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Frederick William Lee.

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THE ELECTRIC STRENGTH OF AIR

UNDER uo::Tiruous potentials an:^ as

INFLUENCED BY TEMPERATURE.



THE ELECTRIC STRi^rGTH OF AIR

u::der continuous potentials and as
influenced by temperature.



Clssertation

Submitted to tha Board of University Studiet
of The Johns Hopkins University

in conforriity with the require-iert? for the
De,:^rfie of Doctor of Philosophy.



By

Frederick W. Lee



■une, 191$.



']A-.n']\



THE ELECTRIJ oTREiJGTH OF AIR

u::d£r co::tikuqus potektials and as

i::flu£i:ced ey tsi.peraturb.



Table of Contsntg.

Page

I. Abstract. 1

II. Introduction and Historical Setting. 2

III. Description of Apparatus. 4

IV. Prfsliminary Sxperiirenbs. 10

V. Rseults of Obser-"'atioyi8, 15

VI. Wire Data and Tube Covstsnte. 18

VII. Difcu?sioR. 19

VIII. Go-clusions. 20

IX. Bibliography. 22



THE SLECTRIO bTRSNGTH OF AIR

UNDER GONTIIJUOUS POTENTIALS AND AS

INFLUSNCBD BY TEM P ERATURE.

I. Abecraot.

Tne papsr descriues a series of experiments or^ the in-
i'iuence of ieu.pei'a Jure on ooroua forwing continuous poteatials.
Ta8 ooserva tions nave oeen made on three sizes of v.ire of diam-
etsre .Oiiol cm., .UBU3 cm., and ,0333- cm., and in sacn case at
several values of temerature within the range 5*^ C. and 70° C.
At each temperature the pressure has been varied 'rors a value in
the neighborhood of that of the atirosphere do\vnwards, reaching in
tne extreme cases bhe value 6.03 cm. of ir.ercury. Within the range
of values reached, as indicated above, the general form of the law
of corona, as developed experinientally by a number of other ob-
servers, ii? found GO be fulfilled. There are separate families of
curves for positive and negative potentials as obtained by varying
the pressure for each constant value of ternperature.

The observations show that under co^^stant conditions as
to pressure and temperature a higher value of negative potential
than popiti^e potential is required to form corona.

As plotted graphically, the results ssem to indicate that
when larger 'Aire? are used corona appears at the sarae values of
ooth positive e>^d negative potential. The observations, however,
have not bean extended sufficiently to show this identity of value.
This conclusion is at •"•ariancs vcith ihe observat io-'^s of a nunber of



other axperi-anters , in particular those o* V;, 3. Brown, who con-
cluoea ifii.!, wiwh larger values u-i aiameoei- oi v.-re negative coroia
may appear at ij^wer valuae tnan positive corona.

Ti.e expdriiiieni;3 puustantiate tne empirical laws developed
oy Wi.ite-.ead and Pee^, clthougu tiie constants of tna equatior.s in-
volved are iii^uer than any heretoi'ora observed. Tiiere is so;iie indi-
cation tiiat at ten^pera tares ^r\ i,he iidigl.oorr.ood oi 7U*^ C. a depart-
ure rro.u Che e...pxrxcal laws .entio-ed may set in.

II. Ivitroduction ana Historical Setting;.

The law of corona foriaation on round wires in air has oeen
derived enipiricaily fro;:, the observatiwns of a number of experimenters.
It involves the eiecoric i-^teieity at the !?urtace of ti.e wire, tne
radius of the wire, and the relative density of tr.e air.

The empirical law i^as been investigated over' quite wide
rang:;>8 of values of radius of wire and air aensity as dependent on
pressure. Comparatively few atterapte have oeen raade uowever to
study the influence of temi^erature on corona formation. T..is paper
describes a series; of experiments in ^/.rhiGh corona voltages have been
neaeursd at several temperatures within the range of 5 C. xo 70 G.,
tne -pressure oCi-g varied in eacl: iase froij» tnat of at.;.v>spi.ere down-
ward, in tne extreii^e cases to 6,03 cm. of ...ercury.

;Jontinucu8 potential aaa oeen used t;irour-,hout -he worK.
Various investigators have shown that with alterna ti/ig corona volt-
ae,es tne maximum values of tiie wave are very close Ly, if not identi-
cally, the same as the continuous voltag,e values.

The law of corona nad its origin in the laws governing

2
tue Bpciri^mti, between plates. Townsecd , upon t/.e exi^eriments par-



voltage in axr as given oy one I'oi.T.uIa

V = 3& P S + 1700 (1)

where V = voltage oetween liwo parallel plates,
P = presaure of the gas,
S = the distance Dst'jvsen the plates.
Tiiis foriiiula oy ■iividing ohroug- oj P S gave use to oiiS
relation

Sinue ii.e electric *viteusity between the plates ie uni-

iorm a'^d therefore 1. = E» Tae relation co'^cer.trated the atten-
S

tion upor the electric inxensity and not upon the total voltage.
Townsend in his development of secondary ionization lirsu showed
chat in a corona tuoe the distance S in tne ^uove formula is that
of tiie path which an ion traverses in che greatest lield and there-
fore is next to the wire. Under tais consiaeratior, the auove ioraula
foi tne corona tuue is, -

i = A . J^ . -^ (3)

p r« /r

Where A and B are constants of the ohenoiiuena and r^ is the
radius o/ the wire. Tnis equation giving the law of coro.'a formation
was first developed experimenbally oy Vjhitehead, Peek and otiiers, and
Townsend nas snown that it may ue derived iroin his experiments on
gaseous ionization at low press'ures and that it is ic accord wa.th
tue taeory of ionization oy cotiision.

ttnitenead first detenainad these constauis in a corona
tuoe lor air unaer normal con aitio-iS. feoK la&er showed xnat tne
aej.8*ty or tne prestfu.e ana tue tojupera i-ure togeti.er afxectad tae
lorrnaoion oi coroua and ostaolished the density factor o •



S -■L'.fL (4)

V^iiere P = prespui'e of one gas Ir centi:..s&ers of ...ercary,

y = ausolute teKipei'aiure in degreas centigrade.
TiiS corona for^^ula fcn^n reaucea so t;ie lora

f = A + -^ (5;

wnich it has at present.

Wl'iitenead ana Peeit carefully i-nvestigsted ti.e values

of A and B With alternaiir^ poientiai oy changi^ig S , r,, humidity

3

of the air and the material 01 the -Aire. Later vi/hitehead and Brown

also determined zue coMstants A and B with continuous potentials oy

6 4 5

changing tne diau.eter of i>r.e wire. Harwell , Watson and Soj.ai'fers

also worked wiL*. -lixect potentials upon corona, Tfasir "etnod of

detecting corona was mostly Visual and with the metnods and material

available a 6 that time they secured qualitative results wnich later

experiments under ;.'iore favoraole conditions to a larger degree con-

3

firmed. It remained for 'la/iiitenead and Pullen to develop a tuoe

which gave more se -.sitive indications of corona.

This exparimental investigation aims to continue the wotk

3

of Whitehead and Brown with co itinuous potentials. The immediate

purpose was to investirjate the constants A andB with positively
and negatively c-^grged wires under different temperatures and vari-
aole pressures at each temperature.



III. Description of Apparatus.
The source of high continuous potential in these tests
was secured froiu a "kenotron" or rectifying vacuum tube, in con-
nection with a hign volt-ags transformer and condensers. The



* Name given oy Genaral Electric Goj^pany to*rectif ying
Fleming valve.



accoiiipanyin^ diagram of con'^ecticre indicctep the iran-'ier in '.vhich
they v,'ere connected rrd how they v.ere cortrolled.

The pllernator wes e tv;o armature gensrator designed to
operate et either six hundred cyclep or at three thousand cycles,
and wfe directly connected to e three horse ocver, 220 volt con-
tinuous currer't rhunt ir.otor. The rotor power, ps well ae the field
excitation v^ere L^ecured froK a 450 ar.ipere-hour , 110-220 volt storage
battery to insure steadiness of alternating potential.

The voltage to the trar.sf oriner was controlled by a variable
iron reactance. This reactance conpisted of tift'o colle mou'^ted upon
the leg? of a core type trsnsf oraier ir. ^ahich the core could oe ad-
justed. The adjusuLjent was accomrlished by a screw naving a pitch of
twenty threads to the inch, end in this way the reluctance of the
magnetic circuit *as cr.anged; ana iaLo in turn ciianged the ratio of
voltage upon the transformer and the reactance coil. The (generator
voltage reaair.ed constant. In this way the high potential voltage
could be raised in a contiguous iiian>^er with an exceptional aegree of
adjustment.

In the high tension circuit oi the transformer a kenotron
was connected va series with a oautory of couienserfe , o: wnic^: each
unit u&a. a capacity of .01 ni.f. Tne Kenotron was designed ior 'tOOUO
volte and l/lO aiiipere. In tr.is experiu.ent tae voltage was lir.itea by
the condensers to 20000 volts. The current taken by the exciting coil
of the kenotron was Six aniperes for iOU/4 elecuronic emission. ^..e
resistance of tne heavln,i, fiiai^ent, a ten mil tungsten wire, was one
Oiim. The construction and tneory of t..e Kenotron are n^w q^ite well
understood and txioroughly descrioed oy Dushman . A Siivail storage
uatter^ of s^x cells in series witn a small caroon co^^pression rheo-
stat was suiiicient to px-operly excite ana ieg..iate t^e filament



current. T^; kenotron to^efciier with its exciting Dpparatue were
carefully ir-eula&ed iro:: fue ground by wooden j. sulatore snd log,ical
grouping*

Tiie conaengere were oT i.he oil insulatsd high potential
type end were grouped inio two uatteries, jne of six units, tne other
of four units each. The combined capacioy of the first condenser
battery was .015 ni.f. and the corpbined capacity of tne eecond con-
denser battery was .01 ni.f. Tne needle gap of each condenser was
adjusted to 10000 -^olts, tiiereby maxiig the voltage across two con-
densers 20000 volts, aetween these two batteries vise connected an
inductance of .027 nenry. According to Hull the voltage fluctuation
would oe, •

V = S-iJL. ( ^ ^ ] (6)

■^^ volt
Wnere C = total capacity ir, farads,

w = 2"Tr f = 2-irx 600 angular velocity in radians per sec.
L = coefficient of self i>^duction in ne^rys between

cocdenser batteries,
i = current taKen froi- the second condenser uattery.
Tne current i whicn was used in the voltage ij^eaeuring cir-
cuit never exceeded .01 ampere. Fro.u the aoove forniula it is seen

that the correctio.i factor is negligiole. Tnat this coi:Tcided with

3
experiir.ental observations was tested by Brown .

The voltage was tueasured with a precision Yifeston voltn-.eter

in series wxth a resistance of 1,428,000 onrus of msnganin v^ire ?"d of

1,343,000 once of lavite resistances. Aoout 50 watts was the naxi-

niurn rate of energy dissipation in the foriu of heat in this circuit.



T-'.e q^-adrevit slectroriS'ter con»-ected a? s^ow« v.as r.ot used for ie-
tectin.^ ccroi^a voltage, but for oLliei' purpose to be described Itvter.
The resisbtirice ijp this circuit did not charge -.vithin the range of
oi-'e percent in this experi-iisnt a? tssted by a V/hea tsbo'-^e 'i^rid^s
upor the sUEiva tio'n of the irdividuel units composing this resistance.
(see photograph f/l).

The iorona tube as constructed for this pxperiment 'A".i3

3 3

similar to the O'-'es constructed before by Vvhitehead and Pullen .

The main chamber w?s constructed fron a piece of 6" steel piping
19" long. This pipe was carefully bored out in a lathe so as to
receive the concentric cylinder supports and perforated corona
cylinder. (oee iiegrar. of construction- and dimensions of the tube.)
Around the inner 4" brass cylinder •>:?? another 'xetal cylinder care-
fully insulated from it. This insulated cyli-'^der received a sr.'-all
fraction of the corona current and, i" conjunction with a galvanometer,
served to indicate the presence of corona. (See diagrp.r.- of connec-
tion of auxiliary apparatus.) The tube was closed at each end by a
fiber head which also supported the central wire. Glass, one inch
thick, was first tried for this purpose, but it was found that it
was unable to -.vithstand the stress due to internal pressure. The
fiber used for this purpose was thoroughly boiled in paraffin to ex-
pell all iroisture. Glass windows were inserted into this fiber to
observe the condition of the wire at all tirr.es. Around the outt-ide
of this cylinder were wrapped coils of copper tubing to serve as

heating or cocli'-'g coils to the apparatus. The^e coils of copper

on
tube were further insulated as to heat/the outside by ?n asbestos

heat insulating compound. Directly a'^ofe and belcv the inside brass



cyli^r^er, tut out of the electric risld, were plpced tv,o copper wire
coils each hpvi-Tg a reeists'^ce of two ohn-.s . Thsse two coile v-ere
conriecte-i i-i eerie? and served zs s resistpri-ce tberiaometer . A iherr;.o-
couple was slso placed in the tube v;ell avtey ivonr. the iron walls, to
measure the temperature of the gas. The tkerr.o-oouple was acade v;ith
a very entail hsat capacity po 3s to quickly acquire the surrounding
temperature, snd thereby give a^i irdicatici of nny quick change of
teriperature of the gps inside of the tube. All of the Joints ware
carefully packed rith string first it:rersed in a packing compound,
I-'-'Sulated tersiiv?,! ? v;ere taken froir the tube by the aid of epark
plugs. The pipe joints were all V;ell litharged before finally ad-
justing them. All of the cylinders were oriachined to align cev?.trally
and the adjust!i,ert was within one-hundredth of ai- inch. Upon the
outside of the tube was placed a spring, as shown in photograph f2,
which served to keep the wirs teut uith ch'anges of length of tube
arising fror the unequal expansion of the tube and its wire.

The teirperature of the circulating water used in maintain-
ing the tei;iperature of the tube was controlled by a special thercio-
static regulstor. This rsgulato.r consisted of a glass tuba of 3/l6"
bore, sealed st one snd, filled with mercury and toluol, as indicated.
Since the coefficient of volumetric tenperature expansion of mercury
is too sfeII for sensitive regulation, toluol ve?- also placed in
'this tube, as shown in the diagram of connections. The jiieroury acted
a? 8 sesl for the very volatile toluol and at the same time ser'"'ed
as 3 conductor in all the control circuits. The circulating: wsber
was heated with an ir.rersion iron wire rheostat and was capable of
carrying, whe*^ subir.er^ed, fifty arperes,. This current was auto-
n.atically interrupted oy a remotely controlled switch, when the



water hai riper to itp ^^rede tsmi-'- ed terperature . The control
circuit of tiiis sivicch viae operaleJ through a telegraph relay circe
it was necessary fco interrupt thif co^atrol circuit st a difference
of poter.tial of 110 volts d.c. A six volt storage battery v.^-is used
tc excite t::if relay coi'>"ected ir, series with a resistsnce. When
the n.ercury in the tube had risen sufficiently high it shortcircui ted
the battery connection to the relay and thereby opened the rer.-jotely
controlled switch. As soon as the ter.^perature receded this short
circuit '.vas rev.oved and allowe'd the heating current to re-estsblish -
itself. The final temperature of the circulating water was sdjusted
by resns of a regulating screv;. The height 'of this regulating screw
deterr.ined the asount of expansion whioh the rrercury and toluol could
have before actuating the control circuits.

This constant temperature v;ater was circulated by a posi-
tive rotary pump through the heating coils of the tube. By this
method the corona tube reached a final steady teirperature sfter one
and one-half hours. The difference in tenipers ture of the circulating
water and of the gt3S in the tube v.as three degrees when operating in
a steady state. Temperatures lower than roon temperatures were se-
cured by using ere eked ice in the, circulating water supply tank. The
temperature of the circulating v:ater in this case was that of irelting
ice, which was sufficiently constant for the experiraent. In a like
manner the teirperature of one therical junction was kept constant;
only here Dewar flask, or therr.os bottle was used to keep the ice
froE) melting too rapidly.

The vacuum purr,p used to exhaust the corc^e tube was a
small "Geyrk" nodel belted to a irotor. (5ee photograph #2 . ) The
pressure was measured directly with a mercury manometer. Needle



FOLD OUT




N




^







m


■ .4




0' ^-


^



f^H07-OC^/=?/^P^H * -3.



10



Vi'lvet- were used in all of the pressure cortrol tube? to assure
positive clof.irig of the ?ir conriections.

From the photograph ^3 it Is see^*. that v.ith the arrarige-
Ksr.t employed here it was poesible tc use out one lamp for illun:in£-.t-
ing the galvanorceter Kirrors, and 3lro c^e scale sufficed to measure
their deflection. The scale iri'this case vjas constructed fror a
ten foot C?liforrig redv.ood board ben.t irto the arc of s circle of
ten feet radius and rr.ounted ten feet fros'^ the ter^perature measuring
calvanoi^eter. For indicating the presence of corona a D'Arsonval
galvanoireter slightly underdamped was used. This galvanou eter ordi-
narily had a sensitivity of 83 rr'egohms but ';vith the above scale
arrangement it was increased to 2 50 megohms.

It, was found expedient, becEUse of the large anount of ap-
paratus used for operating this experiment, to separate the high
potential apparatus fror. the remainder by " platform. The plat-
form was constructed above the corona tube, as is shown in photo-
graph j^^S. In this way it was posrible to do all of the manipulating
v.ith dispatch and v.'ithout any danger of coming into contact v.ith
high potential circuits.

IV. Prelirainary Experiments.

The method of securin- I'.igh potential direct curre:-''t volt-

•5

age v.'8s substantially the sane as that used by Brov/n". However, it
was found more convenient to control the alternating current poten-
tial across the transformer v»ith a reactance than v;ith the field
circuit of the sltei-nator.

The volt::eter used in these tests was a precision Weston
Eodel f/l064. It WPS, however, checked Vvith a V/eston standard cell
snd a Leeds and :'orthrup po tentior, eter through its entire ran-^e a^-^d



11



was found accurate wi&hin the lirtits of observation.

The resistances of the vol cage measurirg circuit v.'sre
measured with a j^eede &'Northrup precision bridge and checked frorj
tiue to time.

The therr;.o-couple was rade from a junction of "Advance"
and copfsr 'Aire and was calibrated with a stanrJard mercury therrriom-
etsr in a bath of water well agitated and slowly cooling. With the
aid of a resistance in the copper portion of thir circuit it was
possible to adjust the deflection to an exact nultiple of the temper-
ature unit. One degree centigrade corresponded to a deflection of
two dlvieions upo'^, the scale. The resistance of the galvsnoroeter
was 110 ohr:s and the extra resistance in the circuit was 1151 ohr'e,
thereby making a total of 1271 ohms in this circuit.

This thermo-couple was then used to calibrate the resist-
ance therfforr.eter i^ the corona tube. According to Bellinger , this
resistance should have a lirear variation of tempsrature within the
range of terrperature froro 0° C. to 70° C. Its calibration will
therefore check the accuracy of both the variation of mercury ther-
mometer and of uniform.ity of temrjarature distribution in the tube.
The temperature resistance curve upon this basis proved to be a
straight line and hence the errors were below that of observation,
(see curves of temperature calibration). The calibrating was done
by first bringing the tube to a high steady temperature and then
allowing it to cool slowly. It required six hours to secure this
calibration curve.

The question of i-'-'sulatlon was a m-ore difficult one. It
WES found that the rr.etal electrode cylinder around the perforated
brass oyli>Tder had a resistance to 'jround of 20 megohms through



the hard rubber i",sul?tion. This '^.-as charged by replecins^ the
inpulators • with lev. hur'i rubber.

Since Mae fou^iation of gsinerabing high direct pots^^tial
by this Liethod pre?upposss alrost perfect iv-eula-Mor, it W£s neces-
sary to check all. of the leaks to ground ir the ge^^eratirig ?ystea.
This wa? do^e by charging the high potential ?ide of the system to
a difference of potential of forty voltg to ground and obperving the
decr'3-^;'e of potential upon the quadrant eleGtror;.eter. The rslations
^■hich exist by n-:easuring the resistance by the lo?r of charge rethod
are:

V = V„ r - -L t (7)



V ^ - -L



Mhere V = 34.7 potential st time t = 10 eeconds,
Vg == 39 initial potential,
C - .025 capacity in farads,
R - the resistance to ground.
This re:i£tar.ce front the observations a? shown by the
teet was 38 x 10 ohr.s. It is safe to conclude that even under the
highest voltages che leakage current did not iijodify the potential
witnin limits of observation. (See f oriiiula 5^-6) .




13



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Online LibraryFrederick William LeeThe electric strength of air under continuous potentials and as influenced by temperature .. → online text (page 1 of 3)