition of the explosive mixtures of gas engines,
the battery circuit is connected to a mechanical device mount-
ed on the engine support.
ment of such a device.
Pig. 8 -shows the simplest arrange-
In the commercial types a large variety
Fig.
8
30
of arrangements is found, and the connections are made in
many ways.
The switch S could be thrown either on the battery
B or magneto M for the electrical current.
The contact-maker
K fixed on the shaft revolves and makes the electrical circuit complete once every revolution at the point L.
This
making of the circuit sends a current thru the primary coil,
and the action previously described takes place in the coil
and a series of sparks passes in the spark plug and ignites
the compressed gas.
Since the oscillation in an average coil
is very fast, even a very small contact point at L permits a
large number of electrical oscillations to take place and a
large number of reversals of magnetic lines occur causing a
series of sparks to pass between the points of the spark plug.
If the contact-point L is large and the contact
maker continues to be in a position to send current through
the primary coil' a'fter the gas is exploded, the sparks continue
to pass in the spark plug and an unnecessary waste of energy
occurs
To sum up the different points regarding the con-
denser, we find that it minimises the contact spark at the break
of the primary circuit, it sets up the electrical oscillation
and causes a reversed current to flow in the primary circuit
thus doubling the change in the magnetic field consequently
inducing a double electro-motive force in the secondary coil.
It also controlls the period of the hammer by retracting it
by sending a reversed current.
This controlling of
the
hammer allows more time for the oscillations to continue and
•3V
to permit sparks to pass "between the terminals of the second-
ary ooil.
3Z
References Quoted.
X.
Sullivan's American Jornal of Science
2.
Comptes Rendus
3,
Alternating Current Transformer
Volume XXXVI
July 1832
p. 418
Fleming.
Bibliography
1.
Internal Combustion Engines
2.
Electric Ignition for Gas Engine
3.
Induction Coil
Coil
4.
—
H. E. Wimperis
J. A.
Uewman
action of condenser in Rhunkorff's
Electrician, London, November 11, 1910
Secondary current of the induction coil,-- oscillograph
— Journal Franklin Institute, Oct.
Coils — B. F. Bailey — Electrical World
by Clyde Snook
5.
Induction
1907
April 14 and 21, 1910
6.
The Design of Induction Coils
Electrical World
—
—
W. M. 0. Eddy
Deo. 26, 1906;
—
Jan. 5, Feb. 2, 1907
—
7.
Induction Coil in Practical Work
8.
The Design and Operation of Spark Coils
Lewis Wright
F. W. Springer
Electrical World, December 14, 1907
9.
Heating Effect of the Electric Spark
Electrioal World,
10.
The Science of Jump Spark Coil
Electric Ignition
H, A. Perkins
March 24, 1906
Engine, July 1909
11.
—
Jones
—
J.
a. Williams
—
Gas