US2629837A - Radioactive resistor - Google Patents

Description

Feb. 24, 1953 J. M. BENADE ET AL 2,629,837

RADIOACTIVE RESISTOR Filed June 27, 1945 2 SHEETS-SHEET l Kalf; i M Zfs Patented Feb. 24, 1953 UNITED STATES RADIOACTIVE RESISTOR Application June 27, 1945, Serial No. 601,805

7 Claims.

The present invention relates to radioactive apparatus, and particularly to radioactive devices suitable for use as electrical resistances of large ohmic value.

It is well-known, that if a potential gradient be maintained between a pair of spaced-apart electrodes which are submerged in a uniformly ionized gas, a current will ow across the electrodes, and that this current will be almost Aexactly proportional to the potential gradient maintained between the electrodes. The straight line voltage-current characteristic of such devices pertains until a maximum currentl value is reached, this value being determined by the physical dimensions of the device and by the degree of ionization provided in the gaseous medium separating the electrodes. When this maximum value is reached, the device is said to be saturated, and it exhibits the characteristic that current ow therethrough continues at a level value which is not increased by any further increase in voltage short of that which will eiect complete insulation breakdown across the electrodes.

Since radioactive materials, and especially those of long half lives, constitute a most conven- ,ient meansfor obtaining constant ionization, de-

vices of the above described type are readily constructed by supporting a pair of insulated electrodes in a vessel containing an ionizable gas and a quantity of suitable radioactive material. Since radioactivity-and hence the ionization of the gas contained in such a vesselis substantially independent of temperature, and since the resistance characteristics of the device are determined by its physical dimensions, which are fixed and substantially independent of temperature, and by the ionization resulting from the radioactivity, which is likewise substantially xed and independent of temperature, it becomes apparent that such a device may be used as an electrical ,resistor which is substantially fixed in value `regardless of its temperature. That is, the device has a zero temperature coefficient of resistance. This is a most important characteristic, because the obtaining of an electrical resistance with a zero temperature coefficient is extremely diicult `to accomplish by any other known means.

`:Elow in a conductor connected across the elec- OFFICE -trodes Self-generated or self-induced voltages of the order of from about 1/2 to 1% volts are commonly observed, and since the entire voltage drop across the resistor is usually of the order of 6 to 8 volts or less, it is evident that a self-generated or self-induced internal voltage of the magnitude stated is a most serious matter. The present invention is concerned with the overcoming of this diculty, and has for its principal object the provision of an improved novel radioactive type resistor which shall not exhibit any self-induced or self-generated voltage when no external source of potential is applied tothe electrodes thereof.

The features of certain embodiments of the invention described hereinafter are illustrated in the accompanying drawings. In the drawings:

Fig. 1 is a longitudinal sectional view partly in elevation illustrating a preferred embodiment of the invention;

Fig. 2 is a plan view of the device shown in Fig. l;

Fig. 3 is a sectional view taken on the line 3 3 of Fig. 1;

and

Figs. 8 and 9 are graphic representations of the current-voltage characteristics of devices of the general type illustrated in the other figures of the drawings.

The particular embodiment of the invention illustrated in Fig. l includes a cylindrical outer casing or closure II of metal having an integral end plate I3 having a centrally disposed, circular opening I5 provided therein for admitting an electrode support rod Il. The electrode support rod I1 is held in position centrally of, and coaxially with, the outer closure I I by a suitable flanged insulator I9, preferably of glass, which, in turn, is supported upon the upper end plate I3 by an annular clamp ring 2l riveted or otherwise attached to the end plate I3. The insulator I9 is suitably sealed gas tight to the rod Il. A gasket 23 placed between the under surface of insulator I9 and the adjacent upper surface of the end plate I3 provides a gas tight seal at the upper end of the closure I i. The other end of the closure I l is provided with a screw thread portion 25 which is engaged by a suitable cap member 21, as illustrated. A gasket 29 assures the maintaining of a A thin, tubular member 33, of radioactive mate#v rial, which is conveniently made by bending into cylindrical shape a sheet of uranium metal having a thickness of the order of .005to .010 inch;iszz:;

disposed inside the main closure Il adjacent the inner wall thereof.

This kv*tubular member `33i c being of radioactive material,` constitutes the means utilized for providing constant ionization"- Within the closure during thegboperationlof .the` r.

device, and it is also utilized as the other electrode. The member 33 is held in positionA in`-the-- Yclosure Il by means of suitably spaced members 351Whichmay 'be ofmetalorf or insulating'matefrial and which are locatedat bothends of'y the closurei I, andfit iselcctricallyconnected-nto the outergclosin'e Whichsthusservesw-as one4 ofthe terminalsl ofthe device'.

The main outer closurell is preferably made of-metaL- brass being particularly suitable; `this permitsthe closurer to' be vused as one of the ter-'- min'als.VVV The closure' il may; however, be made of'anY insulatingY materialfin `which instance some other*me'ans'fmustfbe'provided lfor completing: .the electrical circuit tothe\outerrelectrode.Y If the radioactivef-means is not oficonducting material, as in the device illustrated in Fig, a 'separate velectrode"-ofconducting material must'be provid thev gas pressure'maintained WithinV the cell, yand the physical dimensions of the vario-us parts of thedevi-ce. Infgeneralglhowever, the resistance offV acellV constructed as. illustrated in Figs. 1-3 and'y containing argon,V att vatmospheric pressure under standard temperature conditions (760' mm.

of.' mercury pressure at 20 C'.) will beV of anorder of magnitudewhich is substantially indicated'by tlieequation Where? A .is theeiective surface areazof :the electrodes-'in square centimeters', and where the spacingiof the-electrodesisgreater than the range of the principal ionizing particlesl emitted lfrom .the

R ohms ionizing. material, these-being alpha particles-in thecaseof uranium. For example, a cellrwherein the outer electrode has an internal diameter .of .about 5 cm., a length of about 7.50m., and

wherein the central collecting electrode isabout 4 in diameter and about 5 cm. long, and which` contains argon at v'760' mm. pressure (at 20, C.) Willf havea resistance of the order of v1012'ohms.

lIf'a cell ofthe general type illustrated'in Fig. vl is'constructed with a uranium outerelectrode 33 and a central electrode 3| of carbon or platinum, the central electrode 3l will be found to acquire a small positive potential due to emanations from the radioactive material, and if the two electrodes are connected by an external conductor, a current"will be observed to :ziiotv insuchfconductor evenewhenno externalyoltage ist applied to the electrodes. Suitable instruments will show that tlie device exhibits a self-generated o1' selfinducedvoltage, similar in character to contact voltage effect, of the order of about .3 volt. The application of external voltage to such a cell will, .'ucwe'v'er,Y yield a Astraight line current-voltage curveas-illustrated by the dotted line numbered i'in 'Fig 3.' Ifthe cell is constructed with the centralv electrode 3 i 'of aluminum, a negative, selfgenerated'forzzselffin-duced voltage will appear at the central electrode, and a current voltage-curve, suchas is illustrated by the lower dotted line 31 in Fig, 8, will 4be observed. vIn such instances the observednegativevoltagef may Abe as large as- 1125 voltsil The saturationvoltage cfa cell of 1the type and dimensions? stated is of the order: offv 6 'to' 8 volts, and when voltages above thersaturation value-are applied tothe electrode', the current through the device flattens oi and becomes stable at a constant value andremains so until 'break-v downfvoita'ge is reached. Thi'siiattening off'f'of thecurrent iiow `through the cellat saturationis illu'stratedby the'currentsvolta'ge curveg in Fig. 9.-?. Sincesaturaticnpf` the cell 'may `occurata voltage rwhich isonl-y a small multipleof the selfigenerated or 'self-in'ducedf voltage', that volta-gels Very undesirable especially vwhen the` resistortis to'be usedfasv is 1often' the case-g in the-grid' circuit of an electronic amplifier..

As previously f stated; the present invention.' is particularly concerned- 'with 'the provision v`of meansfor-overcoming the-inherent characteristic ofvthese'devices-to exhibit "a-self-generated or self-induced'yoltage eiectf Specifically-it yhas been discovered that thiseiect caribe-overcome by thesuse-o one Aor-morecomposite electrodes which are constructed by ycombining a material whichr` yieldsv a positive# voltage eiectl Wherremploye'cl' as'V one'of thelelectrodes'of la'radioactive resistorA with a materialv which-"yields aY negative Voltage eifect when employed in that manner. For example; if the centralor collecting electrode comprises acomposite'member of carbon and alumin'um', or platinum'and aluminum,l Ain the proper relative proportions,y the resistance .cell 'Will exlhibitiZeropotentia1across the electrode wheni no external voltage' is-connected thereto." Thecom'- posite electrode can be constructed conveniently by placing ashort aluminum tube; over a cylindrical carbon o1 platinuml electrode; afs Yillustrated at ill in Fig', 1, or it may simply comprise a: small sheetV or blockv 43 of aluminum or otherv material Which'is fastened to a :dat electrode by any suit- 'ablemeans as illustrated in Figs. 4to` 7.`

It appears preferable vto `employ the' combination of Vdissimilar materials as the collecting elec;- trode although elimination of the selfgenerated ofv 'self-inducedvoltage can-be obtained by the additionl of the necessary area of dissimilar material to either electrode'. Theexactrelativeareas to be usedmust be'determined by experiment, but oncethe values are established fora given type of construction, lthe required' area Vcan be accurately'predicted; Since the negative voltage effect is usua'lly'greater than the positiveefect, it is usually most convenient'to fabricate the collecting electrode ofmaterial' yielding `a positiveeiect and" correctby adding small Apieces of'material'or materials exhibiting a negative effect. Other arrangements are, however, obviously possible and are within the contemplation of the invention. The material used to eliminate the self-generated or self-induced voltage should be conductively attached to the associated electrode and should preferably overlie a portion of the active surface thereof, as illustrated. Other embodiments of the invention are illustrated in Figs. 4 and 5. 'I'he device of Fig. 4 includes a pair of circular electrodes @5 made of flat, sheet uranium and supported in spacedapart, opposed alignment within a glass envelope t1 by means of suitable conducting supports le sealed into the envelope 41. The sealed envelope erably at atmospheric pressure. Since the electrodes 45 are made of uranium, a naturally radioactive material, no other ionizing means is required. Either electrode may be used as the collecting electrode (the lower electrode is so used in the structure illustrated), and the voltage balancing material, which may conveniently comprise a small square 43 of sheet aluminum, is Welded or otherwise applied to the face of the collecting electrode in conducting relationship therewith, as illustrated.

The device illustrated in Fig. 5 is of similar construction except that neither of the opposed electrodes 5I provided therein is of radioactive material, any conducting material being utilized. As a result, it is necessary in this cell to provide other means for obtaining constant, uniform ionization of the gas occupying the space between the electrodes 5|. Conveniently, the inner surface of the glass envelope 53 is coated, as illustrated at 55, with a radioactive material, such as a thin sheet of uranium, a precipitate of a radioactive salt, or other radioactive material. To assure that the constants of the device will not change during its useful life, the radioactive material should be one having a long half life, preferably of the order of years. The half life of uranium, for example, is of the order of thousands of years and this material is thus particularly Suited for use in such devices.

By proper selection of electrode materials and the voltage compensating material, it is a comparatively simple matter to produce a radioactive resistor which does not produce any substantial self-generated or self-induced voltage and which has straight line current-voltage characteristics up -to the saturation value. Further, since the resistance of the resistor depends only on the physical dimensions of the device and on the radioactivity of the ionization producing material, and since both of those quantities are substantially unchanged by any reasonable change in ambient temperature, the device has a zero temperature coefiicient of resistance. Such a device is especially adapted for use in connection with the electronic amplifying and control systems where a stable resistance is of great importance.

In the foregoing there has been disclosed the features of certain improved radioactive resistance devices. Particularly, means has been disclosed for eliminating the inherent characteristic of these devices to exhibit self-generated or selfinduced voltages. The invention greatly extends the eld of utility of radioactive resistors and solves a problem which has confronted the art for a very long period of time. The features of the invention which are believed to be new are expressly pointed out in the appended claims.

" 41 Lcontains argon or other ionizable gases, pref- What is claimed is: 1. A radioactive resistance unit comprising, in

combination, a sealed envelope which contains' an ionizable gas, a pair of electrodes supported in spaced apart insulated relationship within said envelope, radioactive means for eiecting substantially constant continuous ionization of the: gas contained within said envelope, and a covering of electrically conducting material disposedin contactual relationship with a portion of the surface of at least one of said electrodes and con-i fronting the other of said electrodes, said covering being of a different material than the electrodes which yields a self-induced voltage of opposite polarity to that induced in the contacted electrode, thereby eifectivelyancelingat least a portion of the self-induced voltages from the resistance unit. 2. A radioactive resistance unit comprisinggin combination, a sealed envelope which contains an ionizable gas, a pair of electrodes supported in spaced apart insulated relationship within said envelope, one of said electrodes comprising a hollow cylindrical member of conducting material and the other of said electrodes comprising a cylindrical rod of conducting material disposed centrally of and coaxial with said hollow cylindrical member, at least `one of said electrodes being of radioactive material which effects substantially constant continuous ionization vrof the gas contained within said envelope, an electrically conducting member disposed in contactual relationship over a part of the surface of the other electrode, said member being constructed of materials which yield a self-induced voltage of opposite polarity to that induced in the electrode adjacent to electrically conducting member, thereby at least partially canceling the self-induced voltages from the resistance unit.

3. A radioactive resistance unit comprising, in combination, a sealed envelope which contains an ionizable gas, a pair of electrodes of different materials supported in spaced apart insulated relationship within said envelope, at least one of said electrodes being of a radioactive electrically conducting material which effects substantially constant continuous ionization of the gas contained within said envelope, a member of electrically conducting material disposed in contactual relationship on a portion of the surface of the non-radioactive electrode, said member being constructed of a material which yields a self-induced voltage of opposite polarity and equal magnitude to that induced in the nonradioactive electrode, thereby canceling the selfinduced voltages in the resistance unit.

4. A radioactive resistance unit comprising the elements of claim 3 wherein the radioactive electrode of the resistance unit is constructed of uranium.

5. A radioactive resistance unit comprising a sealed envelope which contains an ionizable gas, a pair of electrodes supported in spaced-apart, insulated relationship within said envelope, one of said electrodes comprising a hollow cylindrical member of uranium and the other of said electrodes comprising a cylindrical rod of conducting material disposed centrally of and coaxially with said hollow cylindrical member, the natural radioactivity of the uranium electrode effecting substantially constant continuous ionization of the gas contained within said envelope, and means comprising a member of a conducting material which yields a self-generated or self-induced voltage of opposite polarity to that induced in the central electrode; `conduetively attachedetoV thats electrode 4sosas to substantially eliminate selfnduoedor self-generated voltageV eiectsin. saielI unit.A

ti.1 A radioactive resistance, unit comprising a sealedv envelope .which containsanfionizable gas, a -pair of electrodes supported in Y spaced-,apart insulated relationship. within said, envelope, one Of .said electrodes comprising a hollow cylindrical membercfiuranium and the other ofsaid elec,- trodes comp-rising,acylindrical rod caf/a conductingmaterial which yields, a positive selfinduced or `selfegenerated .voltage when sub. merged vin an ionized gas, said. rod electrode being disposed.` centrali-y` of, andv coaxially with. said` uranium electrode, the-.natural radioactivityV lo1 said y.uraniurri` electrode, .effecting substantially constant continuous ionization ofthe gas., Contained.,.within` said envelope,.and a member of sheetmaterial coveringa portion of and being conductively attachedto saidrod electrode,y the materialof saidla'st mentionedmernber yield'- ingi .a negative self-induced or self-generated voltage effect when submerged in an ionized gas and beingA ofV suchsize .and proportions relative tosaid, central electrode thatself-induced or self'- generated voltage effects are substantially elimi-v nated in. saidl unit.

7. A radioactive resistance unit comprising, in

anionizable gas, 'a pair of electrodes disposed CII trically conducting material disposed in con--` tactual relationship with the non-radioactive electrode,` said member beingY constructed of material which yields a self-induced voltage of opposite polarity and equal potential to that induced in the non-radioactive electrode, thereby effectively canceling out the self-induced voltages from the resistance unit.

JAMESM. BENADE.

EDMUND GooDALE.v l'

WILLIAM P; JESSE. y

REFERENCES CITED The following references are of record in the le -of this patent:

UNITED STATES PATENTS Number Name Date' 1,523,013 Greenslade Jan. 13, 1925 1,530,555 Greenslade Mar. 24, 1925 1,531,301 Metzger Mar. 31, 1925 1,658,568 Moore Feb. 7, 1928 2,195,913 Bachman Apr. 2, 1940 2,212,643 Koros et al. Aug.'27, 1940

Claims (1)

1. A RADIOACTIVE RESISTANCE UNIT COMPRISING, IN COMBINATION, A SEALED ENVELOPE WHICH CONTAINS AN IONIZABLE GAS, A PAIR OF ELECTRODES SUPPORTED IN SPACED APART INSULATED RELATIONSHIP WITHIN SAID ENVELOPE, RADIOACTIVE MEANS FOR EFFECTING SUBSTANTIALLY CONSTANT CONTINUOUS IONIZATION OF THE GAS CONTAINED WITHIN SAID ENVELOPE, AND A COVERING OF ELECTRICALLY CONDUCTING MATERIAL DISPOSED IN CONTACTUAL RELATIONSHIP WITH A PORTION OF THE SURFACE OF AT LEAST ONE OF SAID ELECTRODES AND CONFRONTING THE OTHER OF SAID ELECTRODES, SAID COVERING BEING OF A DIFFERENT MATERIAL THAN THE ELECTRODES WHICH YIELDS A SELF-INDUCED VOLTAGE OF OPPOSITE POLARITY TO THAT INDUCED IN THE CONTACTED ELECTRODE, THEREBY EFFECTIVELY CANCELING AT LEAST A PORTION OF THE SELF-INDUCED VOLTAGES FROM THE RESISTANCE UNIT.

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