Radio age (Jan-Dec 1926)

24 RADIO AGE for January, 1926 The Magazine of the Hour It wont be long before YOU'LL SEE WITH YOUR RADIO TUBES Interesting developments on the thermionic photo-electric cell detailed by Engineer IN LATE years photo-cells are taking a more and more important place in ' abpratories as most precise and sensi-. tive apparatus. With the increase in application of photo-cells the methods of using them are improving or, vice versa, the improvement of methods increases the usefulness of cells. After the development of methods of amplification of phot-electric impulses by means of thermionic tubes, the application of cells extends far beyond the laboratories. We know now, after the works of G. de Prell (Ann. der Physik No. 3, 1923); G. Ferrie (Compes, Rendus November 5, 1923) and others how to amplify the photo-electric impulses as high as a million times by means of a single thermionic tube. These methods are particularly adaptable for amplification of very weak impulses an require carefull insulation and evenspecial thermionic tubes for best results. In order to simplify the installation and adapt the photo-electric cell for use of untrained operators, the following device was developed, having in view the output sufficient to operate directly the average mechanical relays. Photo-Sensitive Tube . This device is the combination of thermionic tube with a photo-sensitive control electrode. It consists of filmanent of oxide coated type inside of open mesh grid enveloped completely by another grid of fine mesh. This second grid is in electrical contact with metallic coating of inside wall of glass container. The fourth electrode of cylindrical shape is around the second grid. These last three electrodes are coaxial. The inside of the cell is coated with photo-emitting substance, for instance alkali metal, and treated in the usual way. Great care should be taken to prevent the alkali metal from condensing appreciably on insulatng parts of the cell. In order to prevent the light from the filament falling on sensitive film, the part of second grid is closed by metal shields and the filament is operated at the temperature below visual emission. Several connections are possible with this cell, depending on requirement of the output. The simplest connection is to let the second grid float and first grid connected to plus side of the filament and positive potential applied to the cylinder in respect to the filament. How it is Used The cell is now operating as three electrode tubes, as the first grid acts only for reduction of impedance of the tube and can even be omitted. (♦Research Department, Westinghouse Electric and Manufacturing Co., East Pittsburgh, Pa.) By V-. K-. Zworykn* ■yOU can truly say now that vacuum tubes have X eyes, at least electrically. The tube shown above is the thermionic photo-electric tube which is adaptable to transmission of photos either by wire or radio. Much experimental work is being done on this phase of transmission and the story by Mr. Zworykn should be of interest to photo-transmission fans. The second grid acquies the negative charge from the electronic flow and blocks the current between the filament and anode. If the light will fall now on the sensitive film, it will discharge by photoelectrons the second grid, the latter being connected with the film. The blocking action will be reduced and part or the whole available thermionic current will flow to the anode, depending on the rate of discharge of film or intensity of the light. With such arrangement, the output of the order of milliampere was obtained, the cell being preferably of hard type. When larger output is desire without going to the higher potentials and without sacrificing the sensitiveness, the other circuit is preferable. The first grid is used as an anode with low potential (order of 30 volts) due to the close spacing between this grid and filament. The second grid is connected through the high resistance to a negative potential with respect to the filament. The cylinder can be connected to the first grid directly or, preferably, have possible potential in respect to it. Can Block Current The second grid is now working as outside control electrode and with sufficient negative potential can completely block the current between filament and first grid. If this potential will be adjustable without light falling into the cell, no current will flow between the electrodes and no voltage drop across the resistance. While illuminated the film and second grid will discharge the photo-electrons to the cylinder and this current will produce the voltage drop across the resistance. The potential of the second grid will be lowered and the current start to flow from filament to the first grid. The amount of current depends upon rate of discharge of photo-electrons, i. e. illumination, and charge of second grid through the resistance. By proper choosing of spacing and mesh of second grid and adjusting the resistance, it is possible to obtain good relation between the intensity of the light and the output within certain limits. The cell in the arrangement, of course, has the time lag which is proportional to the capacity of second grid and the value of the outside resistance. Ine one tube, made as described above, the time lag with this circuit was calculated to be of the order of 1-10000 of a second and this has been verified experimentally up to a frequency of 3,000 cycles. This value can be considerably reduced by diminishing the capacity of the second grid and increasing its voltage factor on the first grid. The continuous output obtained Was of the order of five milliamperes. This limit was due to the heat developed inside of the cell, which distills the alkali metal on the transparent and insulating parts. Of course by proper construction this also can be improved. For this connection the cell, of both soft (Turn to page 54)