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THEORYOF THE IGNITION COIL
MATI LAL D"TT
". S "niv ersity o f Illino is , 1 9 1 1
THESIS
Submitted in Partial Fulfillment ofthe Requirements fo r the
Degree of
MASTER OF SCIENCE
IN ELECTRICAL ENGINEERING
IN
THE GRAD"ATE SCHOOL
OF THE
"NIVERSITYOF ILLINOIS b e
1 9 1 2
"N IVE R S I TYO F I L L I NO I S
THE GRAD"ATE SCHOOL
I HE RE"YRECOMMENDTHATTHE THES I S PREPARED"NDE R MYS"PE RVI S ION"Y
MATI LAL D"TT
ENTITLED
"E ACCEPTED AS F"LF ILLING THI S PART OF THE REQ"I REMENTS FOR THE
DEGREE OF MASTER OF
of Major W ork
ead ofDepartment
Recommendation concurred in
CONTENTS
Intro duct i on
Phys i cal The ory and Princi ples
Of Operat i on
Fundamental Equat i ons in
Trans i ent s
Numeri cal Appl i cat i ons
Conclus i ons
THEORYOF THE MODERN IGNITIONCOIL
I Introduct i on
The modern igni t i on co il,o therwi s e known as
Induc ti on c o il,Induct orium
,or Rhumkorff c o il was regarded
a few years ago principally as a us eful lab orat ory apparatus
and was chi efly us ed t o i llus trate the principl e s o f ele c tro
magne t i c indu ct i on . T o - day i t has be c ome an art i cle of ne
ces s i ty t o thousands o f pe opl e,and the indus try o f manu
factur i ng induct i on co ils has be c ome a very important one .
Among the us es t o whi ch induct i on co i ls are put,may be
ment i one d the wi rele s s te legraph,the exc itat i on of X- ray
tubes,igni t ing the eXpl os i ve mixture of gas and gas ol ine
in the internal c ombus t i on engines of var i ous types,ele ct ri c
gas li ght ing,e t c .
In Sp i t e o f the ei tens i ve us e of the induct i on
co i l and the troubl e ari s ing fr om ignorance in the handling
of su ch c o i ls,very l i t tle has been wr i tt en regarding i ts
princ ipl e s and Ope rat i on fr om an engineering and prac t i cal
po int of Vi ew . Of c ourse,all text b o oks on phys i c s give an
el ementary explanat i on of i t s act i on,and a few highly mathe
me t i cal art i cle s,wr i tten more from the s tand p o int of the
mathemat i c ian than from that of the engineer,have be en
publ i she d . The practi cal enginee r having ordinary mathemat i c ~
al knowledge,finds very l i t tle informat i on on the sub je ct
and hi s knowledge i s l imi t ed t o the usual text bo oks . Even
thi s meager informat i on i s s o s cattered that i t i s qui te b e
yond the reach o f pract i cal engineers . The att empt of thi s
the s i s wi ll be t o pre s ent the who l e subje ct in a l ogi cal
manner and devel op s ome e quat i ons from fundamental princ iple s
t o be of us e t o the every day enginee r .
I I Phys i cal The ory and Princ iple s of Ope rat i on
When we examine the devel opment o f the igni t i on
c o i l,we find that the evolut i on foll owed the general law
" that improvement s in exper imental ins truments advance al ong
de fini te l ines by a pro ce s s o f evolut i on in whi ch rudimentary
forms are succ ess ively replaced by more and more c omple t ely
devel ope d machine s ,
" wes t pe ople who are interes te d in the
induct i on co il think that Faraday made the di s c ove ry o f
ele ctr o - magne t i c induc t i on wi th hi s rude ring trans former,
but the fact is that Jos eph Henry,a young teache r in the
Albany Academy,was an independent di s c overe r o f el e ctro
magne t i c induc t i on . This i s pr qven by an ac count of the
manne r in whi ch he had independently performed a s imi lar
expe riment in the previ ous autumn before re ce iving an account
o f Faraday 's work? "ut Wi lli am S turge on,an eminent Engli sh
phys i c i s t,was the firs t pe rs on t o c ons truc t a co il whi ch
was pract i cally the same in the gene ral form as the induc t i on
c o il o f t o ~ day . The next great improvement was that of i n
s erting a condens er acro s s the "break" o f the primary c i r
cui t of the c oi l . Thi s was rendere d by Fi z eau,
a French
phys i c i s t?’
Thi s appl i cati on of the c ondens e r in the primary
circui t made pos s ible the universal us e of the induct i on
c oil . The mo de rn i gni t i on c o il i s e s s ent i ally the s ame as
the Rhumkorff c o il or inducti on c o il . It c ons i s ts o f two
c oi ls , primary and s e c ondary,and a central i ron c ore . The
latt er i s usually c omp os ed of a bundl e of s oft i r on wires
(insulate d from each othe r by shel lac ). The primary c o il
whi ch i s wound ar ound thi s core i s made o f a few turns of
thi ck insulated c oppe r wi re ; the se c ondary co il whi ch
surrounds the pr imary c o il i s made o f very fine,well i n
sul ated wire of cons i de rable length having a large number
of turns .P
Fig . I repres ents
the pi cture o f such a c o il
showing the de tai ls . The cur
rent fl owing from the bat te ry
" through the b inding s crew A,
the c ontact s crew b and the
hammer h,ente rs the primary co i l ,
where i t acts induct ively on thePhy
1
s e c ondary c oil . Having trave rs ed the primary c o i l i t c ome s
out at f,and re turns t o the battery through the b inding
s crew C . When the current fl ows through the primary co il i t
magnet i s e s the i ron c ore whi ch attracts the i r on hammer or
vibrat or h,thus breaking the primary c i rcui t at c . Thi s
int errupt i on of the primary c i rcui t caus e s the magne t i c flux
t o d i e away . The hammer i s then releas ed thus c omple t ing
the e le c tri c ci rcui t,and the current fl ows again . Thi s i s
repeate d at a great fre quency Whi ch depends on the inert i a
of the hamme r and the c ons tants o f the c oi l . The repeate d
appearance and di s appearance of the current in the primary
co il induce s an al te rnat ing ele ctro - mot ive force of h igh voltage
in the s e c ondary co il . The s trength o f this el e ctr o - mot ive
force dep ends on the rati o o f the number of turns in the
se c ondary c o il t o that of the primary co i l . When the el e ctro
mo t ive force i s large enough t o overcome the res i s t ance be
tween the ne edle po ints at p,a S park pas s e s .
T o make the break more sudden and t o s et up an
el e ctr i cal os c i llat i on in the c ircu i t it i s usual t o put a
condens er a cros s the "break p oints " of the primary ci r cuit .
This c ondens er i s thus s o c onne c te d that i t doe s not a ffe ct
the current rush in making but only in breaking . Owing t o
the magne t i c induct i on in the co il,the current in the primary
co i l cannot ri s e suddenly t o i ts maximum s t rength . It foll ows
the law E77 0— 6
where i primary current in ampere s
E impre s s e d e le ctro—mot i ve force in vo lts
res i s tance o f primary c oil in ohmsr
L a s el f induct i on in henr ie s
6
t t ime in s e conds
The current there fore re quire s an appre ci able fract i on of a
s e c ond t o fl ow through the primary c oi l containing inductance
and re s i s tance only . C onse quently at the make o f the pr imary
ci rcui t the rat e of change o f flux i s relat ively sl ow . In the
cas e of the break,howe ver
,the pr imary ci rcui t c ontains i n
ductanc e, res i s tance and capaci ty
,as a c ons e quence the rate
of change o f flux in the case o f bre ak is greater than in the
at the moment o f "break" as i f i t we re a shunt ci rcui t of
negat i ve s el f- induct i on,only wi th thi s di fference - that
ins t ead of di s s ipating energy l ike a c ircui t of res i s tance
and induc t i on i t re turns t o the primary ci rcui t in the form
of a revers e d current,and increas e s the t o tal change o f i n
du ct i on through the s e c ondary
F "NDAMENTAL EQ"ATIONS IN TRANS IENTS .fio ’ fl CD ' b - ‘w w
Th e induct ion c o il
str ipp ed of al l me chani cal
d etail may‘
b e r epre sent ed
d iagrammat i cally by F i g . 2 .
E i s the con stant e l ec tr omo
t ive forc e impr e s s ed,L and
Ffiaar are the s el f- induct i on
an d re s i stanc e of the pr imary co il . C s a cap ac ity conne c ted
acro s s the switch or int errup ter S e i s th e el e c tromo tive fo r c e
of the c ondenser . Al s o,Lii s the s elf- indu c t i on of the se c ondary
co il, and q i s the sum of the re s i stanc e s
, r; o f the c o il, and
n'
o f the sp ark g ap ,and M i s e qual t o the mutual induction of the
two c o il s,p r imary and s econdary .
F rom the fundamental l aw that in a clo s ed c i r cuit the
algebra i c sum of the ele c tromo t ive for c e s i s e qual to z ero,we
find at the instant aft er op ening the swi t ch 8,that the fol
l owing e quat i ons ar e t rue .
— E LI
g o
._o
eya a z/l bfld
'
3 ¢n d 4 a n d, fir cu e
79 77
N"MERICAL EXAMPLES
To i llus trate the us e of the equati oa rev i ous l y de
duc ed the cons tants of an induct i on co il were de termine d for
numeri cal appli cat i on . The re s i s tance s of b oth the primary
and the s e c ondary c o i l s were de termine d by means o f a r e
s i s tence bri dge whi ch i s fami li ar t o every ele ctri cal engine er .
The me thod us e d in measuring the inductance was the well known
impedance method . Each o f the primary and the s e condary co i ls
was c onne ct ed as shown in Fig . 4. An al te rnating e le ctro - mot i ve
force was impre s se d on the ci rcui t .
The t o tal current in the c o i l and the
vol tme ter was measure d in each cas e
and this current was later corre c te d
by deduc ting the current taken by the
vol tme ter . The current fl owing through
the vol tmete r was de te rmined by the
e quat i on ]; -§L where c i s the voltme te r re s i s tance,and E i s
V
the impres s ed el e c tro - mo tive force . The fre quency o f the ci rcui t
was als o de termine d by a fre quency me te r . The impedance of the
c o il Z 5 there fore the reactance,x “2
2R2 where R
the res i s tance of the c oil,the s elf- induct i on L where
f a the fre quency of the ele c tro - mo t i ve force .
The unknown capaci ty was de termine d by c ompar ing the
de fle ct i on caus ed by i t s full charge in a galvanome te r wi th the
defl e ct i on caus ed by a known capac i ty charge d wi th same vol tage
in the same galvanome te r . The conne ct i ons are shown in F i g . 5 ,
G i s a galvanome ter . The unknown capac i ty was charge d by the
bat te ry and then di s charged int o the galvanome ter c ircui t and the
defl e c t i on no te d . Then the s tandard capac i ty of a known value
was conne cte d in place of CX
and the de
fle cti on was no ted again . Then
08
ds
where d i s the de fle ct i on o f the
galvanome ter and Cthe capac i ty .
The foll owing cons tants were de te rmined
the primary res i s tance r . 1 696 ohms
the primary reactance x . 61 2 ohms
th s e condary re s i s tance r1
3 0 60 ohms
the s e c ondary reactance x1
6580 ohms
capa c i ty C . 1 22 mi cro farads
the primary inductance L . 0 0 1 7 henry
the s e condary induc tance LI
henri es
d the mutual inductance M AJTTSEETE
. l 495 (as sumed)
General Case .
Subs t i tut ing the s e c ons tants o f the c oi l under i h
ves i tgati on ,previ ously determined
,in the s olut i on o f the
general di fferent i al e quat i on 8 and the auxi li ary e quat i on 9 .
’7 5 X o /6 75'
0 0 0 / 7 x zz o g g' 0 0 74 5
K / O—C
x 0 0 7 9 5
I f we refer t o the s impl er cas e,when r
1r c c
,
we find fr cme quat i on 1 7,the pe ri od o f os ci llati on equal t o
2. In a part i cular c oil the terms
L,L1 ,
and M remain cons tant . The deduct i on i s that the
peri od of o s ci llat i on for the c oi l i s di re c tly pr op ort i onal
t o the s quare - ro ot of the capac i ty . The refore the fre quency
i s invers ely pr op ort i onal to the s quare - ro ot o f the capaci ty .
Hence smaller the capac i ty,the larger the fre quency .
The peri od of the me chani cal os ci llat i on o f the
vibrat or of the inducti on c o il i s much large r than the peri od
of the e le c tri cal os ci llat i on . As s o on as the hammer i s attract
ed,the primary circu i t i s broken and the e le ctri cal os ci lla
t i on,previ ously de s cr ibe d; s tarts and goe s on for s ome t ime
be fore the hammer c ompl ete s the c i r cui t again . I t is a phys
i cal imp os s ib i l i ty for a me chan i cal vibrat or t o have the s ame
pe ri od o f os c i ll ati on as that of an el e c tri cal c i rcuit o f thi s
nature,there fore a s erie s of revers al s o f magne t i c l ine s takes
place in the c oi l and s everal S parks pas s be twe en the te rminals
o f the s e condary coi l during one break and make of the primary
c ircui t
I t mi ght als o happen that when the current di es down
in the a ct o f charging the condens er,for the fi rs t t ime
,the
hammer i s releas ed . "ut before i t goe s a reas onable di s tance
in the S pace be tween the at tracte d p os i t i on of the hamme r and
the contact s crew,the revers ed current from the condense r
magne t i s es the i r on core i n the c o i l . Thi s magnet i s e d i r on
iron core,though of reverse d p olari ty
,reattrac ts the hammer
t o a ce rtain extent and dampens or hinders i t s o s c illat i on
and ke eps i t l onger in the S pace . The reverse d current dying
down charge s the c ondens e r and the hamme r i s releas ed again
and s o on .
Thi s pro ce ss c ont inually repeate d keeps the hammer
in space whi le S parks pas s be tween the te rminals o f the s e c
ondary c oi l,unt il the s trength of the current be c ome s s o
small that i t cannot hol d the hammer any l onger in space .
Then the hammer rec ompl etes the primary c i rcui t and s ome more
current fl ows through i t,at the s ame time short - c i rcu i t ing
the c ondens er . Thi s phenomenon i s p os s ibl e be cause the
fre quency o f thi s o s ci llat i on i s s o great . Thus what happens
i s almos t e quival ent t o holding the hamme r in Space . Thi s
i s not no t i ceable when the frequency i s great ."ut by i n
creas ing the capaci ty,the frequency be c ome s smalle r and i t
i s no ti ce d that the hammer be c ome s s luggi sh . The act i on,
explaine d above,caus e s i t as i s evi dent from the di s cussi on .
The mos t inte res ting feature o f thi s phenomenon i s
that the pr imary c oil take s a fresh charge of energy only when
the os c illat ion cannot c ont inue whil e working in uni s on wi th
the o ther me chani cal fact ors involve d in the Op erat i on .
The te rm . 1 752 x 3- 5 1 5 t C o s (1 3 80 0 0 t )
of equat i on 3 1 repre s ents the amount o f the c ondens er charge
whi ch caus e s the os ci llat i on and . 0 0 0 046 s e c ond
i s i ts pe ri od . It i s al s o intere s t ing t o note that the peri od
- 5 /5 t ~ 6 s t
be ing . 0 0 0 046 of a s e cond,the expo nent i al terms 6 E
de c rease ve ry Sl owly and the refore the os c i llat i ons prac t i cal
l y re tain thei r in i t ial s t rength for a large number of re
versals o f magne ti c l ine s .
All thi s i s true when i deal c ondi t i ons are ob tain
able,but such c ondi t i ons do not exi s t in pract i ce . The break
of the primary current i s always ac c ompanie d by a S park even
when a c ondens er i s c onne cted acros s the "break po ints " .
Thus a part of the energy i s l os t . The os c illat i on
there fo re s tarts wi th a small er value o f q and i and di es
down fas ter than the theore t i cal curve s .
Almos t all igni t i on c oi ls,whi ch are s een in the
marke t t o day,are di fferent modi fi cati ons o f the induct i on
c oi l . In the i gni t i on of the expl os ive mixtures of gas engine s,
the batte ry ci rcui t i s c onne cte d t o a me chani cal devi c e mount
ed on the engine support . Fi g . 8 Shows the S imple s t arrange
ment su ch a devi ce . In the c ommerci al type s a large vari e ty
Re ferences Quo ted .
Sull ivan 's Ameri can Jornal of S ci ence July 1 83 2
C omptes Rendus Vel ume XXXVI p . 41 8
Alternating Current Trans former Fleming .
"ibl i ography
Internal C ombus ti on Engines H . E . Wimper i s
Electri c Igni t i on for Gas Eng ine J . A . Newman
Induct i on C0 1 1 ac t i on of condens er in Rhunkorff‘s
C o il Ele c tri ci an,L ondon
,November 1 1
,1 91 0
S e c ondary current o f the induc t i on c o i 1 ,~ os c i ll ograph
by Clyde Snock Journal Frankl in Ins t i tut e ,Oc t . 1 90 7
Induc ti on C oi ls ". F . "ailey Ele ctr i cal World
April 14 and 21,1 91 0
The Des i gn of Induc ti dn C oi ls W. M; O . Eddy
Ele ctri cal World Dec . 26 ,1 90 6 ; Jan . 5
,Feb . 2 ,
1 90 7
Induct i on C oi l in Pract i cal Work Lewi s Wright
The Des ign and Operat i on o f Spark C o i ls F . W . Springer
Ele ctri cal World,De c embe r 14
,1 90 7
Heat ing Effe ct of the Ele ctr i c Spark H . A . Perkins
Ele c tri cal World,March 24
, 1 90 6
The S c i ence of Jump Spark C oi l J . A . Will i ams Gas
Engine,July 1 90 9
Ele c tri c Igni t i on Jones