US 1203495 A
Description (OCR text may contain errors)
W. D. COOLIDGE.
APPLICATION FILED MAY 91 1913.
Patented 001. 31,1916.
funun'uununununflnun Inventor William D.Coo||dge His Attorney k 20 27 Witnesses 1" UNITED STATES PATENT OFFICE.
WILLIAM D. COOLIDGE, F SCHEN'EGTADY; NEW YORK, ASSIGHOB, T0 GENERAL ELECTRIC COMPANY, A. CORPOBATTON OF NEW YORK.
Specification of Letters Patent.
Application filed lay 9, 1913. serial No. 766,549.
To all whom it may concern:
Be it known that I, WILLIAM D. Goomnon,
' a citizen of the United States, residing at Schenectady, in the county of Schenectady and State of New York,have invented certain new and useful Improvements in Vacuum- Tubes, of which the following is a specification.
My invention relates to vacuum tubes and more especially to tubes operated for the purpose of producing Rontgen or X-rays. The tube, which I have produced differs so radically from the tubes of the prior art used for producing Rontgen or .X-rays as to amount not so much to an improvement on prior tubes as to an entirely new variety of tube differing .both in its principles of operation and in its operating characteristics.
In the Rontgen tube of the prior art, it
was necessary that there be at least some free gas in the tube, thereason for which may be explained as follows: The number of ions present in the tube was exceedingly small until a high potential was im ressed upon its terminals. The effect of th1s high potential was to accelerate the motion of the small number of ions originally prese nt, thus causing them, by their coll1s1on with gas molecules to produce more ions, and,
further, to so attract the positive ions that they would bombard the cathode and so cause it to emit electrons. These electrons, directed by the electric forces in the tube, move toward the anti-cathode and in their movement constitute the so-called cathode rays. Upon colliding with the anti-cathode these under- -go,a sudden change in velocity and so give rise to the Rontgen or X-rays. As the ennssion of electrons from the cathode was due solely to bombardment by positive ions, and as these positive ions owed their existence to the presence of gas in the tube, it is seen at once that the X-ray tube of the prior art was absolutely dependent for operativeness upon a proper amount of gas. Without the presence of sufficient gas to permit a discharge to take place between the electrodes the cathode would not be bombarded by positive ions and so no resulting production of X-rays'would occur. The presence of this gas, heretofore considered a necessity in the operation of such a tube, gave rise to certain. features of operation of the tube which are obviously very disadvantageous but which- ,have heretofore been accepted workers in the art as unavoidable evils. Wlll not here attempt to point out all of the troubles which may betraced to the presence of this gas, but will mention a few. of
the attendant diificulties.
(a) The pressure of the gas in such a tube is exceedingly variable, and any change in gas pressure is accompanied by a correspondmg change in the resistance of the tube and in the penetrating power of the Patented 0017.31, 1916. V
rected and, as a result, the exposure orv fluoroscopic examination is made, not with Riintgen rays of the quality desired, but with whatever approximation to that is allowed by the idiosyncrasies of the particular tube in question, in other words, with a mixture of rays differing perhaps very widely in penetrating power.
((1) The tendency to undergo rapid changes 1n gas pressure increases with use, until finally the tubes of the prior art show such instability of vacuum that they are no longer useful, even though they may be otherwise in perfect condition.
1 (6) Many of the positive ions instead of striking the cathode, strike the glass constituting the tube wall around it, and, by their excessive local heating, cause disintegration and cracking of the envelop at this point. This imposes a limitation on the ermissible energy input in such a tube.
Many tubes of the rior art show variability in the location 0 the focal spot. As the focal spot is the source of the X-ray, its movement during a radiographic exposure, means a blurring of the picture. vIt
1s my belief that sudden changes in gas the cathode are impeded in their passage to cording from .duced.
As a cathode ,I ordinarily use a tang-- 'sten; filament and this may be conven-- V the target by impact 'with intervening gas molecules and so do not have, upon reaching the target, the same velocity as those' electrons which have passed through the entire potential drop from cathode to target without meeting an obstructlon. The higher the as pressure in the tube the greater will be t e mfluence of this effect and, consequently the greater will be the lack of uniformity in the penetrating power ofvthe revsulting Riintgen rays.
eifect'm the radiograph and on the -fluores-.
(13) Any electromot1ve force 1n thesource of current supply for such a tube, in a direction which would makethe target function.
as cathode, must be carefully suppressed, as it otherwise leads to changes in gas ressure.
In order to overcome these trou les met with in the X-ray tubes of the prior art, I remove the cause of the trouble, namely,the
as in thetube, and produce the electrons om the cathode necessary to the production of the X-rays, not by a bombardment of the cathode by positive ions due to ionization of the gas y the volta e impressed upon the tube, but by heating ,t e cathode to incandescen'ce by energy .from a suitable source. In tubes made according to my invention I eliminate the action of positive ions as far as it is possible to do so by evacuating the tube to the highest vacuum which can possibly be attained and by thoroughly freeing the electrodes from gas. In the tube of the prior art the pressure of gas was quite appreciable and varied from say one to ten microns (0.001 to 0.010 millimeters) of mercury. With a tube made acto my invention, the tube proper and the electrodes being thorou hly freed s as hereinafter'describe the pres-' sure 0 as may be say five hundredths of a micron 0.00005 millimeter) or lower and in any event lower than that at which the tube would operate as an ordinary-Rontgen ray.
tube. The upper limit of pressure may therefore be placed .rou hly at say about 0.6 of a micron or therea out. .With such a vacuum in a tube ofithe prior art so far as I am aware no current whatever would pass in the tube iently heated by current from a storage battery or transformer with suitable regulating and no X-rays would be prodevices so that the temperature of the cathode or filament may be adjusted at will. I have found that electrons will be emitted from such a cathode, whiclb electrons will traverse the space betweenthe cathode and the target under the influence of the electromotive force impressed upon the tube, and by bombardment of the target, will give rise to X-rays. In this new type of tube positive those used in tubes ofthe prior art, the
tungsten filament, functioning as cathode,
is raised by these same discharges from room temperature to extreme incandescenoe, and may even be melted by the bombardment of the positive ions.
Gas pressure in the tube bein thus eliminated to the highest possible features ofinstability in the tu of the prior art which it can now be seen were due to the presence of the gas in the tube are likewise eliminated. Furthermore, by static or electromagnetic means I cause the cathode rays or electrons to be focused on the anode or target and this focusing effect is steady and unwavering. I
A tube made in accordance with my invention differs from the'tubes of the rior art in the possession ofmost remar able characteristics. Thus there are nomarked changes in the operation of the tube-during a considerable length of time, with the result that X-rays may be constantly eneratedof a'much more uniform degree 0 penetrating power and much more constant in volume and in the location of their source or focal spot. The penetrability or in'other words the hardness. or softness. of the X-rays may be adjusted at will by ad'usting the temperature of the cathode. ere is practically no fluorescence of the glass of the tube and this is important because it indicates that the glass is not bombarded by target or anodeand is therefore not being locally heated thereby. There is only a such a'useless and disturbing factor. in the ordinary Riintg'en tube.- The volta ge cone ee, those secondary cathode rays emanating from the sumed by the tube may in certain forms of embodiment of my invention bemade to renfii'n more or less constant even though the voltage of the source of current supply varies. The focusin efiect is steady and the 811832888 of definitlon of the X-ra photogra is increased. These and ot er features and advantages of my invention will be better understood by reference to the following description taken in connection with the accompanying drawings. The features of novelty I have endeavored to point out in the appended claims.
In the accompanying drawings, Figure 1 is a sectional view of a complete tube equipped with one type of focusing device; Flgs. 2 to 7 illustrate alternative forms of focusing devices, and Fig. 8 illustrates a modified tube having a constant voltage characteristic.
Referring to Fig. 1, showing one embodiment of my invention, the tube or envelop is indicated at 1, consisting as usual of glass, or other transparent material. In the tubular end 2 is sealed an anode 3 which in this case is plate-shaped. It is supported by a stem 4, both anode and stem consisting, preferably, of wrought tungsten. Opposite this anode, which serves also as target, is a cathode 5 adapted to be heated, in-this case a filamentary spiral of tungsten, tantalum, carbon or other highly refractory material. In this case the filament is wound non-inductively, though this is not essential. It is connected to leading-in wires 6, 7, sealed into a glass support 8, the wires being insulated from each other by little glass tubes 8' as clearly shown. The wires 6, 7 are connected to a source of current, such as'a battery 9, or to a transformer, through a regulatable resistance 10. The battery, or transformer.
secondary, should be well insulated owing to the high potential used in operating the tube. Around the cathode proper is a hollow cylinder 11 consisting of tungsten, aluminum, iron, or other suitable conductive material held in position by springs 12, one of which engages with a little knob 13 on the glass. This cylinder or tube is preferably electrically connected to the cathode by means of wire 14, either inside the tube or brought outside of the tube as shown, and containing in circuit a switch 15. In normal operation the cylinder 11 performs the important function of modifying the static field to focus the cathode discharge, as will be explained.
Instead of being tubular the focusing device may assume various other shapes. It may be a perforated fiat disk, as shown at 16, Fig. 2, or a perforated dished or concave disk 17, Fig. 3, or even a simple ring 18, Fig. 4. In fact, even a metallic deposit, such as a film of chemically deposited silver, on the walls of the globe, as shown in Fig. 5 at 19, will act as a static focusing means.
Magnetic focusing means may be used, instead of the static focusing devices, and as shown in Figs. 6 and The magnetic focusing means may take the form of a solenoid, either inside the tube similarly to the static focusing ring, or outside the walls of the tube in the vicinity'of the cathode, as diagrammatically indicated in Fig. 6, at 20, or the cathode itself is wound inductively, as shown by cathode'21 in Fig. 7. i
In the preparation of the tube the glass is first freed from moisture and gas by heating and other approved methods and the electrodes preferably are freed from gas as by heating them to a high temperature in a vacuum. Exhaustion of the tube is preferably carried on by means of a pump which will remove vapors as well as gases, such as the Gaede molecular pump. For the removal of the last portions of gas from the electrodes and from the glass the tube is operated on the pump. The electrodes should be made of refractory metal such as tungsten so that they will withstand the very high temperatures attainable by operating the tube while connected 'to the pump, whereby gas may be, effectively driven out. In case the usual constriction in the exhaust tube is employed, pumping is thereby greatly delayed. In some cases much time can be saved by omittinggthe constriction until the tube has been exhausted to the point where the operation is satisfactory. Carefully dried air can then be admitted and the constriction put in. Upon exhaustion of the dry air it will be found that the condition of the tube, even without further running on the pump, is essentially what it was before air was admitted.
lVhen the electrodes have been sufliciently freed from gas particularly the anode which is of large mass relative to the cathode and the tube has been exhausted to a vacuum of about .05 microns (0.00005 millimeters of mercury) or lower it is sealed off from the pump. It may be operated, in accordance with my invention, by closing the battery circuit through the resistance 10 so as to heat the filament 5 to incandescence. \Vhen a sufficiently elevated temperature has been reached electrons are emitted by the filament. When a sufiicient potential is applied across the tube by completing connection to a suitable source 22, through conductors 23 and 24, current is transmitted, provided the negative terminal of the source is connected to the filament. When the source 22 furnishes alternating current, as for example, a transformer or an induction coil, only half Waves of current are transmitted by the device. The electrons soon charge the hollow cylinder 11 to a negative potential with respect to the cathode unless the cylinder and cathode are connected. \Vhether connected or not, the cylinder somodifies the nature of the static field within the tube that the electrons are directed upon a spot of small area upon the anode 3.
. In my opinion the focusing action is caused by the electrons tendingto move perpendicularly to the equipotential surfaces which may be drawn in the space between cathode and anode. This manner of focusing the cathode discharge is especlalllyeffective as the speed of the electrons is re atively low near the'cathode and therefore their movement is easily directed by the focusing devices. The focal spot on the anode remains fixed during the normal operation of the 'tube. This is in sharp contrast to the operation of the ordinary Riintgen tube n which the focal spot often moved; about so rapidly as to produce blurring of all lines not parallel to the direction of motion, even during very short radiographic exposures. Z
The operation'of X-ray devices made in accordance with my invention-is markedly affected, by .the tem ature to which the cathode is raised. I the temperature of the filament is low only a small number of elec-' trons escape, and therefore the saturation current through the tube is relatively small; Increasing the impressed voltage on the tube will cause substantially no further increase in current. It will increase the velocity of the cathode Is 5 and hence the penetrating power-of the "ntgen rays. If the filament temperature on the other hand is very high, further increase of temperature produces no appreciable effect on the tube ,characteristics, more electrons being emitted by the filament than canbe carried,
by the voltages emploiled, from one electrode to the other; At gher filament temperatures, the space relations of the elements of the tube such as the distance between cathode and anode, the location of the focusing means relative to the cathode, the diameter of the opening in the focusing device, etc. constitutes a current hmiting fac-.
tor, as will be later e lained. Except for these limiting cases, an increasev in impressed voltage will in general cause an increase 1n discharge current and in the potential drop in the tube, thus increasing the penetrability of the X-ray. However, by giving the cathode its main extension in the direction of the axis of the tube, and also suitably proportioning and locating the focusing device, a most important new characteristic may be attained. Such a construction is shown in Fig. 8 in which a filament 25 extends nearly through the whole length oftube 26, being'anchored at its tip by a support 27. With a low impressed voltage, only that part of the filament 25 nearest the anode will furnish the electrons necessary to-carry the discharge current. With higher impressed voltages, a progressively greater part of the filament will contributeelectrons. The increase in the number or electrons thus rendered available by the increase of impressed voltage and the accompanying change in distribution in the static field, tends to reduce the tube resistance and thus counteract the tendency of the higher induction coil or a rectifying device such as a mechamcal selector are use d which gives current impulses of variable voltage.
My novel apparatus therefore, has the following valua lo and striking characteristics. First, the resistance of the tube, and, hence the penetrating power of the Riintgen rays produced in the tube, may be controlled with certainty, and may be instantly increased or decreased at will, by raising or lowering of the temperature of the cathode;
second, a much more homogeneous'bundle' of X-rays can'be produced even though a several-fold increase, or decrease of as pressure takes placein thetube, providing the pressure remains within the limits above indicated; third, as the -dischar e canpass through the tube only when the eated electrode is made the cathode, the tube may be operated on alternating" current almost equally as well as on direct current; fourth, if desired, a tube may be proportioned to give over a predetermined range of impressed voltage X-rays of substantially uniform penetration.
The operator is thus enabled to determine, with a properly calibrated device,
from a curve, or chart, just what current values and voltages he needs for a given purpose, and may be sure that when he has prepared his patient he can get exactly the service he requires from his tube without troublesome and dangerous experimentation.
One of the main causes for the uniformity of the penetrability of the X-rays is to be ascribed to the fact that the operation of the tube isnot dependent uponv ositive ion bombardment of the cathode. other cause is its complete independence of vacuumchanges within the permissible range of pressure. It will also be noted that with the 'high vacuum existing in the tube, the mean free path between gas molecules is much greater than the distance between anode and cathode and, therefore, very few electrons can collide with gas molecules in.
passing from cathode to anode, therefore,
-practically all of the electrons reach the target with uniform velocit Because of the low gas pressure the num er of positive ions is'so small that the filament is not heated at all by bombardment. Even when a very heavy discharge current is sent through the tube, a voltmeter and amineter in the filament circuit indicate no change in the resistance and hence the temperature of the filament. Another observation, 'which has a similar significance, is that at low filament temperatures, where the electrons emitted by the cathode are allused in carrying current, a several-fold increase in voltage brings substantially no accompanying increase in the discharge current. This could hardly pertain, in case positive ions played an appreciable role, for increased voltage would mean higher velocities, and hence, a greater emission of electrons which, in turn, would mean an increase in the discharge current.
The wall of the tube exposed to the anode is not heated by secondary cathode rays from the anode, thereby removing the limitation formerly imposed by local tube heating. Tube fluorescence due to secondary cathode rays is likewise absent. Not only are current carrying limitations thus removed but it should also be noted that a marked gain of efficiency is secured in the tube. For example, in the former type of tube with a platinum target, about threefourths as many electrons left the target as secondary cathode rays as were received by it in the form of primary cathode rays. The elimination of secondary cathode rays also greatly improves the tube as the secondary X-rays produced by the secondary cathode rays striking the tube were a serious disturbing factor as already mentioned.
While I have described and illustrated specific forms of tubes embodying my invention, I wish it to be understood that various changes and modifications may be made in the tube within the scope of my invention. As already indicated the spacial relations of the elements of the tube affect its characteristics. For example, increasing the distance between the front of the focus-' ing device and the incandescent cathode increases the resistance of the tube; decreasing the opening in the static focusing ring or tube increases the resistance of the tube.
Decreasing the distance between cathode and anode lowers the resistance of the tube, but this effect is probably not to be ascribed to any considerable resistance change due to linear separationof the electrodes but to the fact that the glass walls of the tube are charged negatively during operation and thus act as though they represented an extension of the focusin device. Bringing the cathode nearer to t e. anode therefore, would have an effect similar to that caused by bringing it nearer to the front of the focusing tube or ring.
Certain broad aspects of the apparatus and methods herein disclosed are not of my invention but are the invention of Irvin Langmuir and are defined by the claims 0 his allowed application, Serial No. 84 t2, renewed March 14, 1916, and his a p 'cationi Serial No. 876,432, filed Decem er 10, 191
. What I desire to secure by Letters Patent of the United States, is
1. In a device for the production of X- rays, the combination of a tube provided with electrodes, the vacuum in the tube being soattenuated as practically to'eliminate the production of positive ions, and means for generating electrons at the'cathode or negative electrode substantially independently of the voltage impressed upon said electrodes.
2. An X-ray device comprising a gastight envelop, the space within said envelop. being exhausted to a pressure at which substantially no gas ionization occurs, a cathode, an anode, and means for generating electrons at the cathode, the current transmitted by said device with a given cathode temperature being independent'of the voltage over a range of impressed voltage values.
3. An electrical vacuum discharge tube, comprising an envelop, cotiperatin electrodes, at least part of one of w ich is adapted to be maintained at incandescence, the vacuum within said envelop being so high that evidences of positive ionization are substantially absent during operation and means for focusing the discharge.
4. An electrical vacuum discharge tube, comprising an envelop, cooperating electrodes therein, one of which is adapted to be independently heated, and a conducting body surrounding said cathode, said body serving to. modify electrostatic forces in the tube, to control the direction of the cathode rays, the residual gas in said envelop being insuflicient to produce appreciable ionization by collision during the operation of the tube;
5. An X-ray device com rising electrodes consisting of tungsten reed thoroughly from gas, the negative electrode being adapted to be independently heated, at least in part, and an inclosing envelop so highly evacuated that evidences of positive ionization are substantially absent during the operation of the device.
6. An electrical discharge device com rising an envelop, the space within sai envelop being evacuated to a pressure below the value atwhich appreciable positive ionization can occur, electrodes therefor, the
cathode being adapted to be heated inde-' pendentlyof energy transmitted by said device and conductive means in part at least surrounding the s ace between said' electrodes to control e ectrostatic forces in thetube. I
I 7. An electrical discharge device compristobe'heated independently of-the operating current, a cooperating anode and means for focusing the cathode discharge upon the anode. e
9. A Rontgen ray device comprising the combination of an envelop evacuated to a pressure at which positive ionization is substantially absent; during the operation of the device, an electron-emitting cathode, and anode and a source of ener connected to said electrodes having a vo tage in excess of the minimum value at which the current is substantially independent of the voltage.
10. A Riintgen apparatus comprising an envelop evacuated to a pressure so low that evidences of positive ionization are substan tially absent during the operation of the apparatus, electrodes therein, external means for independentlyheating the negative elec-- trcde,'and means for modifying the static field in theneighborhood of the cathode to focus thescathode discharge upon a spot of small area on the anode. p
11. A Riintgen tube comprising an evacuated envelop, main working electrodes there- I for, one of which is offilamentary character, means for heating said filamentary electrode, and an electrical conductor at least in part surrounding the space between said electrodes .for concentrating a discharge from said cathode upon a spot of smallarea'on the anode.
12. A Rontgen ray tube comprising an evacuated -envelop,' electrodes therein, thecathode being arranged [to be heated indedently, and a focusing device surroundmg and electrical] connected with said in-' dependently heate cathode.
13. The combination of a source of current alternating in potential, 'and a Rfintgenray tube operatively connected therewith comp cooperating electrodes at least part of one of which is adapted to independently generate electrons, and an envelop therefor evacuated to such high degree that manifestations of positive ions are absent.
14. A Rdntgen ray tube comprising an envelop, cotiperating electrodes therein, atleast-part of one of which may be independ-' en'tly heated to emit electrons, the space in said'envelop being so highly evacuated that manifestations of positive ions are a .means for-producing a cathode discharge in evacuated to such a degree that bsent,
tive ions are absent, a source of high potential current, connectionsbetween said source and said main electrodes, an independent source of low potential current connected to and'heating the cathode, and means for adjusting the heating current to control the resistance of the X-ray device.
16. In a device for the production of Riintgen rays, the combination of a tube practically no current can pass due to ionizing action of the impressed voltage upon such gases as may remain in the tube, s'itive and nega-' tive electrodes in the tu pr: "ided. with suitable leading-in'conductors, and means for heating the cathode to a high temperature, said means being independent of the impressed voltage.
17. An electrical vacuum tube, comprisin an envelop the space in which 1s exhauste to below the pressure at which residual gas will conduct current, a cathode adapted to" be heated to incandescence, a coiiperating anode substantially deprived of gas evoluble by electron bombardment, and means 'disv posed about an axis passing through cathode and anode for directing the, cathode discharge. v
18. An electrical discharge device-comprising an envelop evacuated to a pressure so low that evidences of positive ionization are substantially absent during ,the o ration'of'the device, an anode, a cathode a apt ed to be heated independently of a dischar passing between the same and the ano e, i
and conductive means surrounding, in part at least, the space between cathode and anode for modifying the static field to focus a cathode discharge. I
19. The method of producing Riintgen rays which consists in independentl heatmg ucing an thecathode to incandescence, p electric discharge between the incandescent cathode and a cooperating anode in a vacuum so highly attenuated that evidences of positive ionization are substantially absent during the passage of current, and focusin ghtladcathcde rays by means of an electrica e 4,
'20. The'method of regulating thepenetrability of Rontgen rays produced by a discharge from an incandescent cathode in a vacuum so. attenuated that electrical conductivitykof the residual gas is substantially absent, whidl consists in adjusting the cathode temperature and imp voltage.
v I. I
21. The process .of producing Rontgen rays by current from a. source alternatmg in potential which consists in emitting electrons at a negative electrode, transmitting 6 thereby a ray-producing current wave of one polarity, while preventing at the same time the formation of positive ions, and suppressing the other half .wave of the alternating current.
which consists in impressing an alternating electromotive force upon electrodes of an X-ray tube and causing electrons to beemitted from one electrode only while preventing any accompanyingmanifestations of positive ions, and pro ectin said electrons upon a target surface, where y one-half wave of an alternating current is utilized and the opposite half wave suppressed.
23: The method of operating a tube for the production of X-rays which consists in maintaining within the tube a vacuum so highly attenuated that practically no current can flow through the tube under the influence of the impressed voltage through the medium of ionization of gas molecules in the tube, and rendering the tube unilaterally conducting by generating electrons at the cathode independently of the voltage impressed upon the tube.
24. The method of operating a tube for the production of X-rays which consists in maintaining within the tube a vacuum so highly attenuated that practically no our-- rent can flow under the influence of the voltage impressed upon the tube, through the medium of ionization of gas molecules in the tube,and rendering the tube unilaterally conducting by generating electrons at the cathode by current independent of the voltage impressed upon the tube. 25. The method of depriving a metal body of occluded gas which consists in gen erating electrons constituting the origin of an electrical discharge, independently of the voltage producing the .discharge, in an evacuated space, the residual gas in which is so attenuated that at the given voltage the gas will not conduct'the discharge without the inde ndent generation of electrons, subjecting t e metal body to the electron discharge, and coincidentally removing gas evolved from said metal body to maintain the gas pressure so-low. that at the given voltage a discharge will not occur without theindependent neration of electrons.
26. The metho of depriving a' metalbody of gas\which 1consists .in generating -elec trons constituting the origin of an electric discharge independently of a voltage producing the discharge, in an' evacuated space in which the residual gas. pressure is so low that at a gsvcen voltage the 'gas will not conduct the harge without the independent 65 generation 22. The process of producing X-rays of electrons, subjecting the metal body to the electron discharge, the energy. of. the discharge.being suflicient to heat said metal body to incandescence, and
coincidentally removing gas disengaged by. said discharge to maintain the gas pressure solow that at the given voltage a discharge will not occur without the independent generation of electrons.
27. The method of preparing a discharge device, comprising an envelop,- an anode and 76-. a cathode which consists in generating electrons at the cathode independently of the voltage producing a discharge from said cathode, the space. in the envelop being evacuated so completely that no energy can 80 be passed at a given voltage between said electrodes without the independent generation of electrons at the cathode, removing gas liberated by said discharge and finally sealing the envelop when the residual gas pressure is so low that the device can be operated without appreciable evidence of positive ionization.
28. The method of depriving a metal body of gas which consists in heating a cathode to incandescence to generate electrons, assing between said cathode and said ody used as anode an electrical discharge in a vacuum so high that at the discharge voltage the residual gas will not conduct current without the independent generation of electrons at the cathode and coincidentally removing gas evolved from said metal body to maintain the gas pressure so low that at the given voltage a discharge will not occur 1 without the independent generation of electrons. p
29. The method of preparing an X-ray device comprising an envelop, an anode and a-cathode, both consisting of refractory ma-. 1 terial, which consists in heating the cathode to incandescence to generate electrons, passin between said cathode and anode a high vo tage electricaldischarge of suflicient energy to heat the anode in a vacuuin so high that withvthe given voltage the discharge is not maintainable while the cathode is unheated, removing gas from the evacuated space liberated from the anode by said dis-' charge and finally'sealing the envelop when the residual gas pressure is so low that the device can be operated without appreciable evidence of positive ionization;
30. The process of operating an X-ray device comprising a gas-tight evacuated envelop, a cathode and an anode, which consists in generating electrons at the cathode independently of current passed by said device while preventing any accompanying manifestations of positive ions, impressing on the electrodes a voltage at least as hifgh as the value-at which substantially all 0 the electrons are utilized, whereby the current becomes substantially independent of the. impressed voltage, and increasing or X-rays b respectively 1ncreasing or decreasing t e impressed voltage.
tially absent durin 31. An X-ray device comprising a gastight envelo an electron emittin cathode, and an ano e deprived of-ionizab e gas, the space in said envelo being evacuated to a pressure at least as ow as five hundredths of a micron of mercury pressure, the current transmitted by said device with a given electron-emissivity of the cathode bein substantially constant with an impress voltagevariable over a working range.
32. A Rontgen ray device comprising a sealed enve1op,"the space within which is evacuated to a pressure solow that evidences of positive ionization are substanthe operation of said device, a cathode o refractory metaliadapted to be heated substantially independentlyof a voltageimpressed upon said electrodes and a 006 rating, anode.
33. An -ray tube comprising a sealed envelo a cathode adapted to be heated indepen ently of a discharge transmittedby said device, 'an anode and a dished focusing member surrounding the' cathode having its concave side facing the anode, said tube being exhausted of gas to a pressure so low that evidences of positive ionization are substantially absent durin operation.
In witness whereof, have hereunto set my hand this 8th day of May,191-3.
' V D. 'COOLIDGE.
Witnesses: Bmmum B. HULL,