US1986806A - Method of operating photo-electric cells having auxiliary anodes - Google Patents

Description

Jan. 8, 1935. EN 1,986,806

' METHOD OF OPERATING PHOTd-ELECTRIC CELLS HAVING AUXILIARY ANODES Filed May 15. 1950 I 5: 1 I EH2.

SENSITIVITY SENSITIVITY SENSITIVITY M Patented Jan. 8, 1935 UNITED STATES METHOD OF OPERATING PHGTO-ELECTRIC CELLS HAVING AUXILIARY ANODES Heinrich H. Geffcken, Leipzig, Germany, assignor to Radio Patents Corporation, New York, N. Y., a corporation of New York Application May 13, 1930, Serial No. 452,05

In Germany May 21,

7 Claims.

This application relates to photoelectric cells, and more particularly to gas filled photoelectric cells.

It has already been suggested to provide a plurality of anode electrodes in a photoelectric cell for reducing the reaction on the operating characteristic of the cell due to terminal voltage variations in the output circuit. These conditions are analogous to the conditions prevailing in the electron tube, where it is possible to minimize the effect of the anode voltage variations by means of screening or suppressor electrodes. However. it was discovered that these considerations cannot be applied offhand from the electron tube to the gas filled photoelectric cell. If the photoelectric current of such cells is plotted against the anode voltage with the ,illumination being kept constant, it will be observed that the auxiliary electrode, arranged between the working anode and the light sensitive electrode, has practically no influence on the operating characteristic of the cell. Under circumstances, even a decrease in sensitivity is observed, and one might be inclined to completely reject the use of an auxiliary anode.

The object of the present invention is, therefore, to provide means for effectively utilizing an auxiliary electrode in a photoelectric cell.

A more specific object of the invention is to provide an auxiliary electrode in a photoelectric cell and means for applying proper biasing potential thereto, to maintain substantially uniform sensitivity of the cell within a predetermined operating range. Another object of the invention is to provide an auxiliary anode for a photoelectric cell with proper biasing potential applied thereto, to produce a substantially drooping shape of the sensitivity characteristic of the cell.

These and still further objects and aspects of the invention will become more apparent as the detailed description proceeds, taken in reference to the accompanying drawing, of which Fig.1 shows a simple circuit diagram for car.- rying the invention into effect.

Figs. 2-4 illustrate theoretical curves-explaining the operation in accordance with the invention.

The invention is based on the recognition that it is of primary importance for the effectiveness of the auxiliary anode in which part of the space, enclosed by the cell, ionization takes place under a given condition. According to the invention,

the biasing potential of the auxiliary anode is chosen of such high value that the characteristic curve for the main anode (anode current plotted against anode potential with illumination being kept constant) exhibits a decrease of its steepness in the neighborhood of the biasing potential value for the auxiliary anode. It has been observed that this peculiar phenomenon may be easily reprcduced by choosing a correspondingly high potential of the auxiliary anode. It is even possible, by choosing very high biasing potentials for the auxiliary anode, to effect a decrease of the steepness of the characteristic curve to such an extent that the result is a negative or drooping characteristic, i. e. that the anode current decreases with increasing potential within a certain voltage range. As is obvious, these conditions are basically different from the principle 01 the screen grid, as used in ordinary electron tubes. The invention also differs from the socalled dynatron principle, as there is no secondary electron emission from the main anode electrodc.

Referring more particularly to Fig. 1 of the drawing, the photoelectric tube shown includes a light sensitive layer 1 of substantially flat shape, which may be applied directly to the glass wall of the photoelectric tube, as shown, an auxiliary netlike anode 2 disposed opposite the light sensitive layer at a distance of about 3-6 mm., and a netlike main anode 3 also disposed at about 3-6 mm. distance from the auxiliary anode. The potential for both anodes may be adjusted by means of a potentiometer 4, provided with slidable tap connections and connected to a potential supply source as indicated by the plus and minus signs in the drawing. The circuit for the auxiliary anode furthermore includes a safety or ballastresistance 5, for instance of about 50,000 ohms, and the circuit for the main anode 3 includes the working resistance 6 of, for instance, 2 meg ohms. With varying illumination of the cell a varying potential drop occurs at the working resistance 6, causing a shift of the anode terminal voltage in the case of ordinary gas filled photoelectric cells, thus resulting in an undesired decrease of the sensitivity of the cell with varying illumination.

Curve '7 according to Fig. 2 illustrates the sensitivity characteristic of an ordinary gas filled photoelectric cell, including a single anode, the sensitivity E (anode current plotted against anode voltage with constant illumination) being plotted against the anode potential Va. This illustrates the well known curve, assuming almost infinite steepness at the ignition point of the cell whereby an independent glow discharge unaffected by the illumination of the tube is produced. It is obvious that the sensitivity of such a cell is highly dependent upon the operating potential. Curve 8 according to Fig. 2 in turn illustrates the characteristic of a photoelectric cell including two anodes and connected in a circuit as shown by Fig. 1. The sensitivity E (main anode current per unit of illumination) is plotted against the anode potential Va. As is seen, this curve practically does not difier from a normal photoelectriccell with a single anode.

However, if, in accordance with the invention,

Gil

the potential of the auxiliary anode is gradually increased, sensitivity curves are obtained as shown by Figs. 3 and 4. In the case of curve 9, according to Fig. 3, the auxiliary anode carries a biasing potential of about volts, and in the case of curve 10, according to Fig. 4, the biasing potential for the auxiliary anode is about volts. It is readily seen that with the proper choice of the biasing potential of the auxiliary anode, in accordance with the invention, within a certain range suddenly a strong decrease of the steepness of the anode current or the sensitivity, respectively, sets in. In this manner, an operating range for the photoelectric cell is secured wherein comparatively small variations of the sensitivity take place, due to the fact that the auxiliary anode reacts on the output current in the desired manner to maintain uniform operating conditions.

The explanation for this astonishing phenomenon may be given in that with suificiently high biasing potential of the auxiliary anode ionization at first takes place in the space between cathode 1 and auxiliary anode 2. If the potential of the main anode 3 is increased and becomes equal to the potential of the auxiliary anode 2, the region of ionization is gradually shifted into the space between the auxiliary anode 2 and main anode 3. The electrons arriving from the cathode 1 are shooting from space 1, 2 through the meshes of the auxiliary anode 2 into the space 2, 3, where they will effect their ionization action. Due to this displacement of the ionizing region from space 1, 2 into space 2, 3, a large part of the gaseous ions is brought into a. field of very low potential drop or correspondingly increased probability of recombination. Besides many of the ions will be neutralized by the auxiliary anode. Thus a decreased amount of ions will reach the cathode, whose electronic emission is no longer increased to the same extent, by ionic bombardment, as previously. The above explanation is confirmed by experiments during which a transition of the auxiliary anode current from a positive to a negative sense was observed within the above voltage range. Regarded diagrammatically, this phenomenon appears to be due to a transitionof the sensitivity characteristic from one position (corresponding to the auxiliary anode) into another position (corresponding to the main anode), indicated by the dotted lines according toFig. 4.

It has been found advisable to use an operating potential for the main anode, which is higher or equal to the potential'of the auxiliary anode. In accordance with the invention the anode potential, dependent on the average value of the illumination at which the cell operates, is adjusted to such a value with the potential variations during operation remaining within the region as specified according to the invention. In this manner, distortions are prevented to a large extent, and the sensitivity of the cell maintained at a substantially uniform value.

In some cases it is preferable to utilize the negative characteristic which may be obtained by providing sufficiently high biasing potential for the auxiliary anode, such as for directly producing a trigger action as is well known in connection with gaseous relays. In this manner, the invention makes it possible to obtain sudden and abrupt changes of the anode current, whenever the illumination of the cell increases or decreases by a predetermined amount, for producing a release such as of a mechanical relay or the like in various manners.

It is obvious that the invention as described, is subject to many variations and modifications, coming within its broader scope, as comprised by the appended claims.

What I claim is:

1. In combination, a photoelectric tube comprising, a vessel filled with a gaseous atmosphere, a light sensitive cathode and an anode therein, an auxiliary electrode, means for applying operating potential, further means for applying a biasing potential to said tube insuflicient to produce an independent glow discharge therethrough, to said auxiliary electrode, of a critical value to secure a drooping shape of the sensitivity characteristic of the tube for a critical operating range.

2. An electrical system as claimed in claim 1,

in which a common potentiometer is provided for securing operating and biasing potential for said tube, and a ballast resistance in the connection from said potentiometer to said auxiliary electrode.

3. In combination with an electrical system, a

photoelectric tube comprising, a vessel filled with a gaseous atmosphere, light sensitive cathode and anode electrodes within said vessel, an auxiliary anode, a working impedance in series with said anode electrode, means for applying operating potential to said tube insufficient to produce'an independent glow discharge therethrough and further means for applying'a biasing potential to said auxiliary electrode, of a critical value to se cure uniform sensitivity of said tube over a critical operating range.

4. An electrical system, as claimed in claim 3, in which a common potentiometer is provided for securing and operating biasing potential for said tube, and a ballast resistance in the connection from said potentiometer to said auxiliary electrode.

5. The method of operating gas filled photoelectric tubes having a light sensitive cathode, a

main anode, and an auxiliary positive electrode comprising applying a positive potential to the tube insufiicient to produce an independent glow discharge biasing the auxiliary electrode to a critical positive potential close to the anode potential to secure a drooping shape of the sensitivity characteristic of the tube over a critical operating range.

6. In a gas filled photoelectric tube having a light sensitive cathode, an anode and a meshlike auxiliary electrode intermediate said cathode and anode, the method of operation comprising applying a positive potential to said anode insuflicient to produce an independent glow discharge through the tube and biasing said auxiliary electrode to a critical potential close to the potential of said anode to secure even sensitivity of said tube over a critical operating range.

'7. In combination with a gas filled photoelectric tube having a light sensitive cathode, an anode and an auxiliary electrode; means for applying operating potential to said cathode and anode insufiicient to produce an independent electric glow discharge; and further means for biasing said auxiliary electrode to a predetermined potential for securing a critical operating range of substantially constant sensitivity of said tube to light variations.

HEINRICH H. GEFFCKEN.

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