US1648183A - Method and apparatus for conducting current - Google Patents

Description

K. H. KxN-GDoN E1' vAl.

METHOD AND APPARATUS FOR CONDCTING CURRENT v Filed Dec. 21. 1922 Kingdom muir, v

ITW/i118 .Lan v l by y n y Their N'lorney.

Nov. s, 1927.

Patented Nov.. 8, 1927.

UNITED STATES vIAATENT orrica.vv

KENNETH H. KINGDON AND IRVING LANGMUIR, F SCHENECTADY, NEW YORK, AS-

SIGNORS TO-GENERAL ELECTRIC COMPANY', A CORPORATION OF NEW YORK.

\ METHOD AND APPARATUS FOR CONDUCTING CURRENT.

Application ledJJecember 21, 1922. Serial No. 608,198.

The present invention provides new electrical devices utilizing positive ions which are generated by a new method at one of y the 'electrodes in a regular, controllable manner independently of and without electron impact. l

Positive ion currents have been obtained previously from an electrode by .scientific investigators but such currents have been too minute for practical purposes and in general have been transient. It has not been possible heretofore to generate positive ions from electrodes ,under reproducible conditions in amounts suiiicient for useful pur-A poses.

We have discovered that positive ions can be generated from suitably chosen gaseous material which comes into Contact with a heated, positively charged electrode, the electrical properties of this gaseous material being correlated with the electrical properties of the generating electrode in a manner to be explained later. When the generating electrodes consist of tungsten, we find that the elements caesium and` rubidium, are. particularly eficient for generating positive ions in useful amounts. The continuous presence of the vapor is assured by having an excess of the solid or liquid alkali metal 3o constructed or regenerated 'from the positive ions by the recombination of the ions with electrons to form neutral atoms of the vaporv during the operation of thel device.

In devices embodying our invention positive ions may be utilizedeither as the sole current carriers, as for example, in the detection of. radio signals, or lpositive ions may be utilized conjointly with electrons for the conduction of power currents at high 'eiiiciency. c

The accompanyingV drawings show in Figs.. 1 'and 2 devices"'operating solely by positive ion conduction;` Fig. 3 is a diagram 4' of circuit connections, and Fig. 4 illustrates a 'device for conducting current by the conjoint action of electrons and positive ions.

The device shown in Fig. 1 illustrates an embodiment of our new device in whiohthe positive ions are utilized. to conduct current, and which is structurally particularly in the device; and moreover, the vapor is adapted to demonstrate the laws ofthe posi-v tive ion discharge.l This devicecomprises a sealed container 1 consisting of refractory glass,.quartz or other suitable material, and cpntalning the filaments 2 and 3, one of which `is used as the ion-generating electrode, the other filament being used in the preparation of the film electrode 4 on the inner surface of the glass bulb. The iongenerating electrode for convenience will be referred to hereinafterv as the genode.

The genode 2 mayv consist of tungsten, molybdenum or nickel. The lilament 3. which is used as a source of metal vapor for forming the film electrode 4,' preferably consists of tungsten, although other materials also could be used. The filaments, 2, 3 are connected respectively to suitable leading-in conductors 5, 6 and'7, 8'sealed into a stem -9 inthe usual manner. A cylindrical electrode l0 surrounds the genode section adjacent the connection thereof to the leads for reasons later explained. The. sealed in conductors 11, 12 serve to conve current to the electrodes 4 and 10. As t e formation of lm electrodes by vapori-zing tungsten is described in Langmuir Patent 1,273,628 of July 23, 1918, and is generally understood, it will not be described herein.

When the material which serves-to gen-1 erate positive ions is introduced, the con.

tainer and the electrodes should be free `from gas and the space within the Acontainer v'should be evacuated. Caesium or rubidium may be-introducedjfrom a reductionV tube y communicating withY the discharge device and-provided with a material capable of evolvlng the desired 'metal For exam le, we may employ a mixture of caesium c loride and a reducing agent such as magnesium or, calcium, the latter being in excess.

After suiicient caesium, or other desired ma terial, has been (introduced to serve as a source of vapor, the tube is sealed ofi' inthe usual manner, as indicated at y13.

When for any purpose a device such as shown in Fig. 1 is to `be'operated solely by positiver ion conduction, the positive electrode at which the ions are to be generated isheated to a sutiiciently high temperature and a suitable positive potential is impressed upon this heated. electrode. Forv example,

. above the critical' value.

the electrode 2 of Fig. l is heated by current Vconveyedby the conductors 5, 6 and a'suitable source of potential 14 is connected by the conductors 15, 16 to the genode 2 and the film electrode 4, the genode being positive. A galvanometer or other measuring, or load device, is included in the circuit 16. The guard cylinder 10 is also connected to the source 14 by a conductor 17, but the current flowing therein is not measured. By this connection the laws of the device can be more denitely established than when the 'current flowing from the filament section cooled by the lead wires is included in the measurement.

The current obtained in the described device depends on the positive ion emission and the impressed voltage. (Hereinafter the adjective positive before ion will be omitted for the sake ot brevity.) The. ion emission depends on the temperature of the ion-generating electrode or genode, and the vapor pressure of the active gaseous material.

The critical genode temperature above which an emission of ions is obtained varies somewhat with the nature of the ion-generating material in the' device and other conditions, but in general itl may be said that there is a deiinite temperature for any given genode material above which atoms striking the genode leave the same as ions. In the case of a genode consisting of tungsten and @containing caesium as the active material,

vthe critical temperature may range from about 1000 to 1200 degreesl C. The ion emission is independent of the genodetemperature providlng the genode temperature is Above the critical temperature, the particular value of which vmay be determined under given conditions in any device, the ion emission obtained is proportional to the vapor pressure. The critical genode temperature varies with the vapor pressure in the c ase oa given iongenerating material. For example, in a devic'e contaming a clean, unoxidized tungsten genode and providedwith a charge of caesium the critical genode temperatures observed at bulb temperatures of C., 50 and 30 C. respectively were 1030 C., 94.0 C. and 880.

The'vapor pressure may be fixed at different desired values by an `external heater, or medium such Vas an oil bath which can be kept lat a substantially constant temperature. This external temperature control de.- vice has been indicated in `Fig. 1 by the dottedoutline 18 about the device. It should be understood that similar heat control can be used in the other forms of the invention.

The choice ofvvapor pressure will depend i on the use to be made of the positive ions. 'In general. it is advantageous to generate positive ions in accordance with our invention under such conditions that there is no accompanying positive ionization by colllsion in the discharge space. In the case of a device such as shown in Fig. 1 employing only positive ion conduction, the pressure may be much higher than when electron conduction also occurs.

When electron conduction accompanies ion conduction and it is desired to control the -clectron current. as in the case of a device of the 4type shown Ain Fig. 4, the pressure should be maintained below a value at which the electron discharge in the vapor would be accompanied by appreciable ionization by collision, that is.I below the pressure corresponding to a temperature of about 70 degrees C. in the case of caesium. The specific temperatur-e will depend on the geometrical construction of a particular device. At 70 degrees C. the vapor pressure of caesium is about 0.0001 of a millimeter of mercury (a tenth of a-micron). The pressure of vapor should not be' so high that a self-sustaining dlscharge may occur between the electrodes at the applied potential.

With any given impressed voltage, the ions generated at the genode may not all reach tron to form a neutral caesium atom. This caesium ion is about 237,000 times heavier than an electron. As the relative velocities are inversely proportional to the square' roots of the masses. the velocity of a caesium lon 1s about 1/487th of the velocity of an electron. Hence, when limited by space charge. the ion current obtainable with a given impressed voltage is about 1/487thofthe electron current obtainablefinf'a given electron tube under similar lconditions.

l The positive space current varies as the 3/ 2 power of the impressed voltage up to a'voltage value high. enough to produce a saturation current and then becomes substantially constant for higher voltages. Itis steadyforfa constantly applied voltage and reproducible for different voltages. For ex-v ample, in a specific device. at a bulb temrature of 0.7 degrees C. the saturation posv ltivepcurrent observed was 2.4v micro-amperes l per sq. cm. of anode surface; at a bulb temperatureof 46.5 degrees C. the current was 0.29. milliamperes per'sq. cm. of anode surface, and at a bulb temperature of 53.5

.degrees C. the observed current was 0.63

milliamperes per sq.'cm. of surface.

As explained by Langmuir in the Transactions of the American Electrochemical Society, Vol. XXIX, 1916', page 125,-there is an absorption of energy when electrons are emitted from incandescent metals. This energy is measurable as heat absorbed, and ma be calculated in terms of a potential di erence in volts` which is a quantitative measure of work donc in separating an electron from an emitting surface reduced to the absolute zero of temperature. This value has been called the electron affinity of the emitting material. This electron ailinity, also known as the work function of electron emission, has been determined for a number of materials.' The value for tungsten is 4.52 volts, for tantalum 4.31 volts, and for molybdenum 4.31 volts. The values of these constants are a measure of the ainity of these respective metals for free electrons which are now generally assumed to exist in conductors. The higher the work function the more tenaciously, so to speak, the respective substance holds on to its free electrons, and therefore, the higher the temperature required to liberate the free electrons.

Not. only are free electrons present in substances, but the atoms, constituting the substance itself, contain a system of electrons. When a free atom of a substance in space loses an electron it becomes positive to this extent and is said to be ionized. It requires energy which may be expressed in volts to remove an electron from an atom. In the case of a caesium atom this potential is 3.9 volts. This ionizing potential is a measure of the electron aiinity of the atom which, it will be seen, 'is less than that of a tungsten surface. Hence, when a caesium atom with an ionizing potential of 3.9 volts strikes-a hot, positively charged tungsten surface with an electron aflinity of 4.52 volts, it leaves the heated surface in the form of a positive lion, having. lost an electron to the tungsten.

Our experiments indicatey that the alkali i metals have the property of forming an adsorbed film upon a metal surface even though the metal surface is at a temperature materially higher than the temperature corf responding to the particular vapor pressure of the alkali metal inthe environment of the metal surface. At a temperature materially below the critical temperature for ion generation, the genode surface will be largely coated with an adsorbed film of whatever alkali metal is present in the device. The electron affinity of such an adsorbed surface of caesium for free electrons is about 1.4 volts, and hencewhen caesium atoms leave a surface largely coated with caesium they do not losean electron, as the evaporating atoms have a greater affinity for electrons than does the surface.

When the temperature is progressively raised the surface of the metal is but partly covered with adsorbed caesium, and the electron aflinity of the surface is increased until some of the caesium atoms will leave the surface as ions. For example, a tungsten surface 20 per cent of which is coated with Caesium will have an average electron affinity of about 3.9 volts as a resultant of the work `into contact with a heated surface of higher electron affinity than the ionization potential of said material, we desire to refer to herein as surface ionization.

Fig. 2 shows a positive. ion radio detector 20 which contains in addition to a genode 21 and a cathode 22 also an intermediate input electrode or grid 23. The grid and genode are connected to the secondary of a radio transformer 24, a variable condenser 25 being provided in shunt to the secondary as is usual. The output circuit 2'6l contains a source of energy 27 represented by a battery, and a telephone receiver 28 connected between the genode and the cathode. Instead of having the telephone in the output circuit, suitable amplification of the audio current output may be provided in the well understood manner. A battery 29 and a variable resistance 30 are shown in the genode circuit 31 to heat the genode to a desired temperature. The bulb 1 is highly evacuated' and contains a quantity of caesium,

or equivalent material. At the ordinary operating temperatures, that is, just above room temperature, the 'vapor pressure of the caesium is about 0.002 of a micron of mercury. .Several times higher vapor pressures are permissible, depending on conditions.

The voltages of the heating circuit 31 and the output circuit 26 are so chosen that the positive ion current is limited by space charge. Variations of the grid potential by the received signals in the circuit 32 vary the current in the output circuit 26 and produce audible signals in the telephone 28. In some cases the g-rid may be omitted as shown in Fig. 3, the genode and the cathode 22 being connected directly in the plate circut 26 which contains a battery 27, a telephone 28 and is connected to the secondary.

of the input transformer 24. The signal current is rectified by the unilateral conductivity ofthe positive ion device and becomes audible in the telephone.

4 electron emitting cathode is employed which is adapted to function independently of positive ion bombardment as illustrated for eX- ample, in Fig. 4. In the device here shown the cathode also consists of a filamentary elect-rode 30, which is connected by an errternal circuit 31 to a cylindrical anode 32 in series with a source of'current 33 and a load device 34. The genode 35 consists of a selfsupporting coiled filament. Heating circuits 36 and 37 are p rovided respectively for the cathode 3() and .the genode 35. These circuits respectively contain heatingbatteries 38 and-39 and variable resistances 40, and 41,-as indicated, whereby the temperature of the respective electrodes may be regulated. The vapor pressure of the caesium 0r other adsorptive material is maintained at aI desired value by the external temperature controller 18.

As set forth more at len th in an application Serial No. 608,217 fi ed concurrently herewith by Irving Langmuir' on devices of the type illustrated in Fig. 4, the positive ion current generated at the genode 35 performs the important function of neutralizing the negative space charge of the electron current'emitted by the cathode 3U. As is well known, space charge is a termapplied to the current limiting effect of the mutual repulsion of thenegatlve electric charges of the electrons which must be overcome by the impressed voltage. In electrical devicesheretofore used a considerable. part of the impressed voltage has been required to overcome space charge and therefore electronic power devices could be operated at good eliiciency only when the voltage consumed by the external load was relatively high. Otherwise, the fall of voltage in the devlce itself represented too llarge a proportion of the impressed voltage. It has been known that this high space charge could be neutralized by the presence of positive ions resulting from ionization by collision of electrons with gas atoms in the discharge space, and in certain industrial electronic devices an ionizable gas has been introduced to cause ionization by electron impact in order to lower the voltage drop inthe device. The presence of such gas, however, is necessarily accompanied by certain limitations, as for example the tendency for'the discharge to get out. of control, and the disintegration of the cathode by excessive positiveion bomy bardment.-

In the device shown in Fig. 4, the ions generated at the genode 35 are capable of neutralizing the space charge of an electron current emitted by the cathodel 30 even though the value of the electron current is many times greater than the value of the positive current; as the 'operating temperature .of the bulb may be maintained below a value at whichgas ionization by electron A impact .becomes appreciable, the conduction of electricity through the evacuated space may occur by. a new principle, namely electron 4 conduction with low or negligible space charge without the limitations imposed the presence of the rapidly moving electron stream for a. sufficient length of time to enable a much largerelectron current to pass. What We' claim as new and deslre to secure by Letters Patent of the United States, is

l. The method of generating 'a positive ion current independently ofY ionlzation by c01- lision which consists is bringing into contact with a positively charged velectrode a vapor of a substance chemically inert with respect to said electrodeand having a lower electron affinity than said electrode, 4heating said electrode above acritical temperature at which positive ions are generated and carrying away said ions by an applied potential.

2. The method of conducting current which consists in bringing into contact with an electrode a vaporizable adsrptive material which is chemically inert with respect thereto and has a lower electron aifinity than said electrode, heating said electrode to 'a temperature above 1000 C. to generate positive ions from said material and carrying away said ions inthe absencey of positive ionization by electron impact.

3. Themetho'd of generating positive ion current in an electric dischargel device containing a plurality of electrodes which con-v sists in providing in the environment of one of said electrodes vaporizable material having a lower electron aiinity than said electrode, heating said electrode to a tempera-v said electrode to a positive potential with respect to another electrode to conduct away 'said ions, and maintaining Said device at a temperature at which the vapor pressure of said material is sufficient-ly high to' produce a substantial ion emission but below the value at which self sustaining discharge can occur in said vapor.

4. The method of conducting electric energy between electrodes in a space evacuated to a gaseous pressure so low that gaseous ionization by electron impact is substantially absent when an electron current flows between said velectrodes at voltages above the ionization voltages of residual gas which consists in heating one of said electrodes to an elevated temperature,cbringing into contact with said heated electrode a material having a lower electron affinity than theI surheatin face of said heated'electrode, thereby genertrode, heating said electrode in excess o a temperature at which an adsorbed film of said alkali metal remains thereon, and maintaining the vapor pressure of said alkali metal suliiciently low to enable an electron discharge to occur therein without appreciable ionization by electron impact.

6. The method of operating an electrical discharge device containing a cathode and a second electrode adapted to be operated at an elevated temperature which .consists in said second elect-rode to a temperature o about 1000 to 1200 C., charging said second electrode positively with respect to said cathode and delivering to the environment of said positive electrode afvaporous material which has a work function of electron emission which is less than that of said positive electrode, and a pressure at which an electron discharge therein will be unaccompanied by appreciable gas ionization by collision. p

7. The method of conducting electricity between electrodes one of which consists of a material having a high electron alinity which consists in liberating caesium vapor in the space between said electrodes, heating said electrode of high electron ailinity at least to a critical temperature at which ions are generated, and charging said heated elec trode to a positive potential with respect to another electrode.

8. The method of generating a positive ion current in an enclosed space containing a nickel electrode and a cooperating electrode which consists in introducing caesium into said space to the exclusion of other gases and vapors, charging said nickel electrode to a positive potential, heating the same to a temperature of at least about 1000 C., conducting away positive ions generated from-caesium vapor at said nickel electrode and maintaining the pressure of said vapor below about 0.0001 of a millimeter of mercury.

electrode a vapor having an electron atlinity prising a container,l a plurality of electrodes discharge device comtherein, means for charging one of said elecf trodes to a positive potential, a vaporizable material in said container having an electron atlinity less than the surface yof said positivelyvcharged electrode and having a vapor pressure at the operating temperature of said container high enough to generate positive ions at said positive electrode in appreciable quantities, but being below` the pressure lat which appreciable ionization by collision will accompany electron conduction between said electrodes.

11. An electrical discharge device comprising co-operating electrodes, means for generating positive ions by surface ionization at one of said electrodes independently of electronic collision phenomena, and means for controlling the amount of said ion generation. v

12. An electrical ldischarge device .comprising an envelope, a pluralltyof electrodes therein, means for establishing a difference of potential between at least two of said electrodes, independent means for heatin one of said electrodes, a vaporizable materia in said envelope out of contact with said heated electrode for furnishing to said heated electrodev a vapor capable of generating being constructed to operate at a tempera-y ture at which the vvapor pressure of said` caesium is too low to permit a self-sustaining discharge to operate between said electrodes.

14. An electrical discharge device comprising cooperating electrodes, a'source of potential connected thereto, means for heating the more positive electrode, land means for delivering caesium vapor into Contact with said electrode at a pressure suiiiciently low to permit electron conduction between said electrodes unaccompanied by appreciable ionization by collision.

15. An electrical discharge devlce comprising an exhausted envelope, containmg .positive ions bycontact with said heated n an elementary material capable of electrical coversion into positive ions, and being capable of regeneration by the discharge` of said ions, electrodes one of'whlch has a surface of higher electron aiinity than said ma- Aterial, and means for heating said electrode of higher electron affinity independently of I 'a discharge between said electrodes.

16. An electrical discharge device comprising anv evacuated container, an alkali metal in said container, electrodes therein one of which has a higher electron affinity than the ionizing potential of said alkali metal, means for charging onev of said elec.- trodes to a positive potential and controllable means for heating said positively charged electrode -independently of an electron discharge between said electrodes.

17. An electron discharge device comprising an envelope, a quantity of caesium therein, cooperating electrodes, one of which consists of a material having a higher electron 'anity than caesium at a temperature higher than about 10009 C., means for heating the latter electrode independently of a discharge' between said electrodes, and means for controlling thevapor pressure of csium. v

18. The combination of anA electrical discharge device provided with means for generatlng positlve ions by surface ionlzation,

-'a work circuit connected thereto, a source of current in said circuit having a voltage so related to said ion generating means. thatl an lon current havingvspace' charge charac.

teristics is produced in ysaid device and means for electrostatically controlling said discharge.

19 An electrical discharge device com` prislng a sealed envelope provided with cooperating electrodes and containing a material having a lowerelectron ainity than a positive electrode, and having at the operating temperature of said envelope a-.substantial` vapor pressure, whereby pos1t1ve ions `may be generated in effective amounts when said positive electrode in said device. is heated to a suiicientlyhigh. temperature` In witness whereof, We have hereunto set' our hands this 19th dav of December, 1922;

KENNETH H. KINGDON. IRVING LANGMUIR.

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