[1]United States Patent Office P a ' c e r - t e d J u n e 2 , 1 9 6 4 3,135,677 DU,@?-AA,P@LE AL-@ODE P3-ZOTEi "T.-iVE SYSTEINI -Tla-.Ty OZ@. @r, is-pi-aer, La3@e Valbala, 2ssi,-nor to ThermoCraft. Elecine Coa-poralion, Ne-tv @xork, N.Y., a corporation of New York- 5 Filed Feb. 2, 1961,,9er. No. 86,648 3 Claims. (C]. 204-196) This invention relates to an impressed current cahodic protection system for hot water tank hea'ters and :s par10 ticularly adapted for use with dielectric lined hot water ta-ilks. By irppressed current cathodic protect@,on is mean', the use of a unidirectional current to prevent corrosion of a metal in an electrolyte by makin.- such riaetal th.- cathode. 15 In recent years the use of glass-lined hot wal,er tanl-.s has b,-come prevalent. "le theoretica'llly a coating of or o,her protective d:electric material, upon the i.iterior surface of such tank-s viill prevent the co-vered jr,etal surface from corrosion, in pract-ice th:ls is not com- 20 pletely accomplished. Minute spo@'s of metal are a-Incrally left uncovered and these bare areas, called "holidays," corrode, causing the tank to de@eriorate and eventualiy leak, even thou-.h the glass-covered port:@on of the tank is perfee,. 25 Sacrificial anodes, such as those of magnes;u-rn, have been -,videly used and gen@-rally perform satisiactorily in pro@ecting the bare metal areas. A number of difficulties arise with the us(-, of such anodes. First, there is the replacement problem. In some walers ma,@n@-sium cor- 30 rodes very rapidly, whl]e in others it will not corrode at all and th-as wiH not give protection. Furth-.rmore the presence of sulphur in th-- water Nvill result i_n a bad tas*e and a disagreeable odor. Local cells caused by iinpuri'ties in the anode tend to consume it and 'thus shorten 35 its protec"ive life. Corrosion products of the a-,lode fall to the bottom of the tank and may cause poppin,@ no.@'ses in .-as hot Nvater heaters. When a magncsiun anode is consumed, the homeonvner is generally not aware of it 40 and, since he does not replace it, all protection is lost. With glass-Ened tanks there is, at most, a fenv square inches of bare metal that requires protection. With this sr-iall area, an @Mprf-,ssed-corre-@at cathod@@'c system is econon-iical whe@i conapared with a sacrificial anode. d5 An object of the ;-nvention is to provide for econon-iical - protection of glass-lined hot water heaters over a long period oi' t'Ime. Another object of my .nvent-ion is to provide a - s@,--tem of cathodic protection in which th-- impressed current is 50 virtually discontinued, as during non-use of the ho--- water tanl,, upon acctl=,ilation of electrolyt-ic gases in the top portion of said tank. A further object of my inven'Llon is to provide a control system for the impressed pr6Lective current N@th:ch 55 passes to a hot water tank, to thereby stop such clrrent, if gases accumulate in the top portion of the tank- to a predetermined extent, and reinstate such current wli,-n ',he tank is used sufficie-ntly to remo-,ie the accumulated @ases. A still further obj,-ct of my iriveition is to provide a 60 hot water heater, the tank of which is protected by two electrodes dep,-nding thereinto, one of which is des'@robly 9 rmed of ti-aniurp and normally maintained as an &,iode i.0 with respect to the tank wall as a cathode, and the o'@her one of Nvhich is desirably forried of pla 'cinum-plated- 65 t-'ttanium and connected so as to substantially @lau-L off the action of the anodic el,-ctrode when gas accumula'cs at the top o'L the tank- to a predetermined extent. These and other obj.-cts and advanta,-es wi'll become and do not defm-- the scope or lim:its of the inv.-nt@@on, reference bein.- had for the latter purpose to ihi-, appended In th-- drawings, wherein I.;ke reference characters denote like parts in the several views: FIGURE I is a dia.arammat@'@C represcri@atio.,i of a hot water tank with a control circuit embodying iny in-,,ention, the m-.ans for heatin@ the water in the ta-@ik not beir-a showr., FIGURE 2 is an enlarged axial sectional view of cor.,trol electrodes embodying my in-,,ention, FIGURE 3 is a chart sho:win.- tio current volta.@c reliationship of titanium el.-ctrodes v-@,'acn in cold ivater aiid when in hot water of various ternperatures, in com,:)ariso-n with that of a titaniu-m elecfrode plated with platinum, FIGURE 4 is a wirin.- d-@'agrani, supplementing FIGURES I and 2, sho@vin- details of oiie e-Tnbodiment of th.- control circuit of said FIGURE 1, FIGURE 5 is a wir@-@ig diagram, also supplementin.FIGUP.E-S I and 2, showing details of another embodiment of said control circuit, FIGURE 6 is a wirin.- diagram, further supp7,,-menti-,lg '- I@IGU.RF-S 1 atid 2, sho-,vng details of a further embodiment of sa,d cor@trol circuit, FIGURE 7 -IS a fragmentary portion of another wirin.@ diagram, to supplement FIGU.IZES I and 2 and show aq additional embodiment of the control devic,-, nd FI-GURE 8 is a wirin,- dia,-ram addi@ionallv supi)lementin-, F.IGURES I and 2, sho@vir.-. details ol@ a further embodiment of the co-@itrol. circuit. The c-arre:it limitine-I rectifying, volta,--, regulatin.-, and co-ntrol functions do not necessarily have to be in the ord-er shov,,n in FIG. 1. R.-ferring to the drawin.-S in detail and first considering the embodime@it of my invention as illustrated i--i FIGURE 1, there is sLio@v.,i, as in vertical, sec-ion, the ta-iik 11 of the hot weer hea'Ler. The n-ieans for the water in the tank is not illustrated but the he;a@c@@n,@ may be by electricity, gas or other des-ired rnea-ns. The tank 11 is assumed to be -lass-'lined, to have a cold water inle'Lpipe 12, and a hot water outl--t p:Pe 13. ExLendin.into the tank a rela@Liv--Iy great distance frorq above and insulated therefrom is a ma@@l durable proLecti-@,o anodic electrode 14, desirably formed of titan:lum. An au-.uliary durable control electrod,- 15, desirably formed of platinum-plaled titanium, also exterids from above and projec@s to -@iornally engage the 'cank-cont-,iined water, but on'ly a relatively small d-.'s+aice into the taik and is insulated therefrom and from the main electrode 14. It was originally thou.-ht that it would be necessary to platinumplate or use platinum facin.- on titaiiium anodes, if used, to nake ther-ri operate satisfactorily, becausa titanium is one of th-- socalled "valve" metals. Such metals have the character-stic that when they are used as anodes in water they build up on their surfaces very tenarioiis dielectric o-ade coatin,-S, which preven'Lpassage of current from an anodic surface to a cathode, but do not prevent the passage of current in the opposite direction. Tne characteristic ol' such metals is that af'ter a critical poiential is reached, in the case of tantalu.1-1 about 70 volts, the oxide film breaks down and rapid corrosion results. In the case of the preferred titanium anode, this breakdown potential is around 10 volts. Referring now to FIGURE 3, there is shown a curve 16 indicatin.- that as the voltage on a tilan@.um anode iri water increases, the current there.Lroin to a cathode increases until a crit,.Cal volta,@e of about 10 is applied and the oxide coatin,-, formed on the t@.taniuri metal surface, breaics dowii. In this case, the breakdown occurs in cold e 0 e cuuenl flow is beappaxent from the following detailed description when 7( wat r with a drop -in v Ita.ae ivhen th taken with the accompa,@iying drawinc.,s. It Avill be under- tween 25 and 30 milliamps/sci. in., and rapid corrosion stood that the drawings are for pur@poses of illuslratio.1 restilts at the consid-,rably lower voltage indicated. The
[2]3 curves 17 and 18 in FTGURE 3 rp-present the results with a titaiiium anode, as in curve 16, except that instead of being iminersed in col@d water, it is inimersed in hot water of different temperatures. For exan-lple, the curve 17 represents the results with a titaniura anode in water at a temperature of about 140', while the curve IS represents the res,,ilts with a titanium anode in water of about 160'. Curve 19 represents the results with a ti,anium anode which is clad or coated with platinum, as by plating. As represented by the dotted continuation of curve 17, the brea@kdown is at the same voltage but at a considerably higher current than when the titanium anode is in cold water, as in curve 16. It will, therefore, be seen that the protective surface film on titanium has peculiar properties. It is resistant to the passage of current into an electrolyte, but has low resistivity Nvhen a second mel'al, such as platinum, makes contact with it. If an atte@@t is made to use titanium as an anode, this resistance to the passage of current into the electrolyte immediately becomes apparent. As shonvu in FIGURE 3, as an increasing anodic current is appled, the vol-ta-,C, to force the current through the surface film, rises until at the Nveakest point the film breaks down and intense local corrosion occurs. If, however, platinum is d@'.Sposed on the surface of the titanium, most of the current passes out throu.-h the platinum at a considerably lower voltage and the titanium is uncorroded. I am making use of the peculiar properties of titanium in a preferred embodim,-nt of the invention to be disclosed. In the embodiment illustrated in FIGURE 2, the main anode 14 is desirably cold-drawn titanium wire. This is passed through an inulaiing sleeve 21, desirabl-y formed of Teflon. Said sleeve serves to insulate the anode 14 from a mounting plu.- 22 desirably carrying threads 23 for conneeling to, and closing a correspopdingly threaded opeilin,@ 24 in, the top portion of the tank 11. The Teflon sleeve 21 also serves to prevent "short patlf' current from flowingfromtheanodel4tothewallofthetankll. The attempt in this case is to create an approxirnately equidistant path from the anode to the walls of the tank. For this purpose, the connection of the plug 22, althou.-h nWL so illustrated in FIGURE 1, is d@-sirabl-y on the axis of the tank ILI, if the tank is cylindrical, with a ver-tical axis, and the anode 14 desirably terminates a distance from the bottom of the tank corresponding @tpproximately with the tank radius. The connection with the plug 22 is made watertight by employing packing 25, desirably of teflon, which is compressible by a nut 26 threaded into the plug 22 and apertured to receive the anode 14, the sleelie 21, and another sleeve 27. This Wher s,@veve 27 surrounds the auxiliar-y electrode 15, or the extension therefrom to the control device. This auxiliary electrode is desirably platinumpla,Led titanium wire w'@iieh is wound around the Teflon sleeve 21 and insulated from the mounting plug 22 by the sleeve 27, as well as from the compression nut 26. T'he nut 26 serves to cornpress and make leaktight, not only the anode 14 but also the auxiliary electrode or "trigger wire" 15. The purpose of the trigger wire 15 is to sense the presence of iindesirable gases, indicated at 28 in FIGURE 1, in the top portion of the tank 11. Such gases would be normally due to the electrolytic process and thus consist of hydrogen and oxygen. If they accumulate to a sufficient extent, the auxiliary electrode 15 will not touch 'the waler and thus cannot conduct any current. The fact that this happens makes it possible to control the current to the anode 14 so that only a very minimum amount of gas can ever accumulate in a hot water heater, even when idle. During the normal operation of a water heater, all of the electrolytic gases produced, if any, are dissolved in the water flowing through the tank. it is only during periods of extended idleness that the water then becomes saturated and the electrolytic gases separate at the tOP 3,135,677 4 portion of the tank. It is during these periods of idleness, such as vacation periods, that it is desired to shut off the anode current. When the gases accumulate, this action is effected by means of the trigger or auxiliary electrode 15. Shut-off is accomplished, in accordance with my invention, through suitable relay-type action to be disclosed. Where the current used for protection is in excess of that permitted, in accordance with FIGURE 3, or where the voltage must be higher than 10 volts, platinum 10 platin-, or platinum cladding of a portion of the anode will reduce the voltage required to supply the necessary current. Referring now again to FIGURE 1, there is ihustrated a - generic form of a ponver supply which includes current15 limiting means 29 between the anode 14 and p e 3 o a source of al' ernatinl-, current, such as conventional house lighting power. The other pole 31 is grounded, as indicated at 32, and electrically conneeted to the tank 11 by means of lead 33. Current from pole 30, after the cur20 rent-limiting device 29, passes through a rectifyin- device 34 and then may pass through a voltage regulating device 35 havi-.ig an electrical connection 36 to the lead 33. A connection is then made through a control d@-vice 37 which is, in turn, connected to both the main anode 14, 25 by means of a lead 38, and the auxijiary or trigger electrode 15, by means of lead 39. The function of the control d,-vice is to discontinue the passage of current to the anode 14 when the trigger electrode 15 is uncovered, or at least discontinue the passage of such current except to 30 a very minute extent. It will be understood that the electrodes 14 and 15 are desirably axial with respect to one another ard the tank, as viewed in FIGURE 2, rather than side by side as viewed in FIGURE 1. 1 have shown five wiring diagrar-is employing the same 35 basic functions to accomplish the same purpose, these wirin.a diagrams being respectively ill@as@Lrated in FIGURES 41 5, 6, 7 and 8. In each case the connection to the anode 14 is such as to supply positive current thereto until such current is blocked by actio-Ti of tlle auxiliary 40 electrode 15. In accordance with FIGURE 4, as supplementing the showing of FIGURES 1 and 2, t-he current-liniitin.a device 29 is a resistor 41 which may have a value of about ' 6200 ohms. The reelifyin-, device 34 is a diode 42 which A5 may be one cell only, and the voltage regulatin- device 35 is a group of desirably selenium cells 43 or a snicon carbide varistor performing the same function. Such a re,-Ulating device prevents a hi,@h voltage build-up by passing more current at a hi-,her voltage to ground, thus -,tabiliz50 ing the voltage being suppl;ed to the control device 37 Nvhich, in this case, is PNP type transistor 44. Said transistor 44 will conduct current to the anode 14 from is collector 45 when the trigger or aiixiliary electrode 15 is connected to its base 46, as through lead 39 and resistor r,5 47, which may have a value of about 2500 ohms. The current flowing through the resistor 47 to the trigger electrode 15 will desirably be in the n-,igbborhood of 200 microamperes, whereas t:he current flowing to the main anode 14 will desirably be in the neighborhood of 5 to 60 10 milliamperes. A resistor 48, which may have a value of about 3000 ohms, that is, greater than that of the resistor 47, is used to reduce the current flow to the, anode 14 when the trigger electrode 15 is uncovered by the accumulation of gas 28. 615 The general concept behind th.- present embodiment, is that a regulated source of unidirectional current is applied to the PNP transistor 44. If there is not enough gas in the tank to cause an uncoverina of the auxiliary elec70 trode 15, the transistor will be biased toward conduction and power will be fed to the anode 14, thereby providing cathodic protection for the tank 11. If, on the other hand, gas accumulates to such an extent that the auxiliary electrode 15 breaks contael with the water in the tank, the 75 transistor 44 will block and prevent an effective current
[3]from flowing to the tank througli the anode 14 and thus no more -as will be accumulated. During normal operation of t'ne embod;m@-nt of FIGURES 1, 2 and 4, current is flowin.- from the pole 3-0 of the source of power, through resistor 41, diode 42, transistor 44 and lead 38 to the anode 14, and th,,is to the water in the tank 11, givin.- cathod-.'c protectio@i to the bare areas of th.- tank sur-L'ace. By mear@s ol' resistor 41 the c=ent flov-i is limited and Llic cifective voi'ta,@e is dropped from that of the conventionai s-Linplv say abolil 110 volts, to approx;mately 4 or 5 volts. This act;on is effectine because the resistor 41 is h;gh enou.-h for that purpose, say about 6200 ohirs. At the rectifyindiode 42 the c=ent that is allowed to go to the trans:lstor 44 is in the form of a halfwave unidireelional elrrent, the plus component beina fed to said transistor. The ne.-ative coirponent oi- the current is i)revented from causing a high inverse voltage, as by a rec,ifyir.,g diode 49 wh;ch may a@'so be one c-@ll and connec,ed '@o ground 32, as illustra@L,-d, and puls the ne,@ative component of the alternatin@ current baek on the line behind the diode 42. In this way it is possible to use sin@je cell selenium diode reelifiers in plac.- of multi--i)late rectifiers. It is thus also possible to regulate the voita,@e that the transistor 44 sees to a value below its breakdo-,vn poirt. The element 43 re,-ulates the fonvard volta--c on the transistor by acting like a relief valve. If it comdrises selenium cells in series, ear@h adds .4 volt to the relief valve pressure so that any time the volta.ac on the line be'Lwe,-n the dio-.1e 42 and the trans-@'s' or 44 exceeds a val@de of say 5 volts, all of the current will flow to ground throu.-h the varistor 43 of selenium ceils. This is especia-Ily impor,ant v;hen the transistor is in a blocking s,ate du-- to the accumulation oi' gas in the tank. If the element 43 were not there aand the transistor in a blockin.- state, the volta,ac at the transistor 44 would rise to a point where it would break dov@,n and conduct in a forward direction due to such h,gh volta,-e. Tl,,e transistor is nomiaby conducting whe-ii the tank is -'Lull of water because its base 46 is maintained at a minlis potential wi-h respect to its emitter 51, by a proo,-r choice of values for the resistors 47 and 43. Thus nost of the current will pass throu,ah the transistor 44 to the ar@od,-@ 14 and on to the tank. Only a srnall portion of the current, that is, approxi.-nately 5% of that passin@ to the anode 14, is conducted throuah resistors 47 and 48 to the tri.ager electrode 15. If enoa.-h aas accumulates in the top cK,' the tank to uncover the auxiliary clec-Lrode 15, the base 46 of the transistor 44 is no Ion.-Or maintained negative Nvith respect to the emitter since tlier-- is Llien no way of bleeding off the positive charge which com.-s thereto throug,:l th-- resistor 48. NVhen the base 46 is -oosit-1vely char.-ed due to the open circu-@t, th.- trans-istor 44 goes .nto a blockin- state and no effective current goes to the anode 14. Thus for aR practical purposes the anode IA, is then shut off and no more gas can accumulate so that the syslem remains in a safe cond-ition. After this blockin.@ condi-,ion occurs, assume that the water heater is agai-n put into us-.. I-r,-sh water ideed to the tank, the old water bein@ removed, dissolve the gases that have accumulated at the top c@t' the tank a.,-d which shut off the anode 14. The auxiliary aiode 15 is then a.aain in electrical conn,-ctio@i with the water inside of the tank. This connection allows the positive ch-arge on the base 46 of the transistor 44 to be bled o@'f to the ne.aative tank 11 to such an extent that the bas-. becoires negative with respect to the emitt-,r and tiie transistor will a.-ain function. Should gas re-accumulate aid op-,n tLe circuit bet@veen the trig,@er electrode 15 and the tank wall the transistor 44 Nvill a@ain go into a blocking state an@ shut off the power to the anode 14. T'his ability to repeat the performance and turn on ard off, when th.- trig,aer electrode 15 is covered or w-icovered by water, is an irnportant feaure of tb-epr-,sent -@'nvention. The presence of the veitage rp-gulator 43 ma!,e-s it possible o.Tily to use a less expens--,'ve trans;stor 4,1 than would be n-cessary ;1' such were not used. The diode 42 is t@e essential part ol'L the rectify@'@ng dev;ce 34, as the diode 49 could be cl;minated if the transistor 44 were of such construction that it could stand hi.-h poak voltages. However, to use an tran8is@or, the rectifier 49 musl be used in a circuit srch as ilILlstrated iri FIGURE 4. The resistor 4.1 is here esscntial in any evert 10 ' o reduce the value of the current fionving to th-e arode to a predeterrnin,-d, relativ,-Iy srqall, a-triount that cain be tolerated under conmercial conditions. Referrin- row to the embodirnent of my invention illustrated in FIGURE 5 as suppler@ienting the showing of 15 FIGURES I and 2, ,he current limiting device 29a is here a condenser 52 having a capacitarce of approximately .2 to .4 microfarad, if the sipply voltage is that normally used, say around 110 vol'ts A.C. The rectifyirg device 34a is here a diode 42a which may be like the 20 d-.0de 42. n. e voltage regtilating device 35a in this ei-nbodimeti@L does not have an iniportant function but helps in the control a-,id may be represelited by a Zener diode 53, which will serve to lirnit the applied voltage to a predetermined value. 2@@ The con' rol devce 37a, in this case consists of a thermistor @4 and a resistor 55. The thermistor 54 Nvhen cold Nvill have a resistance oi' approximal,ely I megohm. VAen the themiistor is heated to approximately 150' C. its resist,,)nce wi-11 drop to approximately 3000 oinins, thus 30 a]-lowin.- aii efi'ective amo-ant orL etirrent to travel to the anode 14 along lead 33. in order to get the thermistor 54 healed to this high 'cemperature, I here connect the resistor 55 therlnally to the thp-rmistor 44, and electrically to the atixijiary electrode 15 by lead 39. The passage o.' 35 a normal current through the resistor 55 will heat the thermistor 54 to a point Nvhere it will cond,,ict the necessary aniount of current to the anode 14. When the tri,@ ger electrode 15 is uncovered, the curre-tit to the resistor 55 is open eirejited, and cooling o@.L' said resistor takes 40 place, allowing the thermistor 54 to also cool and return to its blocking state. E,@cept as speciiically disclosed, the pres-,nt enibodin-ient may correspond with the first embodiment as indicated by the use of corresponding reference characters. 45 Referring now to the embod@meit of my invention illustrated in FIGTJRE 6 as sapplemen@@g the showing of FIGURES 1 and 2, a different method of current control is disclosed. This inethod is better s-aited for high current requirement systems and uses, as a voltage regul@el50 ing device 35b, a stepdown transforrier 56 to create current at a low volta.-C, say from 5 to 10 volts, which is rectified by rect-ifying device 34b in the form of a diode 42b, -,vh:ch may be like the diode 42. The current may be limited by passina throuch a current Iiiniting device 5,5 29b in the j'corm of a resistor 41b. The control device 37b in this cnse is a PNP transistor 44- b whicn operates in coiin.-ction with r.-sistors 47b and 49b to supply current through lead 38 to the anode 14 and through lead 39 to the ail-xiliary electrode 15, just like the transistor 44 (;o si,-pplies such current in the embodiment of J--IGURES 1, 2 and 4. Except as specifically disclosed, the present enbodiment may correspond with that of FIGURES 1, 2 and 4. Referring now to the embodiment of my invention 65 illi-,s,rated in FIGURE 7, as si-lppler.,ienting the showing of FIGU-RES 1 a@-id 2, there is shown a modification 37c crj' the control device shown at 37, 37a and 37b in FIGUPES 4, 5 and 6, respectively. It may b-- connected to the electrodes 1-4, and 15 by leads 3$c and 39c, respec70 tively, apd to the pole 30 of the source of power through the auxhiaries as with any one of the control devices of the previous e-rnbodimenlls. However, if substiluted for the coitrol d.-vice 37a, a res,'@stor 41c, as weH as a diode 42c, in series with its switch 5,Ac is employed as illustrated. 75 The coritrol device 37c in this insllance consists of bi-
[4]7 metallic switch 54c, consisting of a movable bimetallic contact 57 which, when the temperature is increased above normal io a predetermined extent, say to 150' C., moves to en-a.-e a fixed contact 58 and close a eirc-ait through lead 38c. Thus this switch 54c and a resistor of a value comparable Nvith that of the thermistor 54 of FIGURE 5 when heated, may take the place of said thermistor. In order to get the switch 54c heated to the temperature required to cause it to close, I here employ a resistor 55c connected thermally thereto and electrically to the au,-,iliary electrode 15 by lead 39c. The passage of a normal current through the resistor 55c will heat the switch 54c to a point where it will close and conduct the necessary amount of current to the anode 14. When the trigger electrode 15 is uncovered, the current to the resistor 55c is open eircuited, and cooling of said resistor takes place, allowing the switch 54c to open. Except as, specifically disclosed, the present embodiment may corres@oond with the first embodiment as indicated by the use of correspondirg reference characters. Referring now to the embodiment of my invention illustrated in FIGURE 8, as supplementing the showing of FIG'LJRES I and 2, a different method of or wiring for current control -is here disclosed ' This method uses a step down transformer 56d to provide a voltaa,e, say 5 to 10 volts, as in the method of FIGURE 6, which is rectified by device 34d in the form of a pair of diodes 42d. There is a lead 57 connerting the leads froni the diodes 42d on the other side of the secondary winding and grounded by connection to the return lead 33. The unidirectional current output, to the main electrode 14 through lead 38, may be limited by passing from a center tap on the secondary winding of the transformer 56d through a device 29d in the form of a resistor 41d. The si,-nificant thing abotit the present method is that t.he energy to operate the transformer 56d is obtained through the auxiliary electrode or trig-,er anode 15 by connection of said transformer primary winding 58 thereto through lead 39. When the trigger anode is uncovered, the entire transformer is deener,-ized or open-eircuited. This is because the normal circuit for energizing said transformer is through lead 30, priinary windin.- 58, lead 39, trigger anode 15, the water in the tank 11, and the grounded retiirn lead 33. In other words, the trans-. former priinary Nvinding 58 when operating, is always in series with the water in the tank, as contrasted with the form of FIGURE 6. The secondary winding 59 6f the transformer 56d is center-tapped to form a full-wave rectifier device using a sin@le selenium cell rectifier 42d on each side of said center tap. In this case, the only additional piece of apparatus required is the resistor 41d to limit the current,thereby making the systeni much simpler, more economical and more foolproof than the other forms. Except when specifically disclosed, the present embodiment may correspond with the FIGURES 1, 2 and 4. 3,135,677 Having now described the invention in detail in accordance with the requirements of the Patent Statutes, those, skffled in this art will have no difficulty in making changes and modifications in the individual parts or their relative assembly in order to meet specific requirements or conditions. Such changes and modifications may be made without departing from the scope and spirit of the invention, as set forth in the fouowing claims. I
