US1955335A

US1955335A - Photoglow tube

Info

Publication number: US1955335A
Application number: US333389A
Authority: US
Inventors: Dewey D Knowles
Original assignee: Westinghouse Electric and Manufacturing Co
Current assignee: CBS Corp
Priority date: 1929-01-18
Filing date: 1929-01-18
Publication date: 1934-04-17
Anticipated expiration: 1951-04-17
Also published as: GB353394A

Description

D. D. KNOWLES PHOTOGLOW TUBE April 17, 1934.

Filed Jan. 18, 1929 lNVENTOR fien/e fl Know/e3.

ATTORNEY Patented Apr. 17, 1934 UNITED STATES ATENT OFFICE PHOTOGLOW TUBE Dewey D. Knowles, Wilkinsburg, Pa, assignor to Westinghouse Electric & Manufacturing Company, a corporation of Pennsylvania Application January 18, 1929, Serial No. 333,389

6 Claims.

' shall maintain its electrical characteristics substantially invariable throughout long periods of operation.

A further object of my invention is to provide an electrical-discharge device of the character described above in which the range of wave length of radiation to which it is responsive may be varied between wide limits.

In connection with many branches of the electrical art, it is desirable to provide a relay device adapted to initiate and cut off the flow of current of considerable magnitude in response to the amount of illumination falling upon a particular point or area. The present invention provides a single electrical-discharge tube having one of its electrodes covered by a photo-electrica11y-sensitive substance. When a voltage is impressed between the electrodes of such a tube, a critical voltage exists below which substantially negligible current flows through the tube and above which a current of the magnitude of one-tenth of an ampere is conducted in connection with a glow in the tube, and the value of this critical voltage decreases as the intensity of radiation falling upon the photo-electrica1ly-sensitive electrode in 0 creases.

It is true that a number of substances are known which are photo-electrically responsive and will, accordingly, vary the electrical-current flow through a gas by which they are surrounded in accordance with the amount of illumination falling upon their surfaces. However, in the case of substances heretofore known, it is found that the sensitivity of the substance to light varies with time, perhaps due to a chemical combination between the photo-electric substance and the gaseous atmosphere. In accordance with one feature of my present invention, I have found that, if the photo-sensitive surface comprises an underlying layer of silver or, alternatively, an alkaline-earth metal, or preferably magnesium,

which has superposed upon it an extremely thin layer of an alkali metal, such as caesium, the electrical characteristics of a gaseous discharge path comprising one of the noble gases, such as,

helium, remained unaltered through extremelylong periods of use. Light-sensitive surfaces of the character above described have the further advantage of being capable of simple and quick manufacture by processes well adapted to largescale production.

7 With the foregoing principles and objects in mind, my invention may be better understood by reading the following specification, taken in connectionwith the drawing in whichthe single figure H is a View, partly in elevation and partly a longitudinal section, of a tube embodying the principles of my invention.

Referring in detail to the drawing, the tube comprises a vacuum-type container 1 of ordinary form into which are sealed a cathode lead 2 andan anode wire 3 of the general form shown in the drawing. The lead 2 lies parallel, and closely adjacent, to the wall of the container 1 for a considerable distance, its upper end being bent and I wire 3 but is spaced away from it by a distance 7 of the order of magnitude of the diameter of wire 3.

A tube, constructed as hereinbeiore described, is first exhausted by methods well known in the vacuum-tube art. In accordance with such methods, the tube is heated to a temperature as high as the glass wall thereof will withstand without collapsing, but before sealing off, a noble gas, at less than atmospheric pressure, is introduced into the tube by any method well known in the art. It may then be sealed off from the vacuum pump. Such having been done, a highfrequency magnetic field is produced in the vicinity of the electrode l, and the latter is heated to such temperature that the magnesium 5 thereon is vaporized and immediately deposited on the walls of the tube, making contact with the wire 2 and forming the base for the photo-sensitive film. At the same time, the magnesium acts as a getter for any gases or vapors remaining or liberated during the sealing-off process. The sleeve metal.

6 prevents the vapor of magnesium from depositing on the glass near the joint with lead 3 and thereby insures the insulation of the latter from the magnesium coating.

It will be noted that, while I have described magnesium as the metal to be vaporized, metals of the alkaline-earth group and even aluminum alloys, such as the well known calcium-magnesium alloy misch metal, may be utilized for this purpose. The metal to be used depends upon convenience of manufacture and the particular characteristics of the photo-electric tube which are desired.

Where it is desired to employ a base layer of silver, the latter may be formed by subliming silver from an electrically'heated wire within the container or even by the chemical-reduction methods well known in the mirror-making art.

In order to provide the other element of the pair of metals which are combined in the photoelectric surface which I have invented, several methods and materials are available.

vance with one method, a small amount of theva- In accordpol of a metal or metals may be introduced,-after heating a small tube containing the desired The metals chosen should be suchas have a maximum on their emissivity versus wavelength curve in the region of the wave length of the radiation to which response is desired. .Usuallyresponse is desired to radiationin the yisiblespectrum; hence, 1ithium,.sodium, po-

tassum, rubidium and caesium, which have maxima of emissivity in the visible region, are ordinarily preferable for use.

A way which Iprefenhowever, consistsin painting a weak solution of an alkali-metal trinitride, such as caesium trinitride, on the portion .of the electrode 4 considerably removed from the magnesium 5.

Stillanother method of settingfree therequired alkali metal istoplace a small amount of some salt thereof,which will not yield a deposit when heated in vacuum to about 500 degrees C., on an electrode within the tube and produce a discharge which shall bombard the salt with ions and, consequently, decompose it.

After the vaporization of the .magnesium, it

:may, in general, be necessary to heat a small portion of the tubewall to clear a window so that light may enter and strike. upon the photo-sensitive surface.

In the foregoing manner, a layer of. magnesium 7 will be deposited-on the tube wall and, upon the surface of the latter, will be deposited a thin layer of caesium 8. It will be recognized thatv the proportions of the layers '7 and 8 shownin the drawing are diagrammatical only and do not represent the true magnitude or relative proportion ductivity thereof that ionization by collision will increase to an enormous degree, and a very considerable current will flow between the electrodes of tube 1 accompanied by a glow in. the e.

flow, once initiated, will persist even after the illumination of the tube ceases. hand, if the voltage impressed upon the tube is On the other periodic, falling to zero at intervals, current flow once initiated will persist until the voltage falls to zero for the first time after the illumination is cut off. The gas in the tube will then almost instantaneously return to its substantially nonconductive condition, and appreciable current flow will only begin again when radiation is again incident to the light-sensitive surface.

In accordance with my invention, I have found that the electrical characteristics of the tube, such as the critical voltage fora given intensity of illumination, remain substantially unaltered throughout an active life of hundreds of hours;

Such a permanency of electrical characteristics is obviously of the highest importance from a practical standpoint and is found in the case of no other gas-filled photo-sensitive discharge tubes which carry currents of the magnitude of 1105 one-tenth of an ampere, such as'the above-described device is capable of carrying. Such a performance is to be distinguished from that of ordinary photo-electric tubesinwhich the ourin 'micro-amperes. That such relatively large current flow should not cause variation in the electrical characteristics of the photo-sensitive device is a very remarkable fact, and by obviating the employment of auxiliary amplifying devices;

produces a device of high commercial value.

Another importantpractical advantage of the device above described lies-in the fact that, by

.properly choosing the base layer 7 and the surface layer 8, the particular wave length of radiation to which the device is most sensitive may -be Varied over a wide range; thus, by employing barium as the outer layer, in combination with the magnesium base, a device having maximum sensitivity in the visible region of the spectrum.

is obtainable. By substituting caesium for barium, the maximum sensitivity is displaced toward the blue-green region of the visible spectrum. The-substituttion of rubidium, potassium and sodium as the surface layer successivelydecreases the wave length of maximum sensitivity farther and farther into the ultra-violet. On the other hand, by employing two or more of the alkali *metals, in combination, in the surface layer,sen-

sitivity corresponding to that of both components is obtained, so that a substantially uniform sensitivity over a very considerable range of wave lengths may be attained. In fact, by properly combining and proportioning the various substances enumerated as suitable for the photosensitive layers, a wide range of photo-electric characteristics may be obtained.

While I have described the foregoing particular embodiments of my invention, it will be evident to those skilled in the art that many modifications thereof may be employed to meet special problems without departing from the broad principles which I have described. Accordingly, I

desire that the claims shall begiven the broadest rents actually traversing the device are measuredino reasonable interpretation to which their terms are susceptible in view of the prior art.

I claim as my invention:

1. A glow tube comprising a photo-sensitive electrode having an outer layer of a metal of the alkali metal group and an underlying layer of a metal of the alkaline-earth group surrounded by an atmosphere adapted to glow in operation comprising a noble gas at less than atmospheric pressure and another electrode cooperating with said first mentioned electrode.

2. A glow tube comprising a photo-sensitive electrode having a layer of ametal of the alkali metal group and an underlying layer of a metal of the alkaline-earth group surrounded by an atmosphere adapted to glow in operation comprising a noble gas at less than atmospheric pressure and another electrode cooperating with said first mentioned electrode.

3. A glow tube comprising an anode having a relatively small exposed surface and a photosensitive cathode comprising an outer layer of a metal of the alkali group and an underlying layer of a metal of the alkaline-earth group, said electrodes being surrounded by a noble gas at a substantial pressure adapted to glow in operation.

4. A glow tube comprising an anode having a relatively small exposed surface and a photosensitive cathode comprising a layer of a metal of the alkali group and an underlying layer of a metal of the alkaline-earth group, said electrodes being surrounded by a noble gas at a substantial pressure adapted to glow in operation.

5. A glow tube comprising an anode having a relatively small free surface, a gaseous atmosphere at a substantial pressure adapted to glow in operation and a photo-sensitive cathode having a relatively extensive surface and comprising an underlying layer of a metal of the alkalineearth group and an outer layer of a metal of the alkali group.

6. A glow tube comprising an anode having a relatively small free surface, an atmosphere of a gas at a substantial pressure adapted to glow in operation and a photo-sensitive cathode having a surface area at least ten times the free surface of said anode and comprising an underlying layer of magnesium and an outer layer of caesium.

DEWEY D. KNOWLES.