A Study of Vacuum Switch Arc Phenomena with the Cathode Ray Oscillograph Citation Hayward, Claude DeWayne (1929) A Study of Vacuum Switch Arc Phenomena with the Cathode Ray Oscillograph. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/0stk-t522. https://resolver.caltech.edu/CaltechTHESIS:08212024-152601231 Abstract No abstract. Item Type: Thesis (Dissertation (Ph.D.)) Subject Keywords: (Electrical Engineering) Degree Grantor: California Institute of Technology Division: Engineering and Applied Science Major Option: Electrical Engineering Thesis Availability: Public (worldwide access) Research Advisor(s): Unknown, Unknown Thesis Committee: Unknown, Unknown Defense Date: 1 January 1929 Record Number: CaltechTHESIS:08212024-152601231 Persistent URL: https://resolver.caltech.edu/CaltechTHESIS:08212024-152601231 DOI: 10.7907/0stk-t522 Default Usage Policy: No commercial reproduction, distribution, display or performance rights in this work are provided. ID Code: 16660 Collection: CaltechTHESIS Deposited By: Benjamin Perez Deposited On: 22 Aug 2024 19:03 Last Modified: 22 Aug 2024 19:19 Thesis Files PDF - Final Version See Usage Policy. 11MB Repository Staff Only: item control page

A STUDY OF VACUUM SWITCH ARC PHENO.i ENA WITH THE CATHODE RAY OSCILLOGRAPH THESIS by Olau.de D. Hayward In Parti al Fulfillment of t he Requirements for t he Degree of DOOTOH OF PHILOSOPHY California Institute of Technology 1929 This thesis describes the result s of an investigation of the a.re and glow discharges oecuring between the contacts of a vacuum swi tch when an alternating current is interrupted. The investigation was made by taking cathode ray oseillogranw of current vs. time 9 voltage vs . time, and voltage vs. current. Circuit volt ages of 2200, 15,000P and 30,000 volte and currents of from Os4 to 14.0 amperes r.m.s. ai.; both lagging and leading power fac.tors were emp loyedo At different til'.es, both aluminum and coppek"' swi teh contacts were used. The function of t.he a rc in acting as an electrical tis.hock absorber , " preventing inductive voltage surges from being set up by the switching operation 9 is discussed , a.nd tabulated results are g iven. shoWing that a. rathei· loose connection exists between the current at which the arc e:ittingu.ishes and the maximum value of the i.nduced. voltage surge ., but that the average arc extinction current and a lso the average induced voltage surge a.re both quite definitely lower for aluminum c·ontacts t han for copper ones~ :Previous swi telling experience indicates tha t the voltage surges are mu.ch higher when tungsten contacts are used. From these facts the t heory is advanced that the arc extinction cu.rrent and the voltage surge depend upon the vapori zation temperature of the contact metal and the use of magnesium or zinc is suggested. The volt age-current oscillograro.s indicat3 a stora 5--e of ener gy in t he di schart:,re itself and a. subsequent release occuring in some cases several times while t he discharge lasts. No explanation has as yet been found for this surprising resu lt 1 but it seems to be authentic, since every effort to eli minate it was unsuccessful. If this r esult may be regarded as a true one, radical changes must "Qe ma.de in the acce pted arc theory~ UlTRODUOTI ON 4'he investigation, described in this thesis, of the are and glow phen.omena which take place when two metallic electrodes, carrying an alternating eur:rent at moderately high voltage., are separated in a vacuum was undertaken as a pa.rt of the Vacuum Switch research program whieh is being conducted in the H.llion Volt Laboratory at the Californ.i a Institute of 'fechnoloa. Preliminary t-eats {l) :followed by more exhaustive ones at higher cu.rren-t.s and vol tageshave proved the soundness Gf' the fund.a.- mental idea of breaking an electric cu.rrent in a vacuum. Currents as high as :f'if'teen. thousand amperes at fourteen thousand volts and two thousand amperes at ninety thousand volts have been mwcessfully b-roken by a vacuum switch having relatively small physical di mensions. In all the tests niade it has been the kva. eapaci ty of the power source rat.her than failure of the vacuum switch which has set the upper limit of voltage and ourrent reached. When the vacuum switch has been evs.euated to below the maximwn operating pressure (about 10- 3 mm. of Hg.) and the contacts h&ve been properly outgassed 9 the alternating current is invariably broken in the fil•st half eye-le with a small momentary are. It should be bome in mind that in studying this arc it is not desired to eliminate it • for the are acts as an electrica l shock absorber, preventing voltage shocks from being transmitted to the system by the swi tcbing operatio11. ASsuming that the swi teh is opened at some point of the current \vave ether t han the zero point, the presence of the arc allows the current t-o continue to flow,. gradually decreasine; to l a val.ue near zero on the sine wave o if there rrere no ar~ and the current dropped to zero i mmedia tely u_pon opening the contacts.• the enormous rate of c ~ of current i th t ime • acting th&1ough the ind:u.cta.nce of the circuit~ would cau.se very high voltages to be induced. High voltage. su.rges of this kind have a. very detrimental effect on the system, causing flashover a nd puncture of insula.tien . If the arc cou ld be made to continue until the current reached zero instead of extinguishing .shortlt before, ideal Std. teh operation with no line voltage surges would result. AS a. matter of fact, however, the arc form of discharge does not continue to zero current but change s to a gl ow fo:rm when the cur.rent !alls 'below a certain minimum value, as wi ll be eX!)lained in more detail later. It was with tl:!.e idea of studying this transition and the hope of finding means of eliminating in the vaou,um St.Ji. t-eh the voltage surges whi ch aeeompany it in all kinds of switches that this re search was begun . Owing to the late arrival of the cathode r ay oscillogra.ph u.sed in this work and t he ti me del~ involved in making it operate satisfactori l y and in making certain changes necessary t-0 a.de.pt it to the wo r k, the investigation may by no means be regarded a.s finished.If the resu.l ts so far obtained may be relied upon, phenomena have been uncovered which cannot be exp lained by the aeoepted arc theory and which certainly deserve further attention. THEORY Fu.n~amentally, the electrieal diseharge cha.racteris,t ie.s for electrodes in all different kinds of gases and vapors and a.tall pressures ranging :from a fraction of a millimeter of mercury to atmospheric or higher seem to be quite similar. With suitable changes in scale and relative proportions the curve given by Compton( 2 ) and shown in fig. l might be adjusted to fit any of the se conditions. In the vacuum swi toh the medium in which the di sc hc'lr ge oecurs is principal ly vaporized metal from the electrodes. .Examining this curve we see that three types of' discharge are recogn1z-ed: corona., glow, and arc. Corona discharge occurs when the gas or vapor roe di um is ionized for only a part of the di stance between electrodes. With this type of discharge rela tively high applied voltages cause only rel atively small currents (fiel d. currents) to flow. If the voltage is raised ., however, ionization progresse s i n a cumulat ive manner and the current rises at an e'lter ine re-asi ng r ate until a. point of insts.bili ty , A, is reached at which the eu.rrent continues to increase even t hough no further increase in voltage i s made. At t his point the discharge chane;~s from corona to glow . In the glow form of discharge t h e gas or vapor is ionized for the who le of the distance between the electrodes and the current is carried by the positive ions and electron s coming from the gas or vapor with the exception of a few electrons which are emitted from the cathode by a photoelectric effect o f' by a seconda.ry emis-sion effect resulti ng from positive ion bombardment of the c athode . As may be seen from the portion AB of the curve• the gl ow discharge h as a negative res i stance charac t eristic, that is, the voltage decreases as the current is increased. To maintain this form of discharge the current mu.st be limited by external resistance or reactanee in the circuit. The arc form of discharge is distinguished from the glow by the presence of a bri ght spot on the cathode, the ttcathode spot. u of electrons. commonly called This spot seems to be a very abundant source These electrons produce positive ions by collision with the gas molecu.les,which, having a slower vel ocity than the electrons, build up a positive space chti.rge, which in turn allows the saturation current to flow from the cathode spot and across the space between electrodes with very little pote:ntial drop. This is illustrated in the curve in fi g . l by the closeness with which t he arc characteristic CD approaches t he ordinate ax.is. Thi a potential drop is scarcely greater th!:ill the minimum ionizing potential of the gas or vapor involved and usually ranges in the neighborhood of from ten· to thirty volts. J. H. Hamilton{ 3 ) has found -tne p'otential drop to vary from ten to t wenty five volts in the case of vacuum switch arcs for copper,. aluminum, and tungsten electrodes. Concerning the mechanism of the electron emission from the cathode, two theorie s have been proposed: thermionic emission., and the pul ling of electrons frorn the cathode by a high field at, its surface. J. J. Tho.rnpson< 4 ) and St ark( 5 ) first suggested the former theory and Langmui r{G) the latter one. K. T. Compton ( 2 ) believes that either kind of emission may take place and in many cases a combination of the two occurs. 4 The transition from gl ow to are di scharge or vice-vers& seems t o be more or les s erratic. As indicated by the portion TB of the eurve, t he glow f orm of dischar ge may sometimes persist into hi gher current s eve n t hough the .s ame current cou l d be c arr i ed at a. much lower voltage by the are for m of di sehar ge. Conversely, as indicated by the portion TO, the arc may continue t o hang on even though the .current is reduced below the va.lu.e at t he transition point T at which the disoha.rge should normal ly e hange to the glow form. The whole transition region is one of great instability an-d one in which oscillat i ons commonly occu r. £\PPP..RATUS A. THE CATHODE RAY OSClLLGGRAPH. The cathode ray oscillogr-aph is a very suitable instrument fer investigating electric arc and gl_ow discharge phenomena 1:?ec,au.ee of ita abil,ity to follow the extremely r-ap i d. variations of vo ltage amt current t.hat occur., Frequencies o-f the order of one hundred milUon Qycles per seeond have 'peen recorded wi th this device. The u.ppeJ:" limit of frequency which it is poss ible to r ecord ie set by the sensitivity of the photographic film ~ather than by any inertia effect of the cathod,e ray. The oscillograph used i~ this re s earch was a General Eleetrie Company deve1o_pment detailed description of which may be found elsewhere('!). In general. it may be said that it is of the Dufour cold cathode type arranged to have the photographic film inside the vacuum cha!I\9er so that the cathode r ay impinges directly u.pon it. This inc-.reases its sensi ti vi ty over the Braun type of QSCillogr11ph many fold. instru.ment.. parts: Fig. 2 shows the essential parts of the For descri ptive purpose s t hey may be divided into three the cathode tube, the deflecting tube, and the film bello The c athode tube shown at the left hand end in the figure is made of glass and -c,o ntains the flat aluminum disk cathod-eo 'Fhe deflecting tube, also made of glass,, contains the electrostatic deflecting plates and is supported in si de a metal cy linder which has holes cut in it to allow eleotrorna.gnetic deflecting coils to be placed a.bout the tube. Between the cathode tu.be and the deflec t ing tube are mounted two pinholes which limit the cathode ray discharge to a narro\l beam. contained in a box lined with sheet copper for the purpose of eleetrostatie- shielding. Connections to the defleeting plates and coils are made through bushings in too back of the box. The film be ll is made entirely of metal and !las an end plate whi ch may be removed f'or the purpose of changing the film which is contained in a metal film box quite aimila.r to that of an ordinary cane-r a ., The film used is- -s tandard 4n x 5" camera roll film on which six exposure-a may be made before i ·t 1$ necessary to let air into the vacuum chancer and reload: . Pumps a-re provided with t he oscillograph which are capable of evacuating the chamber to t he working pressure in about fif•teen minutes. The cathode ray trace m~ be observed vi sua.lly thro.ugh a port hole in the side of the film bell on a f.lou.rescent sere.en which is .swung in front of the film. The circu.i t provi,ded for exciting the cathode is shoBn in fig. 3a. The cathode is connected thrt;>ugh a steadying resistance of 60, 000 ohms to a small X-rS3 type transf'or'mer the se-condary voltag-e of which may be varied from ~ kv . max. to. 65 kv. .max. by adjusting a :meosta-t in the primary eircui t . Also included in the ,primary circuit is a synchronous switch which eomple·te.s the 0.ircui t during only one-half cycle in every t wenty eycles . It is intended that the operating switch be closed while the synchronous switch makes one contact a.nd then be opene-d again before the next contact is made . Thus the c athode i .e e;tci ted by a negative half sin.e wave ot potential. The sweep coil is intended to provide a time a.xis for the osci llograms by c ausing the cathode beam to sweep horizontally a.cross the film under t he influence of the varying magnetic field which it set.s up. ~he sweep coil is connected through a ~eaet.a ooe to 'the same a.lte1~na.ting current source that su.pplies the cathode transformer . Since the sweep ~oil circuit is almost p'tlXe ly reae ti ve O the 0uF1~ent in ii; and hence the magnetic field of' the m:ueep eoil la.gs the voltllge by nearly ninety degree s and hence is passing thro:u..gh Z.el1'o when the. eathode voltage is near ite manmwn. For & short period neal' -'i:b.is tin-a the cathode voltage is ,praetioally constant and the ~etic fie:td ooT the sweep eoil is va1:~y1ng th:n::ru,gh ze.:ro at a p ractically uniform rate. so that a working combination @f con~tant cathode voltage e..nd linear time axis is obtained. This system. however, has two diatine't disadvantages which render it entirely unsuitable tot· recording t!--,.e V&euum, switch arc characteristics. First)} the ma.,."d.1uum leng't..ri o!" time during which e. transient ~ be recorded is but a sma.ll fraeUon o:f' a half cycle~. whereas it was desired te re.cord phenomena 000'1 :Ir:ring ove-r one or two eo~lete eycles .. Second, ouly transients ney be recorded which oecu:r du.rina, the pa.rt ef tho cycle that. the voltage is near its crest . This second disadvantage might have ooen ove:eeome by u.sing some kind of a ~ol tage ph.ase Shifting de~iee capable of supplying sufficient current to operate the .osei llograph; but no such deviee was available. Another metlwd would consist in operating the osoillograph from a diff'erent phase of a three-phase source.. This !'!Bthod, however t) does not cover the complete ey-ele and attempts t,(lj ase it ilere without enoourae_,""i ng resu.l·ts . To overooim the above described troubles it was decided to excite the cathode from a steady)} high voltw.se~ diree·t current sow.."Ce. Tb.i s kind. of e~i tation was obtained. by making the eireu.it ehanges illu:st~tated in fi g., ~. The .synclttonotts swi ten was eliminated and the high voltage ·: frnm the transformer .r ectified by :means. of a kenetron» usb1g a. e.02 mfd.,. condenser fe1· smoothing~ IJfu..e swee-p coil :cirGuit was also c:hanged as sh.own in the eeme, f:igLlre. A paj.r of contacts were motu'.lted on the vactrur.'1 switch operating lever· E>lld ar:r-a.l'lfted to close the eireuPG connec,t ing a 24 ·v,. sto~age llattery in series wi t-h the sweep coil and the J?eactance when the vacuu.m switch was epene-d.. '!'he initial part <rf the transient• e.x ponen.tial b'.l:lilding-up eurl'Gnt o.f t~e :react.a.nee provide a a pr.astieai:t.v 'lm1f'o·r m time rate of change·. of eurr.e nt for. tlJe sweep coil. In 01-der t o .meve . the. starti ng position of th~ cathode spot from the e'ente-r 1>0· the le.ft h~d edge of th~ film a small s'teady direet current (bias e1.2Xr;~ i}- W$s 00:nt threugh the m1eep coil from t.he bias current {:li,_r.cu..it .shown i .n the tigu.l\'e .• The a.t Qve de~dbed system gave excellent results -and w:~s used in. ta.king praetieally all the, osai llogl;'ams.. I ts only drawbaek lies in the fa.ct that the li.fe ef' the cathode was very materially short ened. owtng to t:t;le g1~eatly ine:t:eaoe.d time of voltage applie,~tion. This trouble was (1ompletely eompens:e;lred for by designing and construct,.. ~ng a new oath.ode tu.be in whleh the eathode may be readily renewed. Fo.r a. da.~c.r iption of this. tube see the a.ppE?ndi~a 'i~ee, kinda of oaeillograril$ \'/ere taken: cv. rrent-ti~ .. and volta,ge....c,;u:l,"ent.. voltage-time, For the voltage... eurrent oseil1Qgrams some means of preventing the film :r~om being blaekenec;t by the r.epea.te.d retracing of· the voltage and Ct1rr¢nt ax.es by the sine cu.rrent wave before the swi toh was opeaed and the sine voltage wave afterward w.a.S! desired.. This ivas aeoom,9lished by the "eleotron shu.tte-r" who&e cir.e mit 1 s shown in fig. 30. This "shutter" consisted of a coil wound on a cardboard tube which was attached to t he cathode tube opposite the first pinhole as may be seen in fi g . 2. The coil wa~ connected to a o v. storage battery through a double set of cont acts mounted on the operating lever of the vacuum switch. These contacts were so arranged that the shutter circuit was closed when the vacu.U.ll'l sivi tch was in either the open or closed position, but was o_p en during the time the swi teh was in the process of opening. The magnetic field set up by a current flowing in the shutter coil would deflect the cathode r ay as it passed through the first pinho le so that it completely mi ssed t he s econd. and so would not reach the film. 'thus it is easily seen t.hat the c athode r ay iou.ld strike t he film only duri ng the ti me the vacuum swi tch was opening and so trace 01it the arc characteristic without needless retra cing of the voltage and current axes. B. THE VACUUM SWI TCH. The vaeulun switch used was quite similar in construction to one used by F. o. Lindvall in previous work and descri bed in his thesis( 8 ). Briefly, as shown in fig. 4~ it c.ons isted of a glass bulb i nto which two coppe.r rode carrying the current were brought through the medium of copper to glass seals.. One of the rods was ri gidly mounted while the other was connected by a flexi ble c opper bellows in such a way that it could be moved longitudinally from outside the vacuum chamber, causing 10 the contacts mounted on the inside ends of the rods to be pressed together or se.p arated . !ootion was transmi tte.d. to the movable rod by a transvei·se operating lever.. Jf~nu.al operation wa.s accomplished by means of an insulating rod which could be attached to this lever. For automatic operation the vacuum switch wa.s arranged to be pulled open by a steel coil spring attached to the opex·ating lever. The switch was held in the closed position against the tension of the spring by a toggle c.ateh whieh e.ould be tripped by an electric solenoid .. The solenoid was energiz:ed from a direct current source through a synchronous switch wbich could be adj usted to open the vaeu.-uro. swi toh at an::, desired point on the current -la.ve .• Both copper and aluminum contacts were used in the switch., the area of contact in a-aeh c.a.se being 0.03 s q.o in. The switch chamber was evacuated through a liquid air trap by a two stage mercury diffusion pump backed by a rotary oil fore pumpo The degree of va.cuu.m was determined by an ionization gaUgo-e constructed f'rom a type 201 A tung~ten filament radio vacuum tu.be a.n.d sealed into the pwnp tube as shown in fig. 4. Assuming the c-e.lil)ration for this type of gaug-e obtained by Lindval.l {S) to be correct~ the pressure in the switch during the investi gation ranged from 5 to 10 x 10-5 millimeters of mercury. Mo attempt was made to determine the variations in the characteristic2 with residual gas _p ressure. ll O. THE V.AOUUJ! SWITCH POWER CIRCUITS. Oscillograms were taken at three dii'ferent voltages: 15,000, 30-p.OOO, and 2200 0 The 15,000 volt circuit consisted of a 15,000/110-220 10 kva. distribution transformer 0 the high voltage side of which was short-circuited by the vacuum switch. The current was limited on the low voltage side by an iron grid resistor or a water barrel r heostat . This circuit is shown diagramatically by fi g . 5a. As may be seen, the oscillog:raph current deflections were obtained by passing all the current through the oscillograph current coil, while voltage deflect ions were obtained by connecting one pair of the eleotro,atatic defle.etion plates aci'oss a portion .near the grounded end of a 300.000 ohm mu.ltiplie,r resistance conneeted across the vacuum switch terlllinals -e This multipli er resistance consisted of fifteen 20,000 ohm c arborundur~ rods mounted on a wooden board and connected in a zig-zag manner to reduee the i nductane.e . A condenser mult i plier was tried before adopting the resistance but the combination of the condenser with the vacuum switch arc and the inductance of the leads forne d an oscillating circuit the oscillat ions of which completely uias ked the arc charact eri stic. For some work the current coil was placed between the vacuum sv1i tch and ground a s shown in fi g,. 5b. Thi s was done to prevent the eharg.i.ng current of the transforroor capaci tanoe to g:rou..rid and the cu.:rrent taken by the multiplier resist~uce from flowing in the current coilo The 50,000 volt c i rcuit was similar to t he 15~000 volt circuit enept that t wo identical transformers were used with their h igh voltage coile connected in series. This· eircn,.i t is shown i n fi g ., Sc., 12 No attempt was made to vary 11Ghe p:ow1· f'a.ctor of' ei th.er the 15..,000 v. or the 30.,000 v . cireuitfto The power factor a.s me·a.sured on the input side of the t:ra."l.sforme.r was approxirriB.tely 0 .. '10 l:ag f'Ol' the 15 ,000 volt circrui. t and O. 60 lag for the 30,000 vol t ci Nui.t . The 2200 volt circuit consisted or a. 550 kva . indueti on regulator, l5., ooo/o-~oo volt&, connected t hrough the vaeaum. swi tch to a 50 ·kva. 2200/110,,.220 vol t transformer which was loaded on the low voltage Side as shown in fig. 00.. Inductive loads of power t'a.ators rang ing from o..oe to unity could be obtained by applying a load oonsisting of various combinations of rea.cta.nce and resi s tance. Leading :powei· ;factor loads were obtained b :y stepping the vo.ltage up to 7500 volt·s by rooans of a 10 kva 15,000/110-220 volt transformer working at half voltage and oonne·c ting static oondensers,. A change in the position of the g-round from that used in the 15,000 and 30,000 velt circuit& was neaes.sary owing to the f act that it was not possible to remove the ground connection from the regulator .• The magnitude or the currents iaterru.pted varied from 0 .. 4 to 14 ampere$ r . m. s. Larger eu.rr-ents were not used owing to the limitations of the laboratory equipment . It was realized tha·t; these currents we:re far below the maxim-am which the switch was capable of breaking but this was not d<.i8iIS9d to be of great i mportance in this more or less pre:t.iminary work since i't was the phenomena occurring when the are eningu.ishe·S that ,ve· we re intere sted in and this takes place ju.st be:tore the current wave passes through zero. The work may have some a.dtti tional practical interest owing to the f act that these eurrents a.re 13 of t.he order ot magnitude of exe.i ting current s of transforms.rs or ehart.,>i.ng currents to bus structures and short unloaded tr.ansnli ss.i.on lines . QU.eerly enou.gh,11 the interru,ption of these kinds of esurrents has been found, in -the ease of oil eircmit breakers, to eau.se the are to persist for a longer time and to cause worse voltag$ surges than do mu-rents of fu.ll load magni tnde. Be.c au se of this fact they have- been called nstic.ky cu.rrentatt by eircui t breaker design and ta.st rran. 14 RESULTS The following data sheets were prepared mainly for the purpose of determining what conclasions i could be drawn as to the connection between the value of current at which the arc exti.ngu.i shes and the maximum value of the induced voltage surge. together with the variation of these quant ities with contact material, circuit voltage, and power factor. On accou..l'lt of the meager q_uanti t_y of the data and its great variability no very accurate eonclusion.s nw.y be drawn, but at least some interesting trends are indicated. The prefix CU to an. oscillogram number indicates that it was taken with copper contacts in the swi tch 11 while AL indicates that a.lUJllinum conta.ets we.r e used. The 100 series is for oscillograms of voltage as ordinate against time as abscissa.. The 200 series is for current as ordinate against time as abscissa. The 300 series is for voltage as ordinate against curr ent as abscissa. Table 1· shows a variation of the are extinction current for copper contacts 0f :from 0.8 to 3.7 amperes, with the average at 2.5. The rnaximwn value of the v0l-tage transient varies from 19 to sor,.- etb.ing over 48 kv. 0 the maximum of the Hi kv •. sine wave voltage be;t~. 21 kv.. In a. general t:ray i t may be said that low values of arc extinction current are accompanied by low valu,es of transient induced volt a ~ and vice-ve1•a:12,, although several exceptions occur. Table II snows the corresponding values for aluminum contacts. In this case the a rc extinction current is lower, varying fro m 0.6 to 2.0 amperes with the av.er age at l. 2 o l5 The w.aximwR value of the transient induced voltage is also. lo t(!rii varying trom 6 kv~ -to sorllethins greater than 34 kv o Table III shows the values ubtained for copper eontaeta on the 2200 volt a.nd t he 30 kv... ei rcuit s. circuit the arc extinction cm.rrent b It appears that for the 2200 volt higher than for the 15 kv. while the ratio of the maximum value of the transient induced voltage to the maximum value o:f the sine wave voltage is also higher. For the 30 kv. circuit t.11.e are extinction cu.rrent is slightly higher than f' or the J.5 kv . circuit but t.he ratio of transient to sine wave volt~e is lower .. F;i.g'!;,'Wa 6 ia a eurrent...time oscillogram for copper contacts. !t illustrated very well the rapid rate a.t which the current falls off after the are has ex.tingu.ished. A twenty thou.sand cycle timing wave is sh.ovm traeed on the same film for eomparisono Fig-ure 7 is also a current-time oscillogram but for aluminum contacts. The current in this case was only 0. 8 ampere r.m.s . or l .l amperes .max.,. which ie slightl1 below tlle average value at which the arc extinguishes for aluminum contacts . Figure 8 is a vol ta,ge-time osei llogram for almninum contacts. It shows the transient voltage rtld ck" at the ti me the arc extinguishe s and also another voltage th.a t oocured earlier., perhaps at the time the cont~-ts first separat e, indieating a condition of glow discharge existing before the a.re for.tned. oscillogram for copper contacts. Some of the voltage-time osci llograms obtained do not show this earlier voltage. li Figure 9 is a similar • Figures 10 and ll are · osoHlograms :for ·eopper contacts taken with vol t age erdina.tes and cu rrent abscissas. If the nelee-tron shutter" had not been used the · sine wave al ternati.ng current flowing through the vacuum switch before opening wou l d ea.use t he cathode ray spot to i!?etrace it self in a hori zontal line across the film ones every half cyel ~ o • Simila1•ly 11 the sine wav,e voltage aci"oss the- awttch after the ch•cuit has been eo.zr.plet,ely opened would cause · t he cathode ray $pot to retrace a vertica l line bi treeh.ing t h~ C'Ul'rent trace at right angle,s,, While the arc and glow dischar ges are t aking place , the vo-ltage-current c haracteristic f ,o r them wi ll be traced having the above desedbed v.-01 ta.ge Md current lines as axe-s. The "electron Shu.ttern pl'events blaelr..ening of the film by preventing the apot from t~e.cing over axes mo~e than once. • • ~in f i g . 10 the trace be€;:i.ne off the ri.ght hand edge of the fil m befo:i:-e the contacts separate. It t r a1re ls a.cr oss t he •film from right to left breaking i:nto some f'ar.m of glow discharge when the current has de.seended to ab out one ampere . This discharge has a voltage first in one dir-ect ion and then in t he other before the current h as reache.d zero. The glow discharge continues with osei llations while the current goes t hrough z.e r o and inerea.ses i n the opposite direetion, the voltage remai ning in the se.i--oo direction. At some v.a lue of eu;rrent •so high as to be oi'f t he film the discharge ohoo.ge s to an arc, i n which form it may be seen coming back on the film at t he left hand edge . The arc continaes until the current has fallen to · 0.8 ampere at which value the a rc extingttishea and a voltage is induced having a maximum value of 19 kv . 111 Since the power f.aeto.r of the eirouit is o.:10 l .~ the transforme;r voltage at this tJme is in the opposite direction and so the trace retrerses and after making a saw-toothed :figure st.arts tracing the v.ertlee.l a.xis. No e~lruiation of tile saw-toothed figure has been found; it .appe:\iirs. in the same form in all t.he v:oltage... current os-c.illoe,"Truns t a ~ o-n the 15.k*-• ci-reu.i t, :fa:r both eap_per aiild alumimun eontaet2-. 'flhe form of tl;l.e figure was different when the 2200 volt and too 30 kV:- ei. rc'l:li ts -were. aw! tched as ·w,;y b-e seen in fig-u.re-s 12,.. )..3, 14!} an~ l-6. It i.s hard ta conceive of energy being stared b3 any kind of dischal."ge in the vc~uu.m switch , but, if the os-cillograms ~y be tX'U.Ste-d~ this - is w,,.at oecttrs-:; beea~e in figure 10 the p,i;-odu.et of volts and amp.e res changes s .i~1 li'lO les•s then thre~ times,. Figu,re 11 ia similar to figur-e 10· and was taken ut1de-1· the same conditions. It may be analyzed. in the same way that figure 10 has been and. e~ibi ts the same phenomenon oi ap_p arent energy st.or.age although the glow and ar.e dis- charges do not take place in the same way nor at the same values of voltage and ew.L'r-ent. lt was thought for a long time that t.his e:ffeot was due to so.ms eli:tran.eous ea.use and mu.ch off'ort was ex,pendeti in trying to eliminate it. The leads to the osci llograph were made short and se_pa,rated a s far as possible, various kinds of shielding were tried,. the sw.a.ll inducts.nee of the voltage mu.ltiplie1· ei .: r,euit ms variedp capacitance was placed a.cross the swi ten. different current coils were used , different measuring cireui ts sucb. as shown in figu.11e 5a and 5b we.re tried, all without finding any indication that the effect tJ us other than a true one. Interaction between the volta~ and current zreasuring circuits is r uled out by the fact that 18 the current-tim~ and wltage-time o.a dllograrAS mq be combined to form obtained. on the 2200 volt circuit. tu the oseillogl,'am of figure 12 the power 0,\'1 factor wasAleading, in tha:t of figure 13 i t was 0 . 06 lagging , while in that of t' igure 14' 1 t was unity. Figurei 15 was taken 3m. the S'O kv. eircui t o 1Phese all snow the same et'feet of apparent energy storage~ in the 2200 volt oireuit oscillograms particularly the volt.a ge oscillate& on both sid~s of' the zero axis while the eu.rrent was _continuously .o f was eleax-ed nave no signi:f:tcance,. they were c aused by bouneing a.f the Values for the current at hich th-e arc exti ngui shes and for the maximum value of the induced voltage transient obtained by measurement of the oscillogra.ms . Copper Contacts . Circuit Voltage :a 15 kv. rms. or 21 kv . mu. Power FactaZ' OsoUlogram number = 0.• 70 l~. ~tmum of Mtnumm are -current volt• transJent kilovolts atnl;l!rea _ .,.,....,.... OtJ...10.l CU-1O2 OU-103 z. i 00-201 eu.202 i.v QU...20$ $ .• 5 3. 2 ) 29 3. 2 } 27 5. l } 35 2. 6 ) $5 l.8 ). 35 3.• & ) 55 o.a- 19 otJ...30& a.2 $17 CU-309 2. 2 40 2. 5 3-6 l •.o 21 l.4 00 eY...Zl-6 te . 5 >43 00..317 2. 1 ) 46 f9 2. i ~ ti CU-502 Average 2. 5 20 Osei llati0-ns TABLE II Values for the current at which the arc exti nguishes and for thG maxi.nm.m value of the induced voltage transient obtained by measurement 0£ the osoillogams~ Alwninu...m Cont.acts. Osci'l.logr.am nurnbe~ Circuit Voltage ::; 15 kv,. r.ms-. ow 2l kv,o ma.x. Power _F!il.Ctor Q 0 .• 70 l&go :Ml.nimwn a?lc ~ent li!lllP! re.s Remarks Naximwn of volta.~ t r-ansient kil,·ovo-lts AL-lOJ. 26 .U,..10.2 *105 ~ o.s amp. rms. ~ ~',; __ AL,,.-20$ 1. 0 1.4 ff Gi :2.6 tf 9t 4.5 " ff 6.0 ~i ff 120"' O.G 1100 1. 0 10~ Syne.h. A!r209 90° ~"Witch 800 Position 700 JJs,.301 1.2 .2l AL,-302 1.. -6 25 ~305 2-. 0 ) 2'1 A!,,,,SM 1 .• 4 ) $4 AI,.,,.306 l .• 6 15 AI,-307 1 .. 4 13 AI,..$09 2. 1 28 AI,,,,31~ 1 .. fj. 28 Average 1.2 21 TABLE Ill 'ltal:ues for th-e <'.ntrr.-ent at whi ch the ~re is ex.tingu,.i shed and for the ma:ximum value of the induced voltage transient obtained by .fl"ieasu.r e.tuent 01' the. o.sei llogrruna •. it.nimum 0::Jcil.lo~am nwaber - arc ~vrent- Budmw:n of vol tagf;a tr~aient amperes kilovolts 5-. l Ve r.3 Lt:,.rge oscillations l0 .. 4 amp. 17'1,-l Lead cu-~1 8. 0 OU..~28 fl fl io kv. . ~ireu,it {see fig~ tle) 4ia. 5 1.-v . ~ ., . ff, tt 'C'G1lPer Contacts. ~~l 1. 2 ) 48., 0 (fIJ... 332 1. 2 4£;,. 2 tt " CU-Zi3 2~., > &3. 4 ff fl· CU-336 4.4 56. 0 n in eu... i37 2.9 > 5S.3 " u OU---Z~S 3. 8 ) &'l . l tt vt OU-Z3-9 3-. 7 ) f55. 3 9t tr CU...$40 2 .. 5 > M.O u '' GU,..M.J. 5.,1 42. §- " n Averafd,J_ 2. s J'-53 a.taps. 60. ~~ Lag DISCUSSI ON OF RESULTS AND SOO'GESTION:S FOR FURTHER RESEARCH The data given in the tables seems to indicate rather definitely that the transient voltage Slll'8'8S caused by switching with aJ.umiaum contacts are less severe than those caused by switching with copper centacts. Although no catll~de ray oseillogl'.'ams ,have as ye:t been taken wi tk tungsten c:0ntactac._ pre-viou.s BWi. te:hi.ng experience with tungsten indi-cates that the voltage surge.s are much wo.rsa than for eith.er the copper o~ alwninwn. Since the datia given in 'the tables indic ate$ t hat low arc e,:x.tinp:t'ion currents are, in general,, accompanied by low voltage surges.. and sin©e t'he arc eo.nduction is b.elie·v ed to tal~e place in atmosphere of v.apori.ze.d contact metal •i t might be· e:x_pe.ete<t that the arc would persist to lower· cur.rents,. with eon"espomlingly lower voltage surges ii'. the v.ap:orizat ien tempe.rature of t-he co.ntaet metal we,re lower. Aluminum., with a vaporization temperature of 2000° Q. causes less severe surges, than .e'op.per. mich ha.a a vaporization tempe-ratu.re of 22000 o. 4'u.ngsten, with a vaport'z at-ion temperature of 3700° C. is known to cause very severe surges. It seems only reasonable to expeet 'that some metal With a, low vaporitation point such as Magnesium (11000) or Zine ('9000 l should ee.use but very mild voltage su.rgas, perhaps no greater than the, ma.xi.mwn of the circuit voltage. llil.gnesium would have the added advantage of a strong ttgetter" action when arced which wou ld improve t he vacuum. In future oscillogra.phie work these tt10 zootals should surely be investigated as contact materials. 23 A good explanation of the apparent energy storage in the glow discharge has not- yet been found. It is well known that the glow part of the discharge chara-0teri-stic has a negative elop~(,see figure I) and this has the effect of a negative resistance , but to the author's knowledte no one has ever be.fore ebta.ined evidence that the negative slope continues across the axes in the w~ indicated by the oscillogram~ or figures 10, ll, 12, 13, 14, and 15~ If the effect .i s a false one and is due to some e.xtrane.o us ca.use, then further work should result in finding this out. On the other hand, if the effect is a true one• th$n certainly more work should be done to find out more about it. Thi s work should include investigation of the phenomena for higher swi teh currents, and more investigation of the effect of power faetor and higher voltages, besides the use of conta.c.tts made of other metals, as desc r ibed in the last paragraph. REFERENCES l. :a ... Wo Sorensen and H. Eo ~ ndenhall, Journal A.I . E.E. 9 Dec . 1926. 2. K. T. Compton 11 Jou,rnal A.. I oE . E ., Nov. 192?. 3. J o H. Hami.lton 9 Thesis, Cal.. tMt,. of' Teoh . , 1<)28. 4. J . J. Thompson, "Conduction of Electricity '!'h.rough Gases, t1 2nd Ed. p.604 . 5. Stark, Phys. Zeits., 6, 51, 150, 1904.• 6. La.ng:rm.u. r, G.• Ee Review, 26, '136, 1925. ?. E. s. Lae, G. E. Review, 31, 404, 1928. 3. F. C. Lindvalll) f'fhesis 11 Oal. Inst., of Teeh., 1928. A?PENDIX A. NEW DESI ON OF CATHODE TUBE As has been stated earlier in this thesis, {see .AP.i?il.RATUS~ CATHODE RAY OSCI LLOGRAPH) the use of steady high voltage direct current for exciting the cathode» replacing the single half-cycle volt~ _pulse source supp.l ied with the oscillograph·;··emised a marked shorten ing of the life. of t he cathode. Since the list price on :new cathode tubes is two hundred dollars, and getting a new tube also involved the ti.me delay of sending to t he factory, it see.med advisable to devise some means of renewing the cathode more quickly and cheaply. Aging or the c athocle consists in the formation of a thick , black: f'ilm coa ting on i ts polish ed surface~ This c.oa ting starts at the center and slowly spreads toward the edges, sometimes forming rather pretty colored and shaded rings. This coating quite def'ini tely interferes with the electron emission because t he intensity of th.e cathode r ay trace on t he f'loure scent screen fades and finally t he trace dis•p.-pea.rs alt ogether when the coating co vers a cons iderable portion of the surface of the c athode . When voltage is applied t o the c athode a t hin peaeil of i oniz.a tion appears to start from t lle center of its surf ace &id proceed a long the Ui.s of the c athode tube.. Undoubtedly the electronic charge on the walls of t he tube is the cau se -0f the discharge concentrating along the a.xis in t h is manner. This pencil of ionization is probably made up of both electrons and positive ions moving in opposite directions. The electrons form t he cathode ray while the poai ti ve ions strike the c athode surface and knock out .more electrons from it. If the residual gas were pure and contained no foreign matter, such asstopcock grease vapor, little or no damage to the cathode would resu.l t. Atoms of carbon from the stopcock grease,. however, stick to the surface of the catho~ when they strike and eventually build up the black coating. In order to make an old cathode operative again it. i .s necessary to remov~ the accumulated film ceating. This e-anno-t be rea-0.il.y a,c .. c.ompli shed in the original c athode tube because the only opening into the tube : is a , 3/8" hole in whi-oh ·t he brass tube carrying t he t wo pinho les is mounte~. Cutting open t he tube and sealing it together again after the eathode has been cleaned causes irregulari tiea in the glas.s tube wall which intex-fere with the a.ct! on. of the ele~tron charge t hereon in concentrating the discharge . A -new cathode tube \Vas then designed and constructed whi-ch seemed t .o solve t he problem quite satisfactorily. This t ube is shown in figu.r-e 16 together with the old tube for comparison. It is se.e n that the coppe~ to glass se-a,l which j oins the glass tube t0 the metal s:u.ppo-rting t'lange in t-he old tube has been rep laced by a ground joint in the new one. This joint fDa3 be. readily pulled apart allowing the cathode to be removed by a. special tool wh ich is thrust down the t\.1.be. The film coating may be removed by rubbing the eath-0de s~f:ooe on a piece of ve-ry tine eme11y pa.per laid on a sheet of plate glass and dampened with alcohol. The cath<>de ~ · then be repolished by buffing with Vienna Lj.me and Jeweler •a Rouge. The new cathode tube see.med to have only one bad feature; owing to the pl~esence of an-other greased j .o int in the t u.be itself 27 the e·athode bl.ae:kened at a. faster rate than in the case of the original tube.. This feature ea.used no gj"eat amount of trouble since it was a s.imple matte.r t o clean the cathode- every tilll9 air was let in to chru,tge t he fil ~1 1f it was t hought necessary . To earry the i mprove.ment a step farther" houeve-r 11 and eliminate even this trouble, still another cathode tube was designeo. and i t is expected that it will be co-nst:ru..cted i -n the ne a~ futu.re,_. A drawing of this lat:e-st cathode tu.be is appended.. I -t is seen that the g:roUlld joint has been eliminated by re-v erting to a cop-per to glass seal,,_ but one of l a rger diameter th~m wa.s used in the origin.a l ta.be ♦. 'Phe aSflembly consisting of the pia.nole tube marked {2} in tha dratnn~ and its, supporting pl-ug (:3) mey be removed after the f' l ~ eoupling has be~ ta.ken apart. leaving a hole of. su.f'fioi-ent size to allow the cathode to be u.ii--s:erewed and removed by the tool al.so shown in. t he drawing,. Another new fea-t-u:re is the tube rr.arke.d {ll). This tU:b..e ie to be connected through a .needle valv-e to the rough vacuum lin.e. It will be noticed that the pi?:lho lea f'o:rm. the only opening between the cathod~ tube and the r~st of the vacu.llill eP.anber through which air m.33 t"low to equalize the pressure . ! f the v-ac-u.wn in the film bell and deflecting tube is made as .good as. t ha pumps will p·r odu.ee~ the vacunm in the cathode tube ma,y be sof~ened te the pr~per degree by opening tlle needle valv-e slightly call-owing a small amo-u...l'l:t of gas to :flow in under the influ.ence of the slightly hig..lier pressure in the for4:t-vacu.um line. By properly .adjusting the needle valve the in.flow t hroug):l it may be made to just balance the ou-t:flow through t he pinholes, aa-id thus 28 the proper vacuum for the ca.th-ode disc;;harge will be mru..n ta:ined eontiuu.ously. ! f tlµ.s c-a n be accoo,.plished one .of the chief' operating pr·o blems 9 tha:'G of mai-n taining ju.at the proper vac:u.um 9 wi:U be solved. !t is alst> believed that having a harde-r vacuum in the def'leoting tube and the. film bell wi 11 :ueault i ,n sbarpe-:t: oscillo,graph traces since less co:llisions will occur between the el~ct.r one in the cathode ray a.fl<i the aiz> molecules i-n the '11aeuum eh~e.r o •• I D B _-+'-T_ C A } L.._-------~=~C~oro~>'>~o======------ 1/o//oge GENERAL GAS DISCHA,RGE CHARACTERISTIC i'igu.re 2. ffiE CATHODE RAY OSCILLOGlUPJl. i 05CILL OG2APH Cl2CU!T5 0 b "ELCCTRON SHUTTER" CIRCUIT C Plgure ft. 'l'HE NJW li4CUUM 5W/TCH Cll?CUIT-5. a.c Osc .,.,. Il i ij ~ Il C - ______c:::::.;--, 1.5 kv ~:::::.-=~ L. ____ .J Oil Circui 'l- ~ ~ ~ C\J C\i v" ~ {;} ttxxx;J ~ - 8rtx7lrer d Figure Pigare 'I. Figure ,. iigo.re 10. 12. 16. ftgu:re 16.. mE OATHODE 'l'tJRES.